National Academies Press: OpenBook

Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography (1990)

Chapter: 3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs

« Previous: 2 State-of-the-Art Overview: Physical Oceanographic Processes, Features, and Methods of Potential Importance to the ESP
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 53
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 54
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 55
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 56
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 57
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 58
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 59
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 60
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 61
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 62
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 63
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 64
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 65
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 66
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 67
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 68
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 69
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 70
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 71
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 72
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 73
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 74
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 75
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 76
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 77
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 78
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 79
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 80
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 81
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 82
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 83
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 84
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 85
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 86
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 87
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 88
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 89
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 90
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 91
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 92
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 93
Suggested Citation:"3 Regional Oceanography and Evaluation of the Regional Studies Program and Washington Office Generic Programs." National Research Council. 1990. Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography. Washington, DC: The National Academies Press. doi: 10.17226/1609.
×
Page 94

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Regional Oceanography and Evaluation of the Regional Studies Programs and Washington Office Generic Studies Programs IN~ODUCrlON Primary responsibility for planning, organizing and synthesizing data from site-specific ESP studies has rested with the four regional offices of the ESP. These offices cover the four main regions of the U.S. OCS (see Fig. 2~: the Alaskan coast, the Pacific coast, the Gulf of Mexico, and the Atlantic coast, and are coordinated by BES in Washington, D.C. The ESP's internal division of responsibility alone justifies separate consideration of each regional program in this review. There are also, however, good geological and oceanographic reasons for considering the four U.S. margins independently. Geologically, the separation of the various U.S. margins is founded on primary differences in the crustal structure of the earth. The basic processes of orate tectonics create . . . . . . . ~ severe' cnarac~er~st~c conunenta' margin forms, which can be simplified into (1) broad shelf- slope-rise with large continental drainages, formed by passive spreading, and (2) narrow shelf- steep slope-marginal trough with steep local drainages, formed by active convergence. The Gulf Coast and Atiantic coast are variants of the passive form, whereas the Pacific coast is a convergent active form with two or three variations. The Bering Sea and Arctic Ocean margins are of the broad type, but glacial effects also have played a major role in modifying the inner portions of the continental shelf. Since the morphologies of the coastline and seafloor have a major influence on circulation in the coastal ocean, these geological differences lead directly to oceanographic differences. In addition, the U.S. margins differ significantly in their mass budgets of terrigenous sediment input and removal. Oceanographic and climatological differences further distinguish the waters of the U.S. continental margins. For example, the first-order circulation of the surface ocean differs on the western and eastern sides of oceans, and ice clearly is more important in Alaskan waters than it is on the other U.S. margins. The net result of these geological, oceanographic, and climatological differences was stated succinctly by Allen et al. (1983~: "In general, observations from geographically distinct continental shelves have shown that the nature of the flow may vary considerably from region to region. Although some characteristics, such as the response of currents to wind forcing, are common to many continental shelves, the relative importance of various physical processes in influencing the shelf flow field frequently is different." This chapter briefly reviews the important physical oceanographic characteristics of each of the four major U.S. continental margins the Alaska, Pacific, Gulf of Mexico, and Atlantic regions and evaluates the adequacy and applicability of the ESP physical oceanographic studies carried out uncier the auspices of each regional office. In Alaska, much of the research was carried out by NOAA's Outer Continental Shelf Environmental Assessment Program under an interagency agreement with MMS. The WO is also evaluated. The panel's criteria for evaluating a study's adequacy are described at the end of Chapter 1. 53

54 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF THE ALASKA REGION The continental shelf off the coast of Alaska can be divided into three distinct subregions defined by the topography and bathymetry of the Alaskan coast. Approached from the southern coast clockwise around the Alaska Peninsula, these subregions are the Gulf of Alaska (Fig. 7), the Bering Sea (Fig. S), and the Chukchi ant! Beaufort Sea in the Arctic (Fig. 9), which are further subdivided into planning areas (see Fig. 3~. The meteorology and circulation of each of the subregions are discussed below, followed by a discussion of sea ice in Alaskan waters. Meteorology and Circulation Gulf of Alaska The shelf in the Gulf of Alaska is in general broad, up to 200 km, and contains several deep troughs. Cook Inlet extends inland from the northern apex of the gulf. The northeastern shelf has two major islands (Kayak and Middleton), and to the west, Kodiak Island is midshelf. To the west of Kodiak Island, the Aleutian Islands form a leaky boundary between the North Pacific and the Bering Sea. The shelf is quite deep, with depths in excess of 200 m within several kilometers of the mountainous coastline. Approximately 20% of the coastal region east of Cook Inlet is covered with glaciers. The only major river to empty onto the shelf is the Copper River, although the total freshwater input to the shelf waters is appreciable (Allen et al., 1983), equivalent to the flow of the Mississippi River. Meteorology The weather over the Gulf of Alaska has large seasonal variations in temperature, wind, pressure, and precipitation, all of which affect the ocean. Winter winds over the gulf result from strong cyclonic systems that tend to be trapped in the gulf. This leads to alongshore winds, onshore Ekman transport, and coastal downwelling. A weak high-pressure system replaces the Aleutian low in summer, which results in relaxation of the coastal winds and cessation of coastal downwelling (Royer, 1975~. Adiabatic ascent of moist marine air over the coastal mountains due to trapped storms results in high precipitation rates and runoff in the coastal drainage areas. As much as ~ m/yr falls in the coastal mountains. Although its significance has generally been disregarded, this runoff is equivalent to the flow of the Mississippi River, with a peak in fall and a weak secondary peak in spring. Circulation Off the shelf on the east side of the Gulf of Alaska is the Alaska Current, which is wide (greater than 100 km) and flows at about 0.3 m/s. This current becomes the Alaska Stream as it turns toward the west-southwest off Kodiak Island. Here it narrows to less than 60 km, with speeds of l m/s (Royer, 1981~. High-frequency variability is not typical of this system, although eddies have been observed, and the consequent flux of momentum into coastal waters has not yet been established. The shelf circulation system, which is generally separated from the Alaska Stream and the Alaska Current by a midshelf "doldrum," is strongly influenced by freshwater input from the coast (Allen et al., 1983~. Along the Kenai Peninsula, this flow is a distinct narrow coastal current flowing westward with typical speeds of 0.2 m/s. However, currents can reach speeds as high as 1.5 m/s in the fall due to maximum freshwater discharge in September to October, resulting in surface salinities as low as 25 parts per thousand at the coast. This maximum current precedes the maximum winter winds by several months. An important role of the alongshore winds, which cause coastal downwelling, is to trap the fresh water along the coast even after being transported hundreds of kilometers from the source. These features are clearly seen in

o me, ll al - o it 8 o _ _ 55 -.1 ~ WN at\\ O o <: o Y °_ CO J o J ~5 . . Con \~ I\ o O o ~ ~- ~ _ D In Ct Cal - o

~ ) - ~r- o - 4D o ,`O - ~ z W`-'\: -o ~ ~ ~ \; llJ I A J :~ I . in ~ llJ a: Z - OCO m ~; ' ' Con CO W7 // o C' oo J to o ~ ~o Z co ~_ . o ~ O ~ O m 0o 0 Q o A_ .~ 0 l or: z ,, _ am m \2 / ~ /; \ ~ / / l / / / ~ / °_ Cot a o . Cd ·_I m 00 by _

57 ; o\ o\ \ At; N) ~ ) ~-,1 ~ 0~ 1 ~ ~ ~ , o _ · ~ _- , 0 ~_ ~ ... <a - . Ct _S ~ Cod coons _s lll - :r: CD If, L-4 o l en G . . Cal O ·~) U) ._ Crx _

58 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF seasonal sea-level cycles at various tide stations as the circulation responds to the seasonal wind stress. The absence of the strong cyclonic wind field in summer permits relaxation of onshore Ekman transport and cessation of coastal downwelling. This allows the onshore intrusion of denser water over the continental shelf bottom. Renewal of bottom water in the fjords along the coast takes place by late summer (Muench and Heggie, 1978~. This type of deep-water renewal is indicated by seabed drifter studies near Kodiak Island, where some drifters released near the shelf break have been found within local bays (Allen et al., 1983~. Measurements of currents in the northeastern Gulf of Alaska indicate that the flow tends to follow the shelf topography in the nearshore (Allen et al., 1983~. With increasing distance offshore, the eddy kinetic energy increases as the Alaska Current is approached at the shelf break. However, in the western gulf, as the Alaska Stream is approached seaward, the mean energy of the current increases so that the relative eddy kinetic energy decreases. This suggests that eddies play a more important role in the northeastern gulf shelf-break region than in the northwestern part. The eddies embedded in the Alaska Stream and the Alaska Current appear to be transient. However, there is at least one permanent eddy in the downstream flow to the west of Kayak Island (Galt, 1976~. This islanc! plays an important role in deflecting part of the relatively fresh coastal current that recirculates behinc! Kayak Island and forms the permanent Kayak Island Eddy. Satellite-tracked drifters released east of Kayak Island in 1976 meandered over the outer and midshelf regions before they slowly headed inshore and were caught up in the coastal current (Allen et al., 1983~. They were then kicked back out onto the shelf by Kayak Island and made circuits in the Kayak Island Eddy. After the drifters exited the eddy, they entered Prince William Sound through Hinchinbrook Entrance (where tankers bring the Alaska Pipeline oil out for shipment elsewhere) and became stranded inside Prince William Sound. The flow continues out of Prince William Sound and around through Montague Strait to the west. The circulation in Cook Inlet is strongly tidal, with a great deal of local variability due to coastal and topographic complexity. Cook Inlet is the only part of the Gulf of Alaska that is substantially affected by sea ice, although elsewhere floating ice near glaciers can be a navigational hazard, as the recent Exxon Halides accident illustrates. Interannual Variability ENSO signals can be seen in the Gulf of Alaska in sea temperatures off Seward, especially deeper in the water column (Xiong and Royer, 1984~. Strong interannual variability also occurs in the position and intensity of the Aleutian low, both as a result of interaction with ENSO and with the Pacific-North American weather pattern (Niebauer, 1988~. Although the effects of these changes in the Aleutian low can clearly be seen in the Bering Sea, especially in ice cover, they are not as obvious in the Gulf of Alaska. Bering Sea The Bering Sea continental shelf is one of the widest in the world outside of the Arctic Ocean. It is 1200 km long and 500 km wide but is fairly shallow, with a shelf break at 170 m. Several large canyons indent the continental slope. Bristol Bay is located to the southeast, and Norton Sound is to the northeast. The Kuskokwim and Yukon rivers empty onto the eastern shelf. The Bering Sea shelf connects with the Arctic Ocean through the Bering Strait with about 1 sverclrup (Sv) flow toward the north (Allen et al., 1983~. However, there are reversals in the flow that can last at least 1 week and are often associated with weather patterns. Unimak Pass is the only large pass from the Pacific onto the shelf, with transport of only about 0.15 Sv into the Bering Sea. Much of the remaining water required to balance the outflow through the Bering Strait comes through the deep water Aleutian passes west of the shelf and flows across the northern shelf south of Cape Navarin.

REGIONAL OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS Meteorology 59 The Bering Sea shelf is characterized by strong variability, including interannual variability, due to its high latitude and to the great variability in weather. Insolation ranges from nearly continuous darkness in winter to nearly continuous light in summer. The wind torque varies by an order of magnitude from summer to winter. The shelf region is ice covered in winter, but the entire Bering Sea is ice free in summer. Circulation Tidal currents dominate the southeastern shelf region, varying from 60% of the horizontal kinetic energy in the outer shelf to 90°h in the coastal domains (Kinder and Schumacher, 1981~. About 80% of the tidal energy is semidiurnal. Farther north, the tides are less energetic. The M2 constituents vary from 0.35 m/s along the Alaska Peninsula to about 0.03 m/s or less in Norton Sound. Mean flow over the shelf is described qualitatively by the dynamic topography, with some low-frequency pulses driven by weather systems. From more than 20 record-years of direct current measurements on the southeast Bering Sea shelf, three mean and low-frequency current regimes have been identified. These regimes are nearly coincident with the previously described hydrographic domains. In the coastal domain coastal water from the Gulf of Alaska flows through Unimak Pass and then northeastward along the Alaska Peninsula. The flow is counterclockwise in Bristol Bay and follows the 50-m isobath northward. Currents are strongest near the inner front and parallel the front at 0.01 to 0.06 m/s with the highest speeds in winter. Although about 969`o of fluctuating kinetic energy is dominated by tides, there are wind-driven events. However, the combination of baroclinic geostrophic currents with residual flow produced by the interaction of the tides with the bottom topography seems to account for the observed mean flow. In the middle-shelf regime or domain that lies between the inner and middle fronts, there is little mean flow (~0.01 m/s) except near the fronts. There are wind-ciriven pulses corresponding to meteorological forcing at periods of 2 to 10 days that are similar in magnitude to those in the coastal domain. Due to the width of the shelf, no coasts are within a Rossby radius of deformation, so there is no coastal upwelling or downwelling, or any associated currents, in this regime. This lack of mean flow, along with the strong seasonal pycnocline, allows the retention of the cold bottom layer throughout the summer. In the outer-shelf regime or domain, there is significant mean flow. The flow along the isobaths toward the northwest is 0.01 to 0.1 m/s and across the isobaths onshore toward the northeast is 0.01 to 0.05 m/s. Because the cross-shelf flow does not usually extend into the middle-shelf regime, the middle front is often a region of surface convergence, and advection is as important as diffusion in cross-shelf fluxes. This outer-shelf current regime has more kinetic energy at periods greater than 10 days than do the two inshore regimes, due probably to propagation of energy from the Bering Slope Current and associated eddies, although no eddies have been found up on the shelf. The Bering Slope Current flows northwestward seaward of the shelf-break front at along- slope speeds of 0.05 to 0.15 m/s. This current parallels the shelf break, flowing from Unimak Pass to near Cape Navarin at a speed of 0.1 m/s with a transport of about 5 Sv. The slope current is characterized by mesoscale eddies that appear to be formed and/or trapped by bottom topography, especially in the undersea canyons that are present in the slope. Eddies have been found not to propagate up onto the shelf. Currents on the northern shelf are dominated by the northward flow into the Arctic, with temporary reversals caused by storms, particularly in winter (Kinder and Schumacher, 1981~. The mean flow seems generally to parallel the isobaths, with currents moving at speeds of 0.1 to 0.25 m/s in the Bering Strait and east and west of St. Lawrence Island. In Norton Sound the flow is generally weak, although instantaneous wind-driven currents flowing at up to 1 m/s have been measured.

60 Interannual Variability PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF In the Bering Sea, in addition to the strong annual variability, extraordinary multiyear or interannual variability in ice cover, air and sea temperatures, and winds over the eastern Bering Sea shelf have been observed (see, e.g., Niebauer, 1988~. For example, sea-surface temperature (SST) was 2 to 3°C below normal and winter ice-cover about 20% above normal in 1976. By 1979, SST was 2°C above normal and ice cover about 35% below normal. This resulted in an interannual ice-cover retreat of about 400 km, or about 40% of the ice extent in the Bering Sea. These observations have led to a conceptual model in which variability in the winter atmospheric circulation, primarily the Aleutian low, appears to be the primary driving force behind the interannual variability in the eastern Bering Sea oceanic environment. This model implies that the interannual signal is not driven by the Bering Sea ocean circulation because of the sluggish mean flow on the shelf and because of the restricted flow through the Aleutian passes. In addition, the flow through the Bering Strait is mainly northward out of the Bering Sea. The variability in the Bering Sea occurs primarily in winter because of the interannual variability in the mean winter atmospheric circulation and because the Aleutian low essentially disappears in summer. Recently, this interannual variability has been linked to ENSO variability (Niebauer, 1988). Hydrography The immense width of the shelf tends to spread the various sources of energy (e.g., tides, wind, and fresh water) over a large area. Over the southeastern shelf the mean flow is low, of the order of 0.1 to 0.2 m/s toward the northwest. Most of the horizontal kinetic energy is tidal. Seaward of the shelf break, the Bering Slope Current flows at speeds of approximately 0.1 m/s, with frequent mesoscale eddies. The hydrographic structure on the shelf, which seems little influenced by the slow mean flow, tends to be formed by the boundary inputs from insolation, cooling, melting ice, freezing, and river runoff, as well as lateral exchange with bordering oceanic water masses. Three distinct shelf hydrographic domains and an oceanic domain can be defined, delineated by water depths and separated by fronts that generally parallel the isobaths (see, e.g., Coachman, 1986~: 1. The coastal domain, inshore of the 50-m isobath, is vertically homogeneous due to bottom tidal shear and surface wind shear overcoming the buoyancy input and mixing the water column from top to bottom. The coastal domain is separated from the middle domain or middle shelf by a narrow (10-km) front, called the inner front, where the tidally mixed bottom layer is separated from the wind-mixed surface layer as depth increases. 2. The middle domain or middle shelf, between the 50- and 100-m isobaths, tends to be strongly stratified and has a two-layered structure in summer due to the vertical separation of the tidal and wind mixing and due to seasonal buoyancy input (insolation and/or ice melt). In winter this region tends to be homogeneous due to strong winter storms and lack of buoyancy input. There is no significant advection, so heat content is determined by air-sea interaction. The salt flux required to maintain the relatively constant mean salinity is due to cross-shelf, tidally-driven diffusion. The middle shelf is separated from the adjacent outer domain or shelf by a weak front, called the middle front, located over the 100-m isobath. 3. The outer domain, located between the 100-m isobath and the shelf break, has surface (wind-mixed) and bottom (tidally mixed) layers above and below a stratified interior. Oceanic water tends to intrude landward along the bottom, whereas middle-shelf water extends seaward in the surface layers so that in the water column of the outer domain, vertical fluxes are enhanced. This interior region of enhanced vertical flux has pronounced fine structure due to the interleaving of sheets of warmer, but more saline, oceanic water intruding shoreward and cooler, less-saline water moving seaward. The interleaving occurs at vertical scales of 1 to 25 m.

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS The shelf-break front, which is manifested mainly in enhanced salinity gradients, separates the outer shelf from the Bering Slope Current water. This water is a mixture of Alaska Stream and Bering Sea water. The Alaska Stream water has its source in the Aleutian passes. . _ ~ _ _ ~ On the northern Bering shell (approximately north of OZ-N), 1ncludlng Norton bound, there are three identifiable water masses (Coachman et al., 1975~. The Gulf of Anadyr water is the most saline and lies west of St. Lawrence Island and on the west side of the Bering Strait. Alaskan coastal water lies along the Alaskan coast to the east. In between lies Bering Sea shelf water of intermediate salinity. The Arctic Chukchi Sea and Bering Strait 61 The Bering Strait-northern Bering Sea-Chukchi Sea system water-mass characteristics and currents are dominated by the general northward flow (Coachman et al., 1975~. The Bering Strait is only 85 km wide and is 30 to 50 m deep. Winds tend to be channeled north or south through the strait. The mean northward flow is due to the sea surface tilting downward toward the north and averages about 1 Sv. Reversals of about 1-week duration have been observed and are significantly correlated with atmospheric pressure gradients over this region. Three water masses are squeezed through the strait from the Bering Sea. Definition of the water masses is based mainly on salinity. Water from the Gulf of Anadyr (actually 20% from the Gulf of Anadyr and 809/0 from the Bering Sea), which is the most saline (32.S to 33.2 parts per thousand (ppt)), occurs on the west side. Bering shelf water, which comes around both sides of St. Lawrence Island and occupies the center of the strait, has salinities of 32.4 to 32.S pot. Alaskan coastal water occupies the eastern strait and has a salinity of 32 opt. Temperatures are strongly seasonal but are typically cooler to the west (more oceanic water) and warmer to the east (more coastal water and freshwater runoff from the Yukon River). There are strong current shears across the Bering Strait. The strongest currents are in the upper layers of the east side and can have speeds of more than 2 m/s. Currents at lower-level speeds are half as strong. The flow on the western side is of the order of 0.5 to 0.6 m/s with little vertical shear. The Chukchi Sea area widens quickly north of the Bering Strait, with Kotzebue Sound immediately to the east of the strait. The shelf is shallow (20 to 60 m), with Herald Shoal (20 to 30 m) about 200 km due north of the strait. Many of the capes and headlands in the region on the U.S. side are high mountains that tend to cause "corner effect" accelerations in winds along the coast (Kozo, 1984~. The three water masses that flow northward through the Bering Strait cross the Chukchi Sea enroute to the Arctic (Coachman et al., 1975~. North of the strait, the Gulf of Anadyr water and Bering Sea water tend to combine and flow northward at 0.15 to 0.2 m/s, splitting around Herald Shoal so that some water goes straight into the Arctic Ocean through Hope Canyon and some turns eastward along the outer shelf of the Beaufort Sea. The Alaskan coastal water also flows along the Alaskan coast, gaining fresher, cooler water from Kotzebue Sound and flowing at speeds of 0.25 to 0.30 m/s along Point Hope, Cape Lisburne, and Icy Cape toward Barrow and the Beaufort Sea beyond. Water also "drains" from the Chukchi Sea through the Barrow Canyon into a layer 50 to 200 m deep in the Arctic Ocean. Me Arctic Beautort Sea Meteorology Winds are a major factor in the timing of ice freezing and ice breakup as well as storm and ice surges along the Arctic coast. They also create nearshore currents. However, along this coast there are subscale (subsynoptic surface-pressure grid) phenomena that are important to the shelf oceanography. Monsoon-type winds occur during the summer, caused by a semipermanent arctic front due to the thermal contrast along the coast (Kozo, 1984~. Heating of the land causes

62 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF a pressure deficit, leading to easterly coastal winds that are in quasi-geostrophic balance with the pressure gradient (Kozo, 1984~. The sea breeze is related to the monsoon. Along the Beaufort coast, sea breezes are characterized by large diurnal sea-land temperature contrasts, clockwise rotation of surface winds, and surface winds opposing offshore gradient winds (Kozo, 1984~. At least 25°h of the time, surface wind direction is dominated by sea breeze. The effects of these winds are the maintaining of coastal (up to 20 km from shore) currents toward the west that cause lagoon flushing, and a general masking of synoptic wind conditions in this first 20 km. In addition, sea breezes along the Beaufort coast are not followed by land breezes, because the land stays warmer than the water, in part because the sun does not set in summer. Circulation The offshore portion of the Beaufort Sea is characterized by a mean westward flow of ice and water at the outer edge of the anticyclonic Beaufort gyre (Aagaard, 1984~. Over the slope and shelf seaward of the 50-m isobath, the flow is eastward. This is called the Beaufort Undercurrent (Aagaard, l9X4~. The Beaufort Undercurrent is characterized by a temperature maximum associated with eastward flow originating in the Bering Sea. This flow seems to be trapped along the outer continental shelf and slope and centered at a depth of 40 to 50 m. Bering Sea water can be traced at least as far as Barter Island. Closer to the coast, local winds appear to dominate the ocean flow and drive it toward the west. This coastal flow appears to be primarily a summer phenomenon. Different circulation regimes occur on the inner and outer shelf, and the 50-m isobath seems to be a boundary. The inner circulation is strongly wind-driven but is also highly seasonal and less energetic in winter. Outside the 50-m isobath, the circulation is consistent (about 0.1 m/s) throughout the year, topographically steered toward the east and not, apparently, locally driven. Near the inshore side of the Beaufort Undercurrent, frequent cross-shelf transport events are possible, with time scales of about 3 days. Although currents and tides along the Arctic coast are relatively small, storm surges due to strong winds can cause considerable flooding (up to 1 km onshore). Orography The Brooks Range, some 250 km inland but parallel to the coast anti with a mean height of 1.5 km, has an effect on the winds over much of the Beaufort coast. Near Barter Island, orographic modification of the winds can cause wind speeds 50% greater than that of the geostrophic wind due to the "corner effect" (Kozo, 1984~. This effect can be felt as much as 350 km away. Mountain-barrier baroclinicity occurs when stable air moves toward and up a mountain without heating from below. This causes the isobaric and isothermal surfaces to tilt away from the mountain range, resulting in winds parallel to the axis of the mountain range. This is mainly a winter phenomenon and is a major reason for wintertime west-southwest winds between Barter Island and Prudhoe Bay. This results in a nearly 180° difference in wind direction when compared to the easterly winds at Barrow. Mountain-barrier baroclinicity depends on high surface albedo and so disappears in summer. This effect has a horizontal extent of 120 km and occurs about 25% of the time in the coastal zone from east of Barter Island to Pru~hoe Bay. In winter, temperature inversions are common in the Arctic. This results in strong surface stability, leading to diminished vertical turbulent exchange and hence to reduced wind stress on the surface.

REGIONAL OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS Sea Ice In Alaskan Waters Ice along the Alaskan coasts shows large seasonal and interannual variability (Niebauer, 1988~. The seasonal cycle of ice cover in Alaskan waters is the largest of any of the arctic regions, averaging about 1,700 km in the north-south direction. Interannual variability in the Bering Sea is as high as 400 km. 63 In general, the greatest northerly retreat of the ice occurs in September to October, when there is no ice in the Bering Sea and the ice has pulled away from the Arctic coast. At this time the ice can be as much as 250 km off Barrow. Under conditions of northerly winds, extensive pileup and run-up of ice onto the Arctic coast can occur (Weeks and Weller, 1984~. The maximum extent of ice in the Bering Sea generally occurs in April. Most of this ice is open pack ice that is driven by current and wind. Fast ice occurs in the Bering Sea in protected bays. In the Beaufort Sea, fast ice is more extensive due to the barrier islands and grounded pileups of sea ice. Winter ice characteristics in the Bering Sea can be described by a "conveyor belt" analogy. Ice growth is primarily along south-facing coasts, where polynyas are created as the predominantly northerly winds advect the ice southward away from these ice-generation sites. The ice is pushed southward until it hits its thermodynamic limit and melts. The sea ice limit thus advances southward as melt water cools the upper layer, but probably moves no farther than the deep water at the shelf break. Ice-drift rates are variable, with the Bering Sea rates being on the order of 20 to 50 km/d. In the Bering Strait speeds as high as 50 km/d have been observed. Although the direction of this flow is generally southward, strong northerly events have been observed, with ice passing northward through the Bering Strait before turning around and flowing back southward. In the Chukchi Sea, drift rates are lower (on the order of 0.4 to 4.S km/ for mean annual drift, but rates as high as 7.4 km/d have been observed (Weeks and Weller, 1984~. In the Beaufort and Chukchi seas, offshore ice circulation generally follows the east to west anticyclonic ocean circulation, which is essentially parallel to the coast. Near the coast, fast ice does not move much, perhaps less than 1 km/yr (Weeks and Weller, 1984~. The thickness of undeformed sea ice increases toward the north, ranging from about 0.5 to 1 m in the open Bering Sea, to 1 m in Bristol Bay, to 2 m off the Arctic coast (Weeks and Weller, 1984~. The Bering Sea ice is almost entirely first-year ice, whereas north of the Bering Strait, the ice is 25-75% second-year or older ice. The older ice is thicker, perhaps with a mean thickness of 4 m. However, deformed ice in pressure ridges has been observed with maximum keels of 50 m and sails of 13 m. These keels can cause appreciable gouging of the shelf when they become grounded. The ice over the arctic shelf is highly deformed, with up to 10 ridges per km due to shearing. Farther offshore, 2-3 ridges per km is more typical. There are fewer ridges in Bering Sea ice due to the free-floating nature of this ice. Although relatively rare, ice islands composed of freshwater glacial ice are also found in the Arctic. They are tabular icebergs with typical thicknesses of tens of meters and lateral dimensions of up to 10 km (Weeks and Weller, 1984~. Interannual variability of ice cover can be quite large (Niebauer, 1988~. In 1976 in the Bering Sea, ice cover was about 15-20% above normal. By 1979, the ice cover was about 15-20% below normal. The edge of the sea ice had retreated about 400 km over this period. Some of this variability has been related to E1 Nino events as well as to atmospheric variability in the North Pacific. Evaluation of MMS-funded Research in the Alaska Region Synopsis Gulf of Alaska From the mid-1970s into the 1980s, data from nearly 130 current-meter moorings were collected on the Gulf of Alaska continental shelf supported by hundreds of current, temperature,

64 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF and depth (CTD) profiles as well as water-pressure-gauge observations, drift cards, and a few Lagrangian drifter buoys. The impetus for this work was to provide a description of circulation in the Gulf of Alaska and a reference for circulation modeling as well as to connect nearshore and deeper-water circulations. Meteorological studies included placing temporary weather stations (approximately 7) supported by ship-borne airsonde observations as well as regular weather service observations. The impetus here was to provide a description of nearshore winds as an aid to prediction of oil-spill transport and to better define steering of winds by orographic effects as input to circulation and trajectory modeling. Modeling efforts included analytic diagnostic modeling as well as three-dimensional, time-dependent, layered circulation models to predict circulation and provide oil-spill trajectories. Bering Sea In the Bering Sea (including, at times, the Chukchi Sea to the north through the Bering Strait), data from approximately 90 current-meter moorings and 2,000 stations measuring CTD along with various drift buoys, bottles, meteorological stations, and ice-drift buoy/stations were collected from the mid-1970s into the 1980s. Sea ice morphology, concentration, openings, and seasonal ice edges were also documented through the use of LANDSAT imagery. The declared impetus for these observations was to provide a description of circulation (including tidal), mixing processes, frontal structure, and ice movement in the Bering (and Chukchi) Sea as input to circulation and oil-spill-trajectory modeling. Additional information was sought to assist in prediction of oil movement at the marginal ice edge as well as spilled oil motion in sea ice. The primary modeling efforts were three-dimensional, time-dependent, layered circulation models to predict circulation as well as to provide oil-spill trajectories. Work was also done on comparing modeled tidal currents to observed tidal currents as well as storm-surge prediction. Arctic In the Arctic Ocean (Chukchi and Beaufort seas as well as, at times, the Bering Sea), data from about 85 current-meter moorings and about 850 CTD stations along with tens of various drift buoys, meteorological stations, tide gauges, and ice drift buoy/stations were collected from the mid-1970s into the 1980s. Work was done using meteorological and satellite imagery of sea ice to look at seasonal and interannual variation in sea ice and fast ice. The drive for this work was to provide a description of the water masses and circulation of the region and to assist modeling the movement of ice and of oil in ice. Some of this work was aimed at understanding the circulation and flushing of arctic lagoons along the north coast of Alaska. The primary modeling efforts were three-dimensional, time-dependent, layered circulation models to predict circulation as well as to provide oil-spill trajectories. Work was also done on comparing modeled tidal currents to observed tidal currents as well as storm surge prediction. Sea Ice Sea ice studies were conducted in the Bering, Beaufort, and Chukchi seas as well as on the edge of the arctic ice pack itself and in the coastal and fast ice zones. These studies included current-meter moorings, a few hundred CTD casts, satellite imagery, and ice drifting buoys. In addition, existing and archived meteorology as well as satellite and ice-drift data were used, and laboratory studies were conducted. Much of the research was done both to understand and to try to predict ice movement and circulation as well as to try to understand how oil interacts with and moves with ice. In situ measurements of the mechanical properties of sea ice were also made.

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS Evaluation of Observational Studies 65 The overall quality of physical oceanographic work, including studies of the atmosphere and ice, done so far in the Alaska region under OCSEAP seems sound based on the relatively large amount of the work that has been reviewed and published in the open literature for a region that is so large, varied, and variable. Some of the Alaska OCSEAP work has been bolstered by other programs (e.g., Processes and Resources of the Bering Sea Shelf (PROBES), Inner Shelf Transport and Recycling (ISHTAR), and Gulf of Alaska Recirculation Study (GARS)~. In addition, the physical oceanographic knowledge and information that was gained under these programs in the Gulf of Alaska seem to have been borne out during the tracking along the Alaskan~coast of the oil spilled from the Exxon Valclez (Jayko and Spaulding, 1989~. The panel notes that the circulation of Prince William Sound has not been well studied. Although MMS's responsibility is for the OCS, it is reasonable to expect, and the panel recommends that MMS know about critical nearshore areas, such as Prince William Sound. Such areas could be affected by OCS activities, and the OSRA requires this information to predict hits reasonably. Increased attention should be focused in the offshore areas of the Beaufort and Chukchi seas, because the background data are sparse. Exploration is taking place there, and it will be difficult to determine the path and fate of the oil spilled, especially in winter, when that whole region is ice-covered. Little research has been done on interannual variability in the oceanic, atmospheric, and ice environment of Alaska. There is significant interannual variability in Alaskan waters, for example in the Gulf of Alaska (Xiong and Royer, 1984) and in the Bering Sea (Niebauer, 1988~. For example, the maximum seasonal advance of sea ice in the Bering Sea decreased about 400 km between 1976 and 1979. This is about 40% of the average seasonal ice extent. Sea-surface temperature, wind direction, and air temperatures showed similar variations and seem to be related to North Pacific atmospheric variability as well as ENSO variability. The Bering Sea slope circulation also has not been addressed very much under OCSEAP. In addition, eddies in this current system have not been observed moving up onto the shelf, but it is not clear if this is always true, or if this current system and associated eddies impact lease sites. Eddies are ubiquitous in most oceanic regions, and it is unknown if the Bering Sea region is that different. Near Barter Island, orographic effects cause wind speeds 50% greater than the synoptic or geostrophic winds, and these winds can be felt as much as 350 km away (Kozo, 1984~. This type of phenomenon needs to be considered in field and modeling studies of the circulation off the north coast of Alaska as well as in the Gulf of Alaska. Evaluation of Modeling Studies OCSEAP modeling of Alaskan waters concerned the ocean circulation as affected by winds, tides, density gradients, ice, and large-scale boundary conditions, such as sea-surface slope. The modeling also concerned oil-spill trajectories. The 1977-1985 modeling program (Liu and Leendertse, 1987) considered tides, wind input, density-induced flows, wind-induced flows, and sea ice. Tides were emphasized. In some cases, the mode} did not cover enough of the regions so that some important physical oceanographic features were not modeled (e.g., the Bering Slope Current). Comparison with actual data was not quantitative in most cases, with the exception of tides. The model and results were not published in the refereed literature. Since 1985, the models of the Alaskan region have been using more realistic driving forces such as Fleet Numerical Oceanographic Center (FNOC) winds, which represent actual varying conditions (Spaulding et al., 1987) better than the technique used by Liu and Leendertse (1987), a series of transitions between certain states of the weather. Data-assimilation techniques have also been used recently (e.g., the Navy/NOAA Joint Ice Center ice observations). It also appears that much more quantitative comparison with field data has been done as well as more statistically valid oil-trajectory studies (compare, e.g., Liu and Leendertse (1987) and Spaulding et al. (198Sa,b)~.

66 PHYSICAL OCEANOGRAPHY OF TlIE U.S. OUTER CONTINENTAL SHELF However, there are still not enough data sets available to fully verify the current models, especially in the Chukchi and Beaufort seas. This is especially true as regards surface circulation. In addition, few data sets are available for the Bering Slope region as well as the area of the Alaska Stream in the Gulf of Alaska. No matter how good the models are, unpredictable variability always will exist. This is especially true with respect to winds, which are probably the primary drivers of oil in water. Thus, the models should be treater! as guides, and the EISs should take into account the unpredictability of the real world. There is a great deal of interannual variability in Alaskan waters that probably has not been taken into account in the simulations. For example, there have not been "anti-E1 Ninos" (now called "La Nina" or "E1 Viejo") since the mid-1970s (Niebauer and Day, 1989) until one in the winter of 1988-1989. In the Bering Sea, E1 Nino causes "warm" conditions while La Nina causes "cold" conditions, often with expanded ice cover. The drivers of these conditions in the Bering Sea are the Aleutian low in concert with the southern oscillation. Another source of interannual variability is the Pacific-North American pattern of weather over the North Pacific. The present method of modeling the circulation takes the interannual variability into account but only to the extent that the FNOC winds represent the atmospheric circulation. This data set is apparently limited to the past 10-12 years, and thus covers only one La Nina, but does cover the three E1 Ninos that have occurred since the mid-1970s. The stated impetus for many of the observational studies in the Alaska Region was to provide observational data for input to modeling, and this did occur (see, e.g., Galt, 1976; Spaulding et al., 1987~. However, many of the observations were not input to the model. The modeling in the Alaska Region is different from that in the other regions in that the modeling and oil-spill trajectories were done by the same contractor in Alaska (as opposed to having the trajectories calculated by MMS based on circulation modeling results from a contractor, as in the other OCS regions). THE PACIFIC REGION The Pacific Region (see Figs. 10 and 11) is characterized by a narrow continental shelf. Freshwater input to the shelf generally increases from south to north. Currents are variable, with the seasonal variability increasing to the north. Meteorology and Circulation Mean and Seasonal Winds Mean and seasonal winds are dominated by the seasonal migration of the North Pacific High and the passage of Aleutian lows during winter. North of about 40°N, the wind direction varies seasonally from a northerly or northwesterly direction in the summer to southerly in the winter. In the northern regions, the winter regime is stronger and lasts for a greater portion of the year than does the summer one, whereas the reverse is true in the south. Further south, the seasonal cycle is still strong, but the monthly mean wind does not reverse direction. Hickey (1979) has given an estimate of the amplitude of wind-stress fluctuations as a function of month and latitude, based on Bakun's upwelling indices (Bakun, 1975~. South of and at 36°N (Point Conception), the variance is greatest in the spring (generally in April), except at 24°N; north of 36°N the variance is greatest during the stormy winter months (in December north of Cape Mendocino and in January at 39°N). The largest amplitude of the seasonal fluctuation of the variance and the largest variance are observed at 48°N. The largest variance during spring and summer occurs near 39°N (Cape Mendocino), where maximum southward stress is observed. The variance during those months generally decreases both north and south of this latitude, with a more rapid rate of decrease to the south. Hence, the variance off Baja California for a given month is always less than that off Oregon and Washington, except during August. During spring and early summer, the variance between these

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROMS i j (~ / ~ClffSCE~' Cry se 2000 m ~ hi. . ~ 200 my .. ~ .1 i'"' ~ //EUREKA ~ _~ ~< \Y'' c: r ~ \ t ; ~ I i. ~14~.-. . .,. . \\ ~ ,1 ,170.: i ~: A_ _ ~7 1 \ \ \ LAIN CHICO \ ~ Am: 2000m \ ~ \~:CRU2 ~7~oNTEllE, O 50 lCO STATUTE belles\ \~> O SO 100 lSO 200 KILO-ETERSI 1~; KEY CAP , PACIFIC REGION ~ US, OREGON I . .1. .! CALIFORNIA ~ ~| 'a;: ~ CENTRAL =1 at C`,, 'a NOR1~1ERN -r a:'-: CALIFORNIA >, POSIT, \ I Let. \, 1 - :~ ,; L Aid' a' SOUTHERN I / CALIFORNIA IJ ~32° \ \ \ \ CALlfORNIA \ ~ ~;~^~A 4> :~ ~ Los &lICEI~E5 _ ~ at, 2000 rn \ aim_ ~ . 200 m V: FIGURE 10 Southern, central, and northern California planning areas of the Pacific region. Source: MMS. 67

68 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF , ~-~ - 7 2000 m _4~- 4,'° _~- . _~- 0 25 waS - ~ - ATOM _t- - 0~600- I KEY MAP ~_S~ 1 ~AC.f'C REC10 - / \;,1. i it_/ V`~4`a~c'O. 1, [ <,,,, ~ 0~ 'N .. 'a ~ ~ ; 'it, ~\__1 _44o ~ ~,.'', 1, ~ 0 25 SO STATUTE FILES ~R ~ 'a 5<1 75 K~O~ET~ ?000 ret J 200 m 1 1 2 4 ° 1 ~i 2 Be WASHINGTON . PORTLAND OREGON 22 . ' ' -! 1 FIGURE 11 Washington-Oregon planning area of the Pacific region. Source: MMS.

REGIONAL OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS 69 two areas can differ by as much as a factor of 2. Although maximum southward wind stress occurs during July and August off Oregon and Washington, maximum variance during the period of mean monthly southward stress occurs during May at those latitudes. Hickey (1979), in discussing the variability of the wind stress off the U.S. West Coast, wrote: Although southward wind stress begins to increase in February at most latitudes, the rate of increase is not uniform alongshore. South of 28°N (Punta Eugenia), the maximum increase occurs from March to April and maximum southward stress is observed in April, whereas at 47 N. the maximum increase occurs from May to June and the maximum southward stress expands both offshore and alongshore as the maximum stress advances northward, so that during June and July, when the northerly wind system is most strongly developed, southward stress exceeds 1 dyne/cm2 in a region 500 km offshore by 1000 km alongshore. Maximum southward wind stress is generally observed 200-300 km offshore, rather than adjacent to the coast. The magnitude of the southward wind stress begins to weaken in May south of 27 N and in August at most other locations, and the region of strongest stress moves southward to a location south of Point Conception (36 N) by December or January. Weakest southward wind stress is observed in August south of 25 N and in the winter months at all other locations along the coast. As the summer wind system weakens, the northerly winds are replaced by the southwesterly winter wind system, from northern Washington in September to Cape Mendocino by November. During December and January when the southwesterly wind system is most strongly developed, the alongshore maximum in northward wind stress is most often observed at about one degree of longitude off the coast. As distance offshore increases, the wind stress is more eastward and less northward, but the magnitude is relatively constant. During March and April, when the southwesterly system is weakening, the wind stress is almost entirely eastward off Washington and Oregon, except right at the coast. Substantial year-to-year variations in mean monthly wind stress can occur, so that in any given year, mean monthly stress can even oppose the long-term stress direction, particularly in regions where the long-term value of wind stress indicates light winds. Bakun (1975) has tabulated daily values of "upwelling indices" which are equal to the alongshore component of the wind stress near the coast divided by the Coriolis parameter. Ike indices are computed from six-hourly synoptic maps of surface atmospheric pressure. Whereas Nelson's (1977) long-term data show that mean monthly wind stress south of 40 N along the coast is always southward, Bakun's data for seven years (1967-1973) show that mean monthly wind stress was northward at 36 N during January of 1968, 1969, 1970 and 1973, December of 1968 and February of 1973; and at 33 N during January of 1968 and 1972, and December of 1967. Superimposed on seasonal fluctuations in winds are higher frequency events that have time scales of a few days (Bakun, 1975~. Although such events generally result in an amplitude variation of the mean seasonal northward or southward wind stress, reversals in direction sometimes occur, so that, for example, off Oregon, northward stress events often occur during winter. The number of reversals in direction at these higher frequencies is a function of the alongshore location relative to the position of the seasonal high and low in atmospheric pressure; that is, although southward stress events can occur during winter at any latitude, the frequency of such events increases towards the south, so that south of 36 N they represent the norm rather than an anomaly. Northward wind stress events during summer are relatively rare south of 45 N. although they become more common in late summer south of Baja California (21 N). (Reprinted with permission from Progr. Oceanogr. Volume 8, B.M. Hickey, The California Current system: Hypotheses and Facts, copyright 1979, Pergamon Press.) The statistical properties of the coastal winds were demonstrated further by Halliwell and Allen's (1987) study of the coastal wind field along the west coast of North America for two summers, 1981 and 1982, and the intervening winter using measured winds and geostrophic winds calculated from the FNOC atmospheric pressure analyses. Halliwell and Allen found that: Summer wind fluctuations are driven primarily by the interaction between two relatively stationary pressure systems, the North Pacific subtropical high and the southwest U.S. thermal low, and by their interactions with propagating atmospheric systems to the north. In particular, propagating Cyclones and associated fronts are often followed by a northeastward intensification of the high, producing strong upwelling events along the California coast. This summer event sequence occurred more frequently and was displaced farther to the south on average during summer 1981. Winter wind fluctuations are primarily driven by the propagating cyclones and anticyclones, and they tend to have larger variance and space scales than in summer. A preference for poleward (equato~ward) propagation exists in summer (winter), and the largest time scales were observed in

70 Circulation PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF summer 1982. Coastal atmospheric boundary-layer processes substantially modify winds within 100-200 km of the coast. Consequently, measured wind fluctuations are strongly polarized in the alongshore direction and have means and rms amplitudes that can vary considerably between nearby stations along the coast. Calculated wind fluctuations are less polarized in the alongshore direction and have alongshore correlation scales about 60% larger than those for measured winds. They represent fluctuations with alongshore wavelengths of ~ 900 km rather well but represent poorly those with smaller wavelengths and those due to coastal atmospheric boundary-layer effects. (Copynght 1987 by the American Geophysical Union.) Strub et al. (1987b) concluded that the seasonal cycles of measured wind stress confirm that the magnitude of the seasonal cycle is maximum around 40°N and that annual mean Ids change from northward at latitudes north of 40 N to southward at latitudes south of 40°N. At some latitudes south of 38 N a semiannual signal is seen in the seasonal cycles of measured wind stress but not in those of the calculated Id stress. Because of the limited length of the directly measured wind data, the presence of a semiannual signal in alongshore wind stress in the south must be taken as a tentative finding. (Copyright 1987 by the American Geophysical Union.) Much information exists on the seasonal currents and property distributions driven by the wind and runoff cycles over the shelf along the West Coast (Hickey, 1979; Huyer, 1983; Strub et al., 1987a,b; Hickey, 1988), with the exception of the nearshore (depths less than 60 m) areas of the Pacific Northwest. The Columbia and Fraser rivers introduce appreciable amounts of fresh water into the ocean, affecting the stratification on the shelf. The rivers of northern California and San Francisco Bay have a small but noticeable effect. At midshelf along the Pacific Northwest, the mean seasonal current is primarily alongshore: northward at all depths in the winter, southward at all depths in the spring, and southward at the surface but weak or northward near the bottom in summer. Fall is a time of gradual transition as the surface flow reverses to go north and the isopycnals become horizontal. Similar behavior occurs at the shelf break, but with the currents enhanced. Further south the spring-summer southward flow diminishes in magnitude and duration, with the surface flow at 35°N exhibiting southward flow for only two spring months. The alongshore flow is well correlated with local coastal sea level in accordance with geostrophy. The seasonal variations in alongshore components of wind and currents also are well correlated, although the wind and current may be opposed because of an annual mean current unrelated to local wind. Writing on the alongshore coastal currents north of Point Conception, Strub et al. (1987b) concluded that the best present picture of alongshore coastal currents describes roughly three regimes between 35°N and 48°N and that: (1) From 450N to 48°N, north of the maximum seasonal wind stress signal, the seasonal cycles of currents (midshelf at 450N and shelf break at 48°N) reach their greatest magnitudes. These currents are southward with baroclinic shear in spring and summer and northward and more barotropic in the fall and winter. The observations of Freeland et al. (1984) suggest that currents over midshelf at 48°N are weakly northward in summer. The seasonal cycles account for a large proportion (30% to over SO~o) of the variance in these currents. The annual mean current in the middle of the water column is southward, opposing the annual mean wind. Superimposed on these regular seasonal cycles are short- period (periods less than 1 month) fluctuations that are weaker in summer and stronger in fall and winter. (2) In a mid-latitude regime, from 390N to FIN, where the seasonal cycle of wind stress is greatest, the seasonal cycles of currents are qualitatively similar to those found to the north but are dominated by the shorter-period fluctuations. In this regime the seasonal cycles account for only 10% to 20% of the variances. At 43°N the shorter-period fluctuations decrease in magnitude in the summer, as they do farther north, but at 39°N they are strong during most of the year. This suggests a geographic transition zone between 43°N and 45 N. from a shelf dominated by...shorter-period fluctuations south of 430N to one dominated by smoother seasonal cycles at CON. The dynamics of this

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS transition remain to be explained. In the middle-latitude regime in summer, strong barotropic fluctuations are superimposed on the mean baroclinically sheared southward flow; in winter, strong baroclinic fluctuations are superimposed on the mean barotropic northward flow. The annual mean currents over midshelf are weakly northward, whereas over the shelf break at 43°N they are weakly southward at 35 m and are northward deeper in the water column. (3) In the south around 35 N the annual mean currents are northward, in opposition to the annual mean wind. The period of sheared, southward currents over the shelf is limited to 1-3 months in spring. During the rest of the year a fluctuating but northward (monthly mean) current exists. The seasonal cycles account for only 10% to 20% of the variances in the alongshore currents, as they do from 39 N to 43 N. The seasonal mean northward current appears (from hydrography) to extend over the slope and may have a semiannual nature, with peaks in early summer and early winter, similar to those found by Chelton (1984) in the undercurrent farther offshore at the same latitude. These peaks are roughly coincident with periods of relaxation seen in the measured winds. Temperature and sea level seasonal cycles have their greatest magnitudes in the middle latitudes (39 N to 45-N). Both drop suddenly in spring? especially at 39 N. and increase suddenly in fall, especially at 43 N. These transitions decrease in the far north (48°N) and south (35-N), as do the seasonal Circles themselves. Temperatures have less short-period variability, and their seasonal cycles account for 30% to 80% of their variances, while sea level cycles account for 30% to 50% of their vanances. The seasonal cycles of alongshore currents lead those of adjusted sea-level (ASL) slightly, and both lead those of the wind and temperature by 1-2 months, confirming results of Hickey (1979) and Chelton (1984), who lacked simultaneous directly measured currents. An approximate 2-month lag between the summer regime in the south and in the north is seen in all variables. This lag has been documented previously in sea level (infield and Allen, 1980), calculated wind stress (Bakun, 1973, 1975), and climatolog~cal wind stress data from ship reports (Nelson, 1977~. (Copyright 1987 by the American Geophysical Union.) E1 Nino-Southern Oscillations and Interannual Variations 71 Knowledge about interannual variations has been obtained from observations of sea level and coastal temperatures. Most of the variation, occurring south of Oregon, can be attributed to the ENSO events in the tropical Pacific (Enfield and Allen, 1980; Chelton and Davis, 1982~. These increases in sea level propagate up the coast from the equator with speeds less than the low-mode coastal-trapped wave celerity. Off Oregon, the 1982 E1 Nino increase in sea level and temperature followed that at the equator by about 1 month (Huyer and Smith, 1985~. The interconnections between the coastal, oceanic, and atmospheric responses are not well known at present. Upwelling ant! Associated Events Brink (1987) has written a concise review of wind-driven coastal upwelling and associated features on the U.S. West Coast. He stated: The process of wind-driven coastal upwelling is particularly important off the west coast of the United States. This is so because the winds are predominantly equatorward (upwelling-favorable) throughout the summer over most of the Pacific coast. Since upwelling introduces nutrients to the euphoric zone, the process is of interest as well to biological oceanographers and fisheries scientists. Over the last decade, interest has focused increasingly on the three-dimensional aspects of upwelling, with added attention to alongshore and temporal variability in the more traditionally studied cross- shelf and vertical structure. The upwelling season off the Northwestern United States generally commences with a single, dramatic shift to seasonally permanent upwelling favorable winds. The event, called the spring transition, is part of a global scale shift in circulation patterns (see, e.g., Lentz, 1987) and is most pronounced north of about 37°N (Strub et al., 1987a). The oceanic response to the wind shift consists of the onset of upwelling, formation of an upwelling front, a drop in coastal sea level and a decrease in stratification over the shelf (Lentz, 1987; Strub et al., 1987a). South of about 37°N, the spring transition is less abrupt (Strub et al., 1987a) and appears to be accompanied by comparably slow changes in subsurface hydrography (Brink et al., 1984~. On a shelf-wide scale, the heat balance near

72 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF 38 N is essentially two-dimensional, with near-surface offshore advection of warm water and onshore advection of deeper, colder water (Lentz, 1987~. Strub et al. (1987a) present a large-scale composite of the transition event which suggests that coastal-trapped wave physics tend to affect strongly the detailed oceanic response. For example, the maximum change in coastal sea level occurs north of where the winds are strongest. The detailed character and timing of the spring transition vary from year to year (Strub et al., 1987a), with the most extreme oceanic responses tending to occur after E1 Nino events (Breaker and Mooers, 1986~. After the spring transition, an upwelling front usually exists over the outer shelf near 38°N (Huyer, 1984~. At 36 N. on the other hand, Breaker and Mooers (1986) report a tendency for the frontal location to continue moving offshore during the entire spring and summer. At both locations, the average frontal locations tend to parallel the local isobaths and/or coastline (Kelly, 1985; Breaker and Mooers, 1986~. Drifter measurements near upwelling fronts often suggest convergence (Davis, 1985a; Barth and Brink, 1987), but really solid evidence is generally lacking. The fronts also appear to be sharpest during weak winds (Davis, 1985a). During the occasional cessations of upwelling favorable winds near 38 N. the front tends to remain stationary (i.e., not retreat toward the coast), and the nearsurface warming near shore is dominated bar alongshore advection (Send et al., 1987~. This advection appears particularly effective because of the propensity of shelf currents In the region to flow poleward unless strongly opposed by equatonvard winds (Winant et al., 1987~. (Copyright 1987 by the American Geophysical Union.) The variously called jets, squirts, and filaments of the California Current system are quite important for shelf-ocean exchange and are currently under intensive investigation (see Fig. 12~. Capes or major discontinuities in shelf depth tend to be locations that initiate these features, but their dynamics and the importance of topographic interactions are as yet poorly understood. They currently appear to be associated with the coastal upwelling process. These features are discussed in greater detail in Chapter 2, in text also quoted from Brink (1987~. Brink (1987) concludes his remarked on these phenomena with the statement: Much more work has yet to be done on the dynamics of filaments off the U.S. West Coast. Indeed, a major field study on this subject (the Coastal Transition Zone program funded by the Office of Naval Research, Coastal Sciences and Oceanic Biology Sections) was camed out during 1987 and 1988. Perhaps the most pressing problems to be addressed include the dynamical organs of the features, the extent to which they exist in other areas of the world (and in other seasons), and how they dissipate and mix their Properties into ambient oceanic waters. (Copyright 1987 by the American Geophysical Union.) Internal Waves - r--r Internal tide beams have been observed off the shelf of Washington with current amplitudes comparable to or slightly larger than typical low-frequency onshore velocities. The shear in these currents could possibly enhance vertical mixing, particularly near the shelf break where they are generated. High-frequency internal waves are commonly found propagating shoreward over the southern California shelf during spring and summer. These waves can transport shoreward surface-floating material that is caught in the resulting surface convergences. Bottom Boundary Layer A review of boundary-layer processes is available in Grant and Madsen (1986~. The nonlinear bottom-stress law yields a mean or low-frequency bottom-stress variation that depends on the existing surface gravity waves as well as the motion of interest. Numerical modeling will need to take this variation into account. Topographic Effects Canyons and Capes Submarine canyons affect the flow above them depending on their scale and the stratification of the overlying water. The bottom flow over the shelf tends to be diverted along

REGIOI~4L OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS FIGURE 12 Jets, squirts, and eddies off the California coast. Source: Mark Abbott and Ted Strub. Used by permission. 73

74 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF the isobaths to conserve vorticity. The penetration of this effect into the interior occurs with a vertical scale H ~ fLN, where f is the Coriolis parameter, ~ is the horizontal scale of the canyon, and N is the Brunt-Vaisala frequency. Recirculation may occur at locations where the canyon depth is greater than any depth occurring in the adjacent shelf. Capes or major discontinuities in shelf depth tend to be locations that initiate features such as eddies, meanders, and squirts that enhance cross-shelf advection. Recent results indicate that high velocities are found along the maximum-density gradient edges of the squirts, much like the behavior of a large-amplitude, small-wavelength meander. However, topographic effects and the dynamics of squirts and jets have not been well described to date. Modeling Several numerical models are available whose results are quite dependent on input and boundary assumptions. Quantitative comparisons of observed currents with predictions for the complete West Coast are nonexistent. Predictions over short reaches have been made based on appropriate boundary conditions being specified on the relevant geostrophic contours. However, it is not clear whether the model can be considered to be predictive or to be acting merely as a reasonable data-assimilation and interpolation scheme. Use of coastal-trapped-wave theory provides qualitative agreement for alongshore components of velocity for frequencies that are not influenced by turbulent exchanges of momentum, heat, ant! salt or by surface heating and cooling. However, the magnitudes of the velocities are consistently predicted to be lower than those observed. The cross-shelf component of velocity is difficult to predict, possibly because of its dependence on the shorter alongshore scales, but possibly for other reasons as well. A recent paper by Pares-Sierra and O'Brien (1989) shows promise in using numerical models to predict current velocities in the upper ocean near the coast. It is possible that a new procedure proposed by Clarke and Lopez (1987), which treats higher modes as locally generated and includes remote forcing for only the lowest-order modes, will improve model results. Evaluation of MM1;-funded Research in the Pacific Region History of MMS-funde~i Research in the Pacific Region Modern process-oriented shelf circulation studies, carried out in the late 1960s and in the 1970s primarily by investigators from Oregon State University and the University of Washington with funding from the National Science Foundation and the Department of Energy, led to a major improvement in the understanding of the dynamics of wind-driven currents and their associated effects over the continental shelf of Oregon and-Washington. More recently, oceanographic studies supported by MMS, ONR, and NSF have focused on the shelf and adjacent waters of California. These studies have included some of the most intensive field programs carried out to date for the purpose of providing detailed information on the dynamics of this region. MMS began its research on the West Coast with a circulation model for the entire domain. Subsequently, MMS has been methodically working its way up the West Coast with oceanographic measurement programs. Two studies have been completed, one is in progress, and at least three contracts are being negotiated at the present time. These studies are listed in Appendix C. The California Shelf Circulation Model is based on a well-known general circulation model (Blumberg and Mellor, 1983~. The model is the same three-dimensional, time-dependent model that has been used in the Gulf of Mexico, the mid-Atlantic and south Atlantic bights, and the Santa Barbara Channel. The model incorporates turbulent mixing via turbulent closure techniques; it includes wind and density forcing and tides. It can be run in diagnostic and prognostic modes. Climatological density fields were obtained and were used to initialize the general circulation model. A vorticity-conserving model (the characteristic tracing model, or CTM) is used to prescribe open-ocean boundary conditions, in an area from 20°N to 50°N and

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS 75 from 110°W to 135°W. The model was used to simulate currents for a year-long period in 1981 during which the CODE experiment took place. The California Shelf Circulation Model was successfully completed in 1985. A final report was submitted, but no refereed papers have been prepared. The Santa Barbara Channel project involved a combined modeling and observational effort. The same general circulation mode! was used; however, boundary conditions were specified by current meters (~16 moorings) instead of CTM. Observations also included CTD, drifter, and satellite studies. A pilot field program preceded the main program. This project has been completed, and one refereed journal article has appeared from the pilot program. No refereed articles based on the main program or on the model-observation comparisons have appeared to date. The Central California Circulation Study was a purely observational program off central California involving 11 moorings, seasonal CTD surveys, and drifter studies conducted over 18 months. Problems occurred with mooring design, current-meter design, fishing, and vandalism. MMS clid not accept the draft final report and requested changes, saying that the analysis of the data was preliminary and inadequate (Sigurd Larson, MMS Pacific Regional Office, pers. comm., March 16, 1989~. The contractor asked for more money to complete the original and additional, more specific tasks, and effectively stopped work on the project for 9 months pending the outcome of negotiations. The original objectives seem to have been stated in rather broad, general terms. According to one of the subcontractors, the project seemed to be funded inadequately to achieve the original objectives. Furthermore, the company that was responsible for the instrumentation problems is no longer in business (A.W. Bratkovich, personal communication. March 21. 19891. Two caners have been published based on the ~t~riv (Shelton et al., 1987; 1988~. More journal articles are forthcoming, including one based on the observation of the actual behavior of oil that happened to be spilled while the array of instruments was in place, and an overview that was requested by MMS (A.W. Bratkovich, personal communication, March 21, 1989~. The Northern California Circulation Study is an observational study similar in scope to that done off central California. The pilot program occurred in 1987. A data report was published in September, 1988 (Magnell et al., 1988~. The Oregon-Washington Coastal Circulation Project involves assessing what data are available in this area and whether additional experiments are necessary. The contract for this project had not been awarded as of March 1990. The report of the statistical characterization project, which involves analyses of all available data on West Coast circulation patterns, is due to be released in July 1990. A contract for modeling the circulation of the southern California bight is still under negotiation. The same general circulation model used in other California modeling studies will be applied to the whole California bight and the results compared with available data for the Santa Barbara Channel and for the Santa Monica-San Pedro basin and shelf area. Evaluation of Observational Studies To date, the contributions by the West Coast programs to state-of-the-art knowledge, as measured by the number of refereed publications, have not been impressive; only one refereed journal article has appeared as a result of the Santa Barbara Channel program, and the lead author on that paper (Brink and Muench, 1986) was not funded by MMS but by NSF through the Organization of Persistent Unwellina Structures (OPUS) program. Moreover, that paper addressed only the approximately 3-month pilot program rather than the year-long study. Even though MMS has increased the available data base by an order of magnitude, the potential return from these experiments has not yet been realized.

76 Evaluation of Modeling Studies PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF The modeling work, if published, could make a contribution by illustrating the inadequacies of a state-of-the-art numerical model, that is, identified deficiencies can provide important clues as to the physics and processes that might still need to be incorporated into the model. To date, the modelers have not performed careful statistical comparisons between the mode! and the observations. They usually stop short of coherence and phase calculations, percentage of variance explained, and so on, even in their unpublished "gray literature" reports to MMS. The quality of the modeling work has remained constant, because the same model and group have been used in each study. This model generally is perceived as one of the few state-of-the-art numerical circulation models currently available. Nevertheless, the California modeling studies have had serious inadequacies that are reported in the reviewers' comments on the final reports (Allen et al., 1985; Brink et al., 1987b). For example, the flow predicted by the California Current model is more equatorward than is the flow actually observed; also, the mass field used for the model is inappropriate because it was not the actual mass field that would have been in equilibrium with surface fluxes of heat and momentum in 1981. Predicted current amplitudes are smaller by at least a factor of two than those observed in the Santa Barbara Channel, probably because of the way the contractor filtered the boundary conditions. Given the above reported inadequacies, the Physical Oceanography Panel believes that MMS puts too much faith in the results of modeling studies. This reduces, both directly and indirectly, the quality of its efforts to predict the fate and trajectory of oil spills. In most cases, at the present time, it would be preferable to use observed rather than modeled statistics. However, the emphasis on modeling has detracted from the ability to obtain high-quality observations. In the Santa Barbara Channel program, for example, the model requirements and resource limitations meant that all but three current-meter moorings were placed along a boundary (at the channel ends or between the islands). Thus, a good observational description of the interior flow was not obtained, and worse, model-observation comparison could be performed only at one interior site. 1 HE GULF OF MEXICO REGION The Gulf of Mexico is a semi-enclosed sea that is bounded to the north by the southern United States; to the south by Mexico, the Yucatan Peninsula, and Cuba; to the west by Mexico; and to the east by Florida. The Gulf of Mexico opens to the Caribbean Sea through the Yucatan Strait, between the Yucatan Peninsula and Cuba, and to the Atlantic Ocean through the Straits of Florida, between Cuba and Florida. Sill depths at the two straits are approximately 1,400 m and 800 m, respectively. Water from the Caribbean enters the Gulf of Mexico through the Yucatan Strait and exits to the Atlantic Ocean through the Straits of Florida. The maximum depths in the Gulf of Mexico are around 4,000 m and occur in the central portion of the basin. The continental shelf of the Gulf of Mexico is broadest along the west coast of Florida and along the northern portion of the Yucatan Peninsula (Campeche Bank). The narrowest portion of the shelf is found along the east coast of Mexico and south of the Mississippi River delta. For MMS planning purposes the Gulf of Mexico is divided into the three subregions: the eastern Gulf of Mexico subregion (Fig. 13) and the western and central Gulf of Mexico subregions (Fig. 14~. Several rivers empty into the Gulf of Mexico, the largest of which is the Mississippi River. Along the fringes of the Gulf of Mexico are numerous marshes and estuaries. Also, portions of the gulf coast consist of barrier islands, bays, and lagoons, all of which are influenced by circulation processes that occur on the adjacent continental shelf.

,77 ~.~ - ~ . ~A ' ' Hi| ' :° ,~''''2:': 4' ,. .;,,~.,..., . . Ul ,, ., ~ · ~ I_ - - E Q o ~o Ed O In J o _ ~ _ On o o X o_ - , O ~0 ~ O o 0 / ''of.\lo ~:0 W ~ _ Arc O ~ 00~ A_ I ~ Aft U' . . 8 in o ·lib o C) ·6 o .s Cat Ct Ct be ·$ Ct - Ct .c o o U. ·Ct Cal i: Ct 8 .D o - C) 4 - U) Ct .~

~4~e o z-~i - - ~n _-a Ax - ~ '~- \ - o. -e - !'% - In In ·Eb o .s It be Ox ~ ~ ce ~ 'e if o. o~ It ~d. -aid mu. ~ 'A ~ -~ Alar :'.';\ -in Sin ~ 1 ~ 1 ~1 O _

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS Meteorology and Circulation Meteorology Surface winds over the Gulf of Mexico exhibit seasonal changes in magnitude and direction. Winter winds are predominantly from the east-northeast, spring winds from the southeast, summer winds from the south-southeast, and fall winds from the east-northeast. Winter winds are typically the strongest. The seasonal change in the winds over the gulf is in response to seasonal migrations of the Azores-Bermuda high-pressure cell that dominates the atmospheric circulation over the Caribbean and Gulf of Mexico. Low-pressure weather systems that move out into the gulf from the continental United States have a large effect on the winter wind patterns. These systems usually occur every 3-10 days. 79 The atmospheric systems that have the most dramatic effect on the wind patterns over the Gulf of Mexico are the intense tropical cyclones (hurricanes) that occur in this region between May and October. The effects of these storms on lane! as well as on nearshore and coastal regions of the gulf have been well documented. The most recent EIS for lease sales in the Gulf of Mexico (U.S. DOI, 19870) lists several instances in which hurricanes have caused the destruction of OCS platforms and breakage of pipelines. These storms can also have an effect on the large-scale circulation of the gulf. Brooks (1983) described the current oscillations that were caused by Hurricane Allen as it moved across the western Gulf of; Mexico. Also, upwelling is known to occur in the wake of hurricanes. This can result in large heat exchanges anti in the introduction of nutrients into the euphoric zone, which can stimulate biological production. Circulation General Circulation Features The large-scale water-mass distribution in the Gulf of Mexico reflects the limited exchange the gulf basin has with the adjacent oceans. In general, the gulf waters consist of three distinct water masses: subtropical underwater, antarctic intermediate water, and North Atlantic deep water. The subtropical underwater enters the gulf from the Caribbean at depths of 200 to 500 m and is found throughout the eastern portion of the gulf. This water is readily recognized by its high salinity, >37.00 opt. Antarctic intermediate water also enters the gulf through the Yucatan Strait and is found throughout the gulf between depths of 500 to 1,200 m (in the eastern gulf) and 600 to 800 m (in the western gulf). This water mass is recognized by a distinct minimum, <34.00 opt, in salinity. North Atlantic deep water is found below 1,200 to 1,400 m throughout the Gulf of Mexico. McLellan and Nowlin (1963) suggested that waters deeper than 1,500 m in the Gulf of Mexico have long residence times (300-500 years) and are not frequently exchanged with outside waters. Hydrographic observations indicate that additional water masses gull- water, tar example-are formed locally in the Cull ot Mexico during periods of intense winter cooling. Numerous studies (see, e.g., Nowlin and McLellan, 1967; Molinari et al., 1978; Hofmann and Wor1ey, 1986) have shown that the general large-scale circulation in the upper 1,A00 m of the Gulf of Mexico is anticyclonic (clockwise). The transport in the northern limb of the anticyclonic gyre is a combination of flow from the Texas shelf and from the southern portion of the gyro. The contribution from the Texas shelf can at times be as high as one-third of the total transport of the easterly flow in this limb of the gyre (Molinari et al., 1978~. The westerly flow in the southern part of the anticyclonic gyre is composed predominantly of water recirculating in the southern gulf, although at times water separating from the Loop Current can contribute to this transport (Molinari et al., 1978; Hofmann and Worley, 1986~. Average geostrophic velocities and volume transports associate with the large-scale anticyclonic circulation of the Gulf of Mexico are 10 cm/s and 5 x 10 m3/s (Hofmann and Worley, 1986~. Additionally, large-scale cyclonic (counterclockwise) circulation gyres are found in the Bay of Campeche and over the

80 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF northern portion of the west Florida shelf (Nowlin and McLellan, 1967; Molinari et al., 1978 Hofmann and Worley, 1986~. Major Circulation Features ~, Superimposed upon the large-scale circulation of the gulf are two major circulation features, the Loop Current and Loop Current rings. Both of these have considerable influence on the circulation characteristics of the Gulf of Mexico. Loop Current. The Loop Current is a swift, narrow current that enters the Gulf of Mexico through the Yucatan Strait. This current can be traced as a coherent feature that extends into the northern portion of the eastern gulf, where it turns to the east and then flows southward along the west Florida shelf. At the southern extent of the Florida shelf, the Loop Current again turns east and exits the Gulf of Mexico through the Straits of Florida. The Loop Current is part of a larger circulation system that feeds into the Gulf Stream along the eastern boundary of the United States. The Loop Current can be readily distinguished in vertical-density distributions down to depths of 1,000 to 1,200 m in the region where it enters the Gulf of Mexico (Morrison and Nowlin, 1977~. Surface geostrophic velocities into the gulf associated with the Loop Current have been estimated to b& 13O0 to 150 cm/s and the corresponding volume transport has been estimated to be 25 to 35 x 10 m /s. Surface velocities diminish somewhat as the Loop Current extends into the gulf and widens. Outflow surface velocities of the Loop Current are on the order of i0 to 100 cm/s and the corresponding volume transport is the same, approximately 25 to 35 x 10 m3/s. It should be noted that the sill depth of the Straits of Florida is shallower than that of the Yucatan Strait. Hence, part of the inward-directed flow of the Loop Current through the Yucatan Strait does not flow out through the Straits of Florida, but rather is deflected back into the gulf. It has been suggested that there is a recirculation of Loop Current water in the region north of Cuba. The extent of penetration of the Loop Current into the Gulf of Mexico has been studied extensively. At times the Loop Current has been observed to extend northward into the gulf as far as the Louisiana and west Florida continental shelves (Huh et al., 1981~. A seasonal cycle in the depth of penetration of the Loop Current northward into the gulf is well documented (see, e.g., Leipper, 1970; Maul, 1977; Behringer et al., 1977~. Increased penetration of the Loop Current has been observed in winter and spring, with the maximum northward extension occurring in early summer. However, Vukovich (l98Sa) has found ring separation that is not periodic. It has been suggested (Maul, 1977) that inflow at the Yucatan Strait must exceed outflow through the Straits of Florida in the upper 500 m in order for the Loop Current to grow in northward extension. The implication is that the seasonal cycle in the penetration of the Loop Current is a response to variations in the large-scale circulation associated with the Gulf Stream system. Along the northern and eastern boundaries of the Loop Current, where it comes into contact with the west Florida shelf, cold-core eddies have been observed to form on the Loop Current front (Paluszkiewicz et al., 1983; Vukovich and Maul, 1985~. These features result in intense but short-lived upwelling events along the west Florida shelf break. Loop Current Rings. The warm-core (anticyclonic) rings that separate from the Loop Current are a major feature of the large-scale circulation of the Gulf of Mexico. Observations (see, e.g., Behringer et al., 1977) show that these rings typically separate from the Loop Current at the time of the maximum northward penetration of this current into the Gulf of Mexico. On an average, one to three anticyclonic rings per year may separate from the Loop Current. Loop Current rings are approximately 300 to 400 km in diameter and have a depth signature that extends to approximately 1,000 m (Brooks, 1984~. After detaching from the Loop Current, these rings move westward across the Gulf of Mexico, and observations have shown that the rings exist as identifiable features for periods of several months (Elliott, 1982~. Geostrophic surface velocities within the rings have been estimated to be on the order of 25 to 100 cm/s, and

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS 81 volume transports associated with the rings are on the order of 5 to 10 x 106 m3/s (Merrell and Morrison, 1981; Merrell and Vazquez, 1983; Hofmann and Worley, 1986~. Consequently, these rings represent a major mechanism by which properties such as temperature and salinity are transported from the eastern to the western gulf. Once in the western Gulf of Mexico, the rings encounter the Texas or Mexican continental shelf. The fate of the rings at this time is not fully understood. These rings represent a mechanism by which the shelf waters in the western gulf are exchanged with the gulf waters originating in the eastern gulf. Shelf Circulation On the Texas-Louisiana continental shelf, west of 92.5°W, the predominant feature of the circulation is a cyclonic (counterclockwise) gyre, elongated in the alongshelf direction (Cochrane and Kelly, 1986~. The inshore portion of this gyre is directed westward (downcoast). An eastward-flowing countercurrent at the shelf break constitutes the offshore portion of the shelf gyre. Flow in the western extent of the gyre is directed offshore, while that in the eastern gyre near Louisiana is directed onshore. The alongshore wind stress is the primary mechanism driving the circulation of this cyclonic gyre. Because the gyre is primarily wind driven, it exhibits seasonal variability in strength and occurrence that reflects the seasonal variability in the wind patterns over the Texas-Louisiana shelf. In July, when the downcoast (to the west) wind stress is diminished, the cyclonic gyre on the Texas-Louisiana shelf disappears and is replaced by an anticyclonic gyre centered off Louisiana. In August and September, the prevailing wind direction changes abruptly so that the predominant winds are again downcoast (westward), and the cyclonic gyre is re-established. Thus, shelf currents on the Texas-Louisiana shelf reverse with a seasonal frequency. The circulation on the continental shelf east of the Mississippi River delta is directed toward the west during the winter. As with the Texas-Louisiana shelf, this flow diminishes and reverses direction in the summer months. Currents over the west Florida shelf are predominantly to the south. Several studies (see, e.g., Molinari et al., 1978) suggest that a cyclonic gyre exists in the northeastern corner of the Gulf of Mexico over the west Florida shelf. Evaluation of MMS-funded Research in the Gulf of Mexico Region Observational Studies MMS supported a 5-year (1982-1987) observational program-the Gulf of Mexico Physical Oceanography Field Study designed to study physical oceanographic processes in the Gulf of Mexico. The focus during the first 2 years and the fourth year of the observational program was the eastern Gulf of Mexico. In particular, processes associated with Loop Current dynamics, eddy shedding from the Loop Current, and interactions of the Loop Current with the west Florida continental slope/shelf region were of interest. During the third year the observational program emphasized processes in the western Gulf of Mexico, primarily Loop Current eddy interactions with the shelf/slope region in the western gulf. The final year of the observational program shifted emphasis to the north-central gulf, offshore of Louisiana. Below are brief reviews of the observational programs in the various regions of the Gulf of Mexico. The contracts for the reports that describe the results of the studies in the various regions are listed in Appendix C. Eastern Gulf of Mexico The components of the MMS-sponsored physical oceanography observational program in the eastern Gulf of Mexico consisted of moored current and temperature measurements; a coordinated ship-airplane hydrographic survey of the Loop Current, with emphasis on the west Florida shelf region; satellite thermal imagery and advanced, very high-resolution radiometer (AVHRR) data; satellite-tracked Lagrangian surface drifters deployed in Loop Current eddies;

82 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF and a ship-of-opportunity program that provided temperature measurements (from expendable bathythermographs (XBTs)) along a north-south transect in the eastern Gulf of Mexico. The moored current and temperature measurements on the west Florida shelf were made with a current-meter mooring array that consisted of a single mooring located in approximately 180 m of water near 27°N and a line of five moorings south of 26°N that extended from the mid- portion of the west Florida shelf offshore into water that was deeper than 3,000 m. The three shelf moorings were located at water depths of 30, 75, and 180 m, and each mooring consisted of three current meters. The two off-shelf moorings were at water depths of 1,700 and 3,400 m and consisted of five and six current meters, respectively. The configuration of this mooring array was such that it provided information primarily on across-shelf motions that occur at large scales. Mooring deployments were for 2 years, which provides current and temperature time-series of sufficient length to consider water motions that result from atmospheric (short-term and seasonal) and LOOp current forcing. An atonal current-meter mooring was maintained near the Mississippi River delta in South Pass Block. This mooring was in 80 m of water and consisted of instruments at depths of 13, 25, 40, and 70 m. Two intense hydrographic surveys of the Loop Current and Loop Current interactions with the west Florida shelf were sponsored by MMS. On each cruise, the hydrographic transects were designed to intersect the Loop Current front on the west Florida shelf side of the Loop Current. The water properties measured on these cruises consisted of temperature, salinity, dissolved oxygen, and nutrients as well as several properties of biological interest. The station spacing and cruise track design from the two hydrographic cruises were adequate to map the Loop Current front and the frontal eddies (also referred to as boundary eddies and cyclonic cold domes) associated with the Loop Current front. In particular, the hydrographic surveys focused on the interaction of the Loop Current frontal eddies with the adjacent continental-shelf waters. The hydrographic surveys were complemented by larger-scale temperature distributions obtained from airplane surveys of the Loop Current and eastern Gulf of Mexico waters. The MMS Loop Current studies program contained a considerable remote-sensing component, which is appropriate given the scale of the features of interest. Sea-surface temperatures obtained from the infrared sensors on the NOAA and geostationary operational environmental satellites (GOES)-were used to study fluctuations in the seasonal cycle of the depth of penetration of the Loop Current into the Gulf of Mexico, to study the separation of warm-core (anticyclonic) rings from the Loop Current, and to study the frontal eddies that form on the boundary of the Loop Current (Vukovich, l98Sb). Also, Lagrangian surface drifters were placed into the warm-core (anticyclonic) rings that separated from the Loop Current. These drifters provided information on the trajectories and translation velocities of the warm-core rings as they moved westward across the Gulf of Mexico. The drifter data were also used to calculate dynamic quantities such as local vorticity and horizontal deformation rates that can be used to investigate changes in the dynamics of the warm-core ring throughout its lifetime, contributing greatly to our understanding of drifter data (see, e.g., Lewis et al., 1989~. Western Gulf of Mexico The moored current and temperature measurements in the western Gulf of Mexico were made with an L-shaped mooring array that was deployed in the region approximately between 24°N and 26°N. The array consisted of five current-meter moorings, all of which were deployed in water depths greater than 2,000 m. Three of the moorings were placed along-slope and three moorings (one mooring is the apex of the array) extended offshore into the gulf. This array design is appropriate to capture the motions of large-scale features, such as warm-core Loop Current eddies, as they move toward and along the slope in the western Gulf of Mexico. The moorings remained in place for approximately one year, which is sufficient to provide current and temperature time-series from which mesoscale motions can be extracted. Extensive hydrographic surveys were made in the region around the current-meter moorings. These observations were complemented by an air-droppeci expendable bathythermograph (AXBT) survey of a warm-core ring in the western gulf. This type of survey provides synoptic large-scale coverage of oceanographic features that is not possible from ship

REGIONAL OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS 83 observations. With assistance from the Mexican Navy, it was possible to obtain frequent surveys of a warm-core ring observed in the western Gulf of Mexico. The study of warm-core rings in the western Gulf of Mexico made extensive use of surface Lagrangian drifters. Surface drifters were placed in rings after separation from the Loop Current, and the drifters remained with the rings as they moved westward across the Gulf of Mexico. The drifter data were used in conjunction with sea-surface temperature measurements obtained from satellite observations, and with vertical temperature distributions obtained from AXBT surveys of the warm-core rings. This combination of data allowed study of the dynamics governing the circulation in the warm-core rings. North-central Gulf of Mexico The general objective of the MMS-sponsored study of the north-central Gulf of Mexico is to develop a data base that can be used to describe the circulation patterns and processes of this region. The measurements and observations being made in this region follow those made in the eastern and western gulf. These consist of moored current and temperature measurements, hydrographic surveys, satellite thermal imagery, satellite-trackecl drifting buoys, and a ship-of- opportunity program. The proposed current-meter mooring array for this region consists of seven moorings deployed along a transect that extends from the mid-continental shelf offshore to water depths of 3,000 m. The shelf moorings are between the 15- and 150-m isobaths, while the deeper moorings are on the 1,000, 1,500 and 3,000-m isobaths. This type of array design provides information on across-shelf motions. It is intended to provide insight on shelf circulation processes and the effects of the larger-scale gulf circulation processes. Evaluation of Observational Studies The MMS-funded, 5-year physical oceanography observational program was adequate to study the large-scale circulation of the Gulf of Mexico. The field programs were structured to provide data on the major oceanographic features in the gulf: the Loop Current and the rings and eddies associated with the Loop Current. With few exceptions, the physical oceanography program in the Gulf of Mexico focused on circulation processes that occur in waters deeper than 1,000 m. Essentially no studies were performed on the Texas-Louisiana continental shelf, or in waters shallower than 500 m, in any part of the gulf. The exception to this is the study of Loop Current frontal eddies on the west Florida shelf. The field program designed to investigate Loop Current processes and interactions of the Loop Current with the west Florida shelf was thorough, given the budgetary and time constraints imposed on the overall program. It can always be argued that more observations and data are needed; however, the data obtained were adequate to describe the basic circulation features, i.e., the frontal eddies. The panel has reached similar conclusions about the observational programs in the western and the north-central Gulf of Mexico. The western gulf program in particular has resulted in a considerable data base on warm-core rings and progress in the development of techniques to analyze and understand Lagrangian measurements. Overall, the MMS studies of the gulf circulation made good use of a combination of data sources, in particular sea-surface temperature distributions obtained from satellites. It should be noted that a significant portion of the results from the Gulf of Mexico physical oceanography program has been published in the refereed scientific literature (see, e.g., Kirwan et al., 1984a,b; Lewis and Kirwan, 1985, 1987; Kirwan et al., 1988~. Also, many of the results of this program have been presented at national scientific meetings such as the annual meeting of the American Geophysical Union. It can be concluded that the MMS study of the circulation of the Gulf of Mexico yielded scientifically credible information. Although the MMS physical oceanography program in the Gulf of Mexico did yield substantial information on large-scale and mesoscale circulation features in the gulf, it did not provide any substantial information on shelf-circulation processes. Given that most oil-drilling

84 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF activities occur over the continental shelf in water shallower than 150 m, the lack of focus on shelf circulation is disturbing. Little attention has been paid to the Texas-Louisiana shelf, where oil exploration and drilling activities are the most intense. However, projects are being started in this area. The Texas-Louisiana shelf region is occasionally influenced by warm-core rings of Loop Current origin, but these rings are not the major oceanographic process governing the circulation on the shelf, ant! the frequency of occurrence of rings in the western gulf is not high. Thus, although MMS has been supporting a physical oceanographic observational program that was adequate to describe some aspects of the general circulation of the Gulf of Mexico, the focus of this program was on circulation features that are of minor importance to the needs of programs designed to perform oil-spill risk-analysis assessments. In summary, the MMS physical oceanography program was designed to look at space and time scales that are too large to be of use for oil-spill risk-assessment analysis. Modeling Studies The MMS-supported modeling studies of circulation in the Gulf of Mexico consisted of the development of models to predict the seasonal water circulation over the southwest Florida shelf, the geostrophic circulation on the Texas-Louisiana shelf, and the basin-wide circulation of the Gulf of Mexico. Brief descriptions of these models are given below. Details of the models can be found in the reports based on the contracts listen! in Appendix C. Southwest Florida Shelf Dodge! The model developed for the southwest Florida shelf (Cooper, 1982) was a linear hydrodynamic model that included vertical and lateral friction effects and a free surface. The model allowed for forcing due to surface-wind stress, atmospheric-pressure gradients, and bottom stress. The vertical structure of the flow was represented by a series of functions that allowed for vertical variation in the velocity fields. The model used a time-invariant density field that was specified from seasonally averaged hydrographic observations made on the west Florida shelf. Model boundaries were specified as land boundaries (inshore) and as either closed or open along the north, south, and offshore boundaries. Model output consisted of distributions of the horizontal velocity components (u and v) on a grid with a 30-km resolution. Steady-state and time-varying velocity distributions were computed with the model. The maximum depth allowed in the model was 200 m. The effect of the Loop Current was included in the model by specifying a velocity distribution along the outer (offshore) boundary of the model. This flow was specified as a constant velocity along the boundary or as a velocity that linearly increased or decreased along the outer boundary of the model. This approach assumes that the Loop Current is essentially barotropic and does not allow for any baroclinic structure or adjustments of the flow. The use of a time-invariant density field places a severe restriction on the usefulness and reliability of the simulated circulation distributions. The assumption made in this approach is that the wind field does not interact with the density structure of the shelf waters. This is contrary to understanding of coastal circulation processes. In general, the calculation of seasonal circulation patterns is questionable. Texas-Louisiana Shelf Mode' The circulation distributions used to perform oil-spill risk-assessment analyses for the Texas-Louisiana continental shelf region were derived from geostrophic velocity calculations. The geostrophic velocity fields were obtained using seasonally averaged density fields for the shelf waters. Consequently, the circulation fields, at best, can only be representative of seasonal circulation patterns. Such circulation patterns are not appropriate for determining the trajectories

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS 85 that may be followed by an oil spill. Oil-spill trajectories are determined by the prevailing circulation, which may not be represented in a seasonally averaged circulation pattern. For example, wind forcing can produce currents that flow in a direction opposite to that suggested by a long time-averaged circulation. The use of geostrophic calculations for determining circulation patterns in continental-shelf waters is questionable, particularly in shallow waters. Furthermore, the accuracy of the density fields that go into the geostrophic calculation will determine the reliability of the derived circulation. Also, the shelf circulation models might not account for such climatological variations as timing in the maximum river runoff and changes in wind patterns, which can vary from year to year. In summary, the Texas-Louisiana Shelf Model is not adequate to meet the stated objectives of oil-spill risk-assessment modeling studies. Basin-wide Circulation Mode! The circulation model used by MMS for the basin-wide Gulf of Mexico modeling studies is a modification of an existing model developed by Hurlburt and Thompson (Wallcraft, 1986~. The Hurlburt and Thompson (1980) model is a two-layer, nonlinear, hydrodynamic, free-surface, primitive equation model on a beta plane, with a realistic coastline geometry and full-scale bottom topography confined to the lower layer. This mode! provides velocity distributions on a grid with a resolution of 0.2 degrees. The model is forced by inflow through the Yucatan Strait and compensated by outflow through the Straits of Florida. Wind forcing was not treated in the Hulburt and Thompson model but was included in some experiments with the modified model. MMS sunnorted a modeling effort that had the overall objectives of modifvina the . _ _ ~ ~ ~ ,, ,, -, Hurlburt and Thompson model so that it had a toner spatial resolution tU.1 degrees) and Included an additional layer (3 versus 2 layers), so that the circulation results could be coupled with a mixed-layer model, specifically the Navy's operational mixed-layer forecast model. It should be pointed out that these are all modifications to an existing model; initial model development was not necessary. A major problem with a layer model is that the model becomes unstable when the layer surfaces intersect the bottom topography, or the surface. Much of the effort in modifying the Hurlburt and Thompson model has been directed at correcting this problem. This is a particular problem for the objectives of the MMS modeling program, because it means that the model is not valid for shallow depths. Indeed, the basin-wide circulation model produced for MMS does not provide usable velocity distributions in regions of the gulf with depths of less than about 500 m. Evaluation of Modeling Studies The circulation models developed for the southwest Florida shelf, the Texas-Louisiana shelf, and the Gulf of Mexico (the basin-wide model) are inappropriate for the objectives of the MMS modeling program. In particular, the first two models are not designed to provide more than a best guess at the circulation pattern. The basin-wide circulation model is interesting from a scientific standpoint, but is of little practical use in meeting the goals of the MMS modeling program. Given the inability of this model to produce realistic flows in the shelf/slope regions, it is not clear what MMS has gained by funding the modifications to the Hurlburt and Thompson model. Furthermore, this modified model is complex, and understanding the simulated circulations produced with the model would require considerable effort. It is not clear that MMS would (or perhaps should) fund such an analysis. It is doubtful that the basin-wide mode! would be of much use in oil-spill risk-assessment analysis. It should be noted that few of the results obtained with the modified Hurlburt and Thompson model have been published in the refereed scientific literature. One interpretation of this is that the modified model does not represent any significant advance over what was learned from the Hurlburt and Thompson studies. However, the MMS Gulf of Mexico modeling effort has attempted to incorporate flux-corrected transport techniques that will remedy some of the problems encountered in layer models when interfaces intersect the surface or bottom

86 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF topography. If this effort is successful, it will represent an advance over the Gulf of Mexico model done by Hurlburt and Thompson. In summary, the MMS modeling program has not been successful. It appears that inappropriate decisions as to what models would be used were made in the early stages of the program. Instead of those decisions being reevaluated at the mid-point of the study, for example, the models were continued, even when it should have been obvious that they were inappropriate. The results of the Gulf of Mexico circulation modeling program have not yet been used by BEM in oil-spill-risk assessments because the work is not complete, and the results have not been verified (personal communication, T. Paluszkiewicz, April IS, 19893. On the positive side, the data obtained from the physical oceanography observational program are adequate to form the basis of circulation modeling studies in some specific regions and for some specific oceanographic features. However, there seems to have been insufficient interaction between the observational and modeling programs. This is not a new problem and should be avoided in future MMS studies. THE ATLANTIC REGION The Atlantic region can be divided into two parts. The region south of Cape Hatteras is known as the South Atlantic Bight, and that north of Cape Hatteras is known as the Mid- Atlantic Bight. In the South Atlantic Bight, the mid- and outer-shelf region is dominated by the Gulf Stream flow, with eddies at the shelf edge providing very strong variability in the currents. The shelf is about 100 km wide. The shelf widens over the Mid-Atlantic Bight, with the Gulf Stream further offshore. There are strong southwestward currents in the slope water and the shelf with, again, strong variability associated with winds, tides, and offshore meandering of the Gulf Stream, mesoscale eddies, and Gulf Stream rings. For MMS planning purposes the Atlantic region is divided into the north Atlantic, mid-Atlantic, and south Atlantic subregions (Figs. 15, 16,and 17~. Meteorology Meteorology and Circulation The meteorological conditions differ in the northern and southern parts of the Atlantic region. Seasonal winds in the South Atlantic Bight stem from circulations around either the Azores-Bermuda high-pressure center (tropical, maritime air) or a high-pressure region in the Ohio Valley (colder and drier air). In the spring, the Azores-Bermuda high dominates, and the flow is to the west over south Florida, turning to the north and northeast over the Blake Plateau. Monthly averaged velocities are 1 to 2 m/s. In summer, the northward flow strengthens. In autumn, there is a transition to southwestward flow as the Ohio Valley pressure center becomes dominant. Warm northward flowing air persists only over the Blake Plateau and offshore. The winter regime is dominated by southeastward winds of about 1 to 2 m/s. However, to a large degree, this mean is the average of numerous weather systems passing through the area. In the Mid-Atlantic Bight, the seasonally averaged winds show strong southeastward flows in the winter, due to the Icelandic low and the weak North American high, but become less organized in spring. Northward to northeastward winds develop in summer (Azores-Bermuda high), shifting to southwestward winds in fall (Canadian high). In the Georges Bank region further north, this pattern persists: the mean winds are eastward to southeastward during the fall to spring months, averaging 3 to 6 m/s, and are weaker and northeastward in the summer. Interannual variability in the monthly mean wind speeds is substantial, partly because the transitions from one regime to the other occur at differing times. Representations of the windfield need to appropriately account for the long-term distribution of speeds and directions in the monthly averages. For simulations of currents and oil-spill motion, the synoptic variability in the wind field is as important as the mean, especially in the more northerly regions. Major wind systems in the

REGIONAL OCEANOGRAPHYAND EVALUATION OF STUDIES PROGRAMS 71° rot _ S NEW ~ _.\ HA~PSHIRF i-~ ·:¢ -45- :-.:/ :.~ .. ~ "&I OF . ~ .~e ·';~1 .:-' :' '.' is, .' - ~ +.; ~ r -41 _39. O tS ~T' loo STATUTE HODS 0 20 50 re 100 K1~0~5 ski., is. tq g \ ) ~J - Jay -Act NORTH Be_ ) ~ATLANTIC ,r~-~ CANAD^ 1 1 (, ~ ~ In.._ ~ UNITED {'~ em-. STATES N\\ ~ \ ATLANTIC (a 'a ~REGIO N FIGURE 15 North Atlantic planning area of the Atlantic region. Source: MMS. 87

88 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF . . , . . , . ~. . , 7C- 75- rue / r5e 72. 71-: rot .~. I ' .2'/ AsSACHU5ETT8 .~- . :.: - S _~.. ~is._ _~. PE HN SYLVANIA .> -40- a; ~. a. 0 's so re loo STATUTE MILES L ' L it: , I · . t) 26 ~75 1~ KILOMETERS ;r ~ ~ L. ~ ~ i, .~ ~NEW YORE | CONNECTICUT 1 1~ I ~ T! ~ ATLANTIC UHITE0 $~ <~ ~ ATLANTIC lo\\ \' ~REGION FIGURE 16 Mid-Atlantic planning area of the Atlantic region. Source: MMS.

REGIONAL OCE~4NOGRAPHYAND EVALUATION OF STUDIES PROGRAMS - -54- ~ ~ J SOUTH \ ~C ~ R O L I N A \ _ 53- "N A'.. 11. . ~. rso ~78- . NOR T ~, - C A R O L I N \ -\` \ ,". \ SOUTHPORti][ ...;.'~1 ../ _~! ~ CHARLESTON _' BUMP ~ ~ ~5 I" STATUTE ~lLtSi if- I - } L O /25 50 as loo KIL~ETER~ : AS //j // / ,/// / - ~CAN4n ~ TIC ) ~ J ATLANTIC N;-r-. . ~ FIGURE 17 South Atlantic planning area of the Atlantic region. Source: MMS. 89

9o PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF South Atlantic Bight have 2- to 14-day periodicities, with shorter diurnal periods also appearing near the coast. These sea-breeze cycles can also be seen near shore, peaking during the summertime. The major frontal events move offshore nearly parallel to the coast, setting up large-scale, alongcoast current patterns. In addition, the varying wind strengths are important in establishing the amount of vertical mixing and the depth and strength of the seasonal pycnocline. The variability in winds in the winter is caused by the fairly regular passage of cyclonic disturbances along a storm track lying in the Mid-Atlantic Bight. In addition, some cyclogenesis occurs off Cape Hatteras, and some storms do come up from the Gulf of Mexico. Fewer cyclones occur in summer, with the storm tracks lying further north. Anticyclonic events are less frequent, but again the storms follow storm tracks lying in the Mid-Atlantic Bight north of Cape Hatteras. In an average year, 5 (Florida) to 20 (New England) cyclone events occur. The synoptic scale variance shows a strong anticyclonic component to the rotary spectrum over the Blake Plateau, especially in winter. Nor'easters, large (1,000-2,500 km), severe low-pressure systems moving from the west or southwest with winds at speeds up to 35 m/s from the northeast, are frequent in the wintertime. Over 10% of the observations in December through March at Georges Shoal show wind speeds higher than 17 m/s; most of these high-wind periods are associated with these nor'easter events. They also contribute significantly to the observed higher average wind speeds. Because the winds generally have a long fetch, the associated waves are on the order of 3 m in height, occasionally reaching 12 m on Georges Bank. Precipitation, consisting of rain or snow, is also heavy. Tropical cyclones and hurricanes may strike many of the offshore sites along the Atlantic coast. Wind speeds of 50 m/s may occur, along with heavy rain. For the purpose of simulating oil motion, winds in nearshore regions have two effects. They can directly produce surface currents by creating a turbulent Ekman layer with surface drifts to the right of the winds. Secondly, the winds in the presence of a lateral boundary may also cause setup of the ocean surface. Currents, driven by the resulting pressure gradients and retarded by bottom friction, can have a magnitude comparable to or larger than the directly driven flows. These flows depend in a complicated way on the direction' strength, and history of the wind stress, as well as on the topography. Wind measurements are most readily available from shore stations. However, the atmospheric boundary layer changes character significantly over water as moisture and heat are exchanged with the sea surface, so that the wind field measured on land is quite different from that taken from stations 20 to 50 km offshore. Schwing and Blanton (1984) found that the ocean stations had more energy by a factor of 4 in the synoptic time scales; the directions were also significantly different. The shore-normal component, in particular, was not well predicted by even an appropriately magnified form of the winds measured on land. Thus, the estimated wind stress based on data from shore stations may easily be too low by a factor of 2 to 5 and may be off by 40 degrees in direction. Circulation The South Atiantic Bight The oceanography of the South Atlantic Bight region is very well summarized in Atkinson et al. (1983~. This region is characterized by a relatively narrow shelf (50 to 120 km) with a water depth of about 50 m, bounded by the coast on one side and the Gulf Stream on the other. South of the topographic feature known as the Charleston Bump (at 32°N), the Gulf Stream flows in about 400 to 600 m of water, with currents of 1 m/s. The stream is deflected eastward by the bump and then returns to the shelf edge near Wilmington (at 33.5°N) and continues slightly farther offshore to Cape Hatteras. In the midshelf region (water depths of 15 to 400 m), the currents are, on the average, in the same direction as the Gulf Stream, although reverse flows can often be found around the low-pressure center formed by the offshore meander. In the shallowest part of the shelf, there is often a baroclinic southward current associated with the 1aye3r of fresh river runoff water. The rivers in the South Atlantic Bight provide from 3 to 7.7 km of fresh water per month, distributed fairly evenly along the shelf. This can significantly alter the flow and stratification near the shore.

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS 91 Currents are highly variable; tides, wind-driven motions, and mesoscale eddies all cause significant fluctuations on the shelf. The tides (predominantly M') have a range of 1 to 3 m along the coast, with tidal excursions of 4 to 20 km in the inner shelf region. Currents may reach 0.4 m/s nearshore. In the midshelf areas, the tides account for 80 to 90% of the cross-shelf variability and 20 to 40% of the alongshelf current fluctuations. Over the outer shelf, tides become less significant, accounting for less than 30% of the variability. Wind-driven currents lead to strong motions-at 5- to 10-day periods as well as mean northward drift. These are significantly correlated with the coastal sea-surface pressure and the alongshelf component of the winds, but are not very coherent spatially. Fluctuations of 0.1 to 0.3 m/s are common. Modeling work by Lee et al. (1984) suggested that displacements of 70 km over ~ days were characteristic of the midshelf region; of this, about half was caused by wind-driven currents. It should be noted that the winds in this region show strong variability from synoptic systems moving through the area. The winds over the ocean are consistently stronger by a factor of 2 than those measured even at nearby onshore stations. Furthermore, the directions may be off by as much as 40 degrees during periods of changing winds. The alongshore winds are usually well represented (except for strength) by the coastal stations, but the onshore or offshore winds may be quite different. Sea breeze contributes significantly to currents near shore. Mesoscale variability is primarily produced by the frontal waves and eddies of the Gulf Stream at the outer edge of the shelf. These disturbances, with wavelengths of 100 to 300 km, displace the shelf break front by 10 to 100 km across the shelf. Cyclonic circulations develop in the troughs corresponding to reverse flow near shore. About once a week' the disturbances grow _ 4, , ~ ,~ ~ . . . . ~ .. . . ~ . ~ , and fold backwards to term a cold pool and a Pattern ot- northeast-southwest- northeast currents. . · . ~. . . . . . . Upwelling at a rate of about 10~ m/s occurs in the cold pool; this may significantly influence the biological and chemical distributions. Transient upwelling also occurs over topographic features. The current fluctuations are about 0.S m/s and are coherent for about 100 km along the Gulf Stream. The average propagation rate for these waves or eddies is 0.5 m/s, dominated by the advection by the strong currents of the Gulf Stream. The waters on the shelf of the South Atlantic Bight are vertically well mixed during the winter, with a strong horizontal temperature gradient occurring across the shelf into the Gulf Stream. During the summer, the shelf area is stratified, and the surface thermal gradient is quite small. Some estimates of horizontal mixing and flushing times exist for this region. Bumpus (1973) has estimated a residence time of about 3 months based on the transport and volume of the shelf waters. The Mid-Atiantic Bight and Georges Bank North of Cape Hatteras, the physical oceanography changes significantly: the shelf is wider (150 km), and the Gulf Stream moves further offshore (200 to 300 km from the shelf break). Georges Bank lies off Cape Cod, separating the Gulf of Maine from the slope water, with most of the water exchange occurring through the Great South Channel and Northeast Channel. There are fairly strong temperature-salinity contrasts across the shelf, with the shelf-slope front along the 100-m isobath in winter separating the two water masses. The front leans outward by about 30 km as it extends to the surface. The density contrast is relatively weak, so that, although the currents along the front are fairly strong, there is not a large baroclinic component. In the summer, a strong seasonal thermocline develops, and a cold pool of water cuts off along the 100-m isobath; there is still a moderate salinity gradient. Mean currents in the Mid-Atiantic Bight are generally along-isobath, with flow along the bottom into estuaries and strong currents in canyons. There is a complicated circulation around Georges Bank, with flow into the Gulf of Maine near Nova Scotia, cyclonic circulation around the gulf, and a strong jet (0.3 m/s) around the northeast side of Georges Bank, which turns to the southwest along the 70-m isobath. Some of this flow appears to recirculate around the bank through the Great South Channel, while some proceeds to the west and south into the Mid-Atlantic Bight. In addition, some of the Gulf of Maine circulation passes through the Great

92 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF South Channel and joins the general alongshelf drift. Further offshore, in the slope water, the mean circulation also has a southwestward flow at about 0.7 m/s at the surface. Tidal currents are strong in the northern region; tidal range varies from 0.5 to 4 m, and current speects can reach 1 m/s in the Northeast Channel, with tidal excursions on the order of 20 km. In shallow areas, these currents are sufficient to vertically mix the water even during the summer and to cause substantial sediment transport. Rectification of tidal fluctuations is thought to be an important driving mechanism for mean flows. Low-frequency motions on the shelf of the Mid-Atlantic Bight are dominated by the wind, which drives energetic alongshelf fluctuations of 2 to 3 times the mean flow velocity, with periods on the order of 5 days. The onshore-offshore flows are at least twice as weak. Alongshelf currents are fairly coherent all along the shelf, but the cross-shelf flows are incoherent over distances of 50 km and across the shelf. Another source of variability on the shelf is forcing from the slope water mesoscale eddy field, the Gulf Stream rings passing down the coast, and the large meanders of the Gulf Stream (and the waves associated with these). Warm-core rings bring strong currents to the edge of the shelf and can force slope water or Gulf Stream water into shallower regions and entrain material off the shelf. Several of these pass by Georges Bank each year. They significantly reduce the residence time for water on the bank and increase the mixing of waters on the bank. Summaries of drogue experiments and model estimates indicate that water is resident on Georges Bank for 40 to 80 days, with the nearly closed circulation being responsible for the long period. Further south in the Mid-AtIantic Bight, the flushing time- is shorter, about 30 days. Mixing is strong on the shelf, both because of the tides, which disperse material over a scale of the tidal excursion in one period, and because of energetic fluctuations driven by winds and by eddy and meander events. Evaluation of MMS-funded Research in the Atlantic Region History of MMS-fundec! Research in the Atlantic Region MMS has funded a number of large physical oceanographic studies in the Atlantic region. Among these are the Blake Plateau Current Measurement study (October 1982 to September 1986), the Florida Atlantic Coast Transport Study, or FACTS (January 1984 to February 1986), the South Atlantic Bight numerical modeling study, and the Mid-Atiantic Slope and Rise study, or MASAR, two of which are discussed below. Appendix C contains a full list of MMS-funded studies in the Atlantic region. Evaluation of Observational Studies These studies have generally consisted of gathering current, hycirographic, and surface imagery data. In most cases, the studies were carefully done and provided good data on the mean and variable circulations in the various areas. The mooring programs have been quite ambitious: the FACTS program began with 41 current meters deployed on 10 moorings, one line of 7 spanning the Gulf Stream and 3 more located along the continental slope upstream. The Blake Plateau study maintained three lines of moorings across the stream at Onslow Bay, Long Bay, and the Charleston Bump for over 2 years. However, the programs all suffered from instrument failures and losses, partly because of the difficulty of mooring work in strong, highly sheared currents and probably also in part from fishing activity. The FACTS program also involved drifting-buoy and drift-card releases. Buoys launched about 50 km offshore of Melbourne Beach stayed within the stream, whereas those launched nearer inshore or in the Straits of Florida tended to move into the coastal waters. Drift-card returns indicated more significant onshore motion, perhaps associated with increased wind influence. No attempts were made to relate either directly to oil motion. Hydrographic data provided tracers of water masses, including river runoff, and indicated the occurrence of strong wind mixing, upwelling, and injection of water onto the shelf from Gulf Stream frontal eddies. Surface satellite imagery was also used to observe and describe eddy

REGIONAL OCEANOGRAPHY AND EVALUATION OF STUDIES PROGRAMS features, both to provide a context and to obtain estimates of the frequency of strong onshore events. 93 MMS has included a fairly high proportion of investigators from universities and research institutions in the FACTS program. The analyses of the individual data sets by the subcontractors have been competent. However, there has been no quantitative synthesis of the various types of data to produce a dynamically consistent description of the flow. It does not appear that much of the information from this study (or any synthesis work) has been published in the reviewed literature. This problem can be seen in a number of other MMS-sponsored programs, although the work on Georges Bank has been published and some synthesis has been presented. In the Atlantic region, MMS has generally made good efforts to coordinate its field programs with other ongoing projects. This has enabled the various scientists involved to gain a broader perspective on their observations and has contributed to the other programs as well. Evaluation of Mo~iel~ng Studies Modeling work in the Atlantic region has progressed in two stages. The contractor (Dynalysis of Princeton) has worked with equations for horizontal momentum, heat, salt, turbulent energy, and turbulent length scale (as part of a second-order closure scheme). Dynalysis first produced a characteristic tracing model CTM, which neglects advection in the momentum and turbulent energy/length scale equations. The equations are solved by integrating along contours of the Coriolis parameter divided by the depth (also known as f/h contours). This diagnostic calculation produces a steady flow pattern under fixed boundary conditions. In the second stage, the CTM is used to produce boundary information for a full primitive equation general circulation model (GCM). The contractor uses diagnostic and prognostic forms of the temperature and salinity equations. These models do not appear to produce significant eddying, despite the fairly fine grid resolution. The contractor is working on incorporating more variability into its model by varying the boundary conditions. It is not clear why instabilities do not appear to be significant, in contrast to the calculations of Orlanski and Cox (1973) for a · · ~ slm1 ar region. The models use data for initialization, forcing, and boundary conditions. There appears to be little relationship between the modeling work and the data collection. The reports do not reference each other, and the modeling does not appear as a subcontract in the same project as the observational work. (MASAR is an exception to this, although the modeling work in that study does not seem to be of the directly applied numerical simulation kind.) The data used in the models come from the historical record; verification of the models has been rather minimal. Environmental Impact Statements It is disappointing to note that the OSRAs produced in 1984 and 1985 continued to use the CTM model, despite the fact that the GCM reports were available in 1981 (Blumberg and Mellor, 1981~. MMS judged the GCM calculations to be too short and to cover too limited an area and, therefore, chose not to incorporate this information into their risk analysis (pers. comm., MMS, 1990~. In addition, large amounts of field data on the magnitude and importance of the variability were available, but were not used in the risk analysis. The EIS for the sales in the Georges Bank region (U.S. DOI, 1983a) exhibits similar problems and is discussed in more depth here as an example of poor use of the available data base. The document has a 15-page summary of the physical oceanography of the region. This draws on both published and unpublished reports and is a good summary of the currents, hydrographic structure, and wave conditions. Then, 143 pages later, risk analysis is finally discussed, and there is a 2-page discussion of the analysis, which is based entirely on modeling as discussed further below. There is no reference at all in the risk analysis section to the preceding summary of oceanographic knowledge.

94 PHYSICAL OCEANOGRAPHY OF THE U.S. OUTER CONTINENTAL SHELF Furthermore, the modeling work, reported in Appendix D (2 pages) of the EIS document, uses the CTM south of 41.5°N and west of 69.5°W. These boundaries correspond to Rhode Island and Cape Cod to the north and to Nantucket Shoals on the eastern edge. Thus, the numerical model does not cover the Great South Channel or Georges Bank at all. Instead, currents based on a "geostrophic assumption," calculated by F.A. Godshall in a NOAA unofficial report and cited in Appendix D of the EIS, were used for the rest of the study area (U.S. DOI, 1983b). None of the simulated trajectories are shown, so it is impossible to judge the performance of this calculation. Again, no comparisons with the actual observations are offered. Thus, we have a fairly elaborate and expensive field study and (perhaps unique to this region) synthesis work in the EIS, giving a rather full picture of the circulation and mixing, all of which was ignored when oil motion was estimated. THE WASHINGTON OFFICE The efforts of the WO, in contrast with the regional offices' goal of data collection, analysis, and synthesis, are focused on supporting regional studies, addressing issues that are common to several or more of the regions (generic studies), and summarizing or documenting previous studies. From 1973 to 1988, physical oceanography studies funded by the WO accounted for 4% of all MMS funding for physical oceanography. The number of physical oceanography studies funded under the WO is extremely limited (see Appendix C). According to the summary list of studies (FY 1973 to FY 1986, 3rd quarter) appended to the Washington Office Regional Studies Plan for FY 198S, only seven studies have been funded for that period, and only two since the previous NRC review (NRC, 1978~. The major effort is an interagency agreement with NOAA for bathymetric mapping services. Another study, proposed in the Regional Studies Plan for FY 1989, which requires study of near- surface physical oceanographic processes, is assessing the use of satellite-tracked surface buoys in simulating the movement of spilled oil in the marine environment. According to the material available, the physical oceanographic studies completed under the WO of the ESP address areas of real concern and have been completed with quality products in a timely manner. An important question is why the WO budget is so small compared with those of its regional counterparts. Several important generic research efforts that have been carried out by the regional offices clearly seem appropriate for the WO. These include, among others, efforts to characterize the transport and fate of oil in the marine environment, to develop models of atmospheric and drill-cuttings dispersion and of coastline-oil interaction, to investigate oil-sediment interaction, to characterize coastal wave dynamics, and to develop methodologies for specifying wind and atmospheric forcing for circulation and trajectory models. It appears that such studies could have been better directed and more efficiently executed, with results that would have been more widely useful, if they had been managed by the WO. It is likely that these investigations were not organized under the WO generic studies program because of the historical strength of the regional offices in establishing the total ESP for MMS. The mandate to complete these overview or generic efforts clearly belongs with the WO. The management structure and funding allocations should clearly reflect that fact. - · ~

Next: Conclusions and Recommendations »
Assessment of the U.S. Outer Continental Shelf Environmental Studies Program: I. Physical Oceanography Get This Book
×
Buy Paperback | $48.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF
  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!