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Oceanography in the Next Decade: Building New Partnerships 3 Future Directions in Ocean Sciences The possibility of and the need for studying the ocean on a global scale provide a major impetus for new partnerships in oceanography. The design and deployment of a global ocean observing system, now being discussed, will be possible only with the cooperation of ocean scientists and governments throughout the world. THE SCIENCE OF OCEANOGRAPHY Oceanography, the science of the sea, serves many purposes while deriving impetus from many sources. All of oceanography—physical, chemical, geological, and biological—is driven by scientists interested in advancing basic knowledge. Ocean scientists have made a number of exciting discoveries in the past 30 years that have changed our view of Earth. The discovery of oceanic eddies has been important for an understanding of ocean circulation, propagation of sound in the ocean, fisheries productivity, and other ocean processes. Verification by ocean drilling that Earth's crust is divided into moving plates that are created at mid-ocean ridges and recycled into Earth's interior replaced the traditional view that the surface was essentially static. Discovery of dense colonies of animals and bacteria at some deep-sea hydrothermal vents demonstrated that organisms could thrive in ecosystems based on chemical energy from Earth's interior rather
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Oceanography in the Next Decade: Building New Partnerships than directly on energy from the Sun. Study of the combined ocean-atmosphere system has provided sufficient knowledge of interannual climate variations that scientists are now able to forecast El Niño climate disturbances months in advance. Over the next decade, oceanography will continue to provide exciting discoveries by contributing new understanding of Earth as a system and by helping us understand how humankind is altering the system. It is now essential (and possible) to study ocean processes on a global scale. The oceanography of the next decade will take place in the traditional marine science disciplines and at the boundaries of these disciplines. New partnerships among oceanographers working in different disciplines should lead to new discoveries about the ocean's role in climate change, the function of mid-ocean ridges, and coastal ocean processes. Additional oceanographic studies in the coming decade will focus on how ecosystems affect global cycles of important chemicals and, conversely, how changes in the global environment affect marine ecosystems. Studies of ecosystems at hydrothermal vents and hydrocarbon seeps will refine our ideas about the conditions under which life is possible and about the origins of life. More of the ocean floor must be explored to determine the extent and nature of deep-ocean vents, their ability to support novel organisms, and their importance in global chemical cycles. Continued study of the ocean's chemistry should bring new understanding of the past state of Earth, how ocean processes operate today, and the contribution of sources and sinks of various chemicals. The study of deep-ocean sediment cores will provide more information about past natural cycles of Earth's climate, with which present climate fluctuations can be compared. Oceanographers will achieve a better understanding of the variability of the circulation of the world ocean. The interaction of climate with this circulation is only poorly known, but there is evidence that the transport of surface water to depth can vary greatly even over as short a time as one decade. Unlike many other sciences driven by scientific curiosity, aspects of marine science have immediate and obvious practical applications. These include, but are not limited to, the control of climate by ocean circulation, chemical and biological reactions to climate change, understanding fisheries productivity, movement of pollutants, and the problem of coastal development in the face of rising sea level. Oceanographers are fortunate to take part in a science that is fascinating, compelling, and intellectually challenging. Oceanography is also a science whose outcome is of
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Oceanography in the Next Decade: Building New Partnerships immediate societal application and in which the financial stakes are potentially immense, for example, the economic impact of a reliable forecast of a sea-level rise. Because the societal implications of the science are readily apparent to policy makers, they may demand answers to purely practical questions in the short term. This pressure can distort the investment in basic science, undermining the quest for basic understanding that remains key to the long-term solution of practical problems. Thus the functioning of oceanography in the United States should focus both on sustenance of the underlying basic science and on specific answers to practical questions of short-term urgency. This chapter summarizes the concerns of basic scientists, with some focus on the interaction of basic science with more practical problems. Several themes are common throughout the discussion, which is divided by classical disciplines. First is the growing sense that the basic science now encompasses the global ocean scale. This capability and the need to conduct global-scale studies have led to the planning of large-scale, long-term cooperative experiments. Primarily planned and executed with National Science Foundation (NSF) support, they focus the work of many scientists on global ocean research. These large programs are usually managed through national or international consortia that involve many scientists, agencies, and often countries. Such programs will explore new questions and test new mechanisms for working together in the next decade. Global uncertainties are rapidly moving much of oceanography from the capabilities and interests of single or small groups of investigators for a limited time to the involvement of many individuals, institutions, and governments for decades. Mechanisms must be developed for these new large-scale efforts to be sustained in a scientifically and technically sound manner, by coordinating the plans of other nations, federal agencies, academic institutions, and individual scientists. Second, all sections of this chapter emphasize the dependence of the subject as a whole continued technical developments. The ocean is remarkably difficult to study, given its size, opacity to electromagnetic waves, and general hostility (e.g, its corrosiveness, high pressures, and turbulence). The health of all disciplines depends directly on the continued development of new tools designed to solve their fundamental sampling problems. In the past decade, oceanographic sampling improved through incorporation of new technologies from other fields, such as remote sensing, material science, electronics, and computer science. A fundamental change arising from the use of these new technologies is
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Oceanography in the Next Decade: Building New Partnerships an increase in the quality and the volume of data collected. Accompanying this change is a significant increase in each oceanographer's capacity to study ocean phenomena, an increase that raises the costs for each oceanographer's science. As the cost per oceanographer for scientific equipment and facilities has increased, the field has responded with increased sharing of facilities, such as ships and submersibles, and equipment, such as the new accelerator mass spectrometer for carbon-14 measurements. The development and shared use of expensive facilities are likely to continue in the future. Yet even with shared facilities, inflation-adjusted research funding for the ocean sciences has remained nearly constant over the past decade, while the number of Ph.D.-level academic oceanographers has increased by about 50 percent and societal pressures to predict man's effect on the ocean have also increased. The growth in the scientific capacity of each investigator and the number of qualified investigators, coupled with nearly constant funding, has resulted in partial funding for some ocean researchers. Third, the resolution requirements of oceanographic models and the complexity of model physics have always outstripped the largest computational capability anywhere. As understanding of the ocean becomes more sophisticated, more sophisticated models are required. The nurture of computational capability is reflected across the disciplines. Fourth, the understanding of the ocean and of the problems of oceanographers has progressed so much in the past several decades that all disciplines are now capable of new accomplishments in a seemingly endless number of areas. The problem is that the potential far exceeds the resources likely to be available, and the difficult task of setting priorities within and across disciplines will be amplified. The foundation of knowledge about the ocean that is now used in policy decisions was gained largely through Office of Naval Research (ONR) and NSF investments in basic research over the past several decades. Yet the demand for quick answers to purely practical questions sometimes obscures the need for investing in basic science, which remains the key to long-term practical applications. Under pressure to provide immediate solutions, mission agencies may be tempted to focus only on the short term. One example of the importance of basic research is a 1961 study that is now contributing to the debate about climate change—the question of whether ocean circulation has two stable states. Both the geological record and numerical models suggest that, at some times
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Oceanography in the Next Decade: Building New Partnerships in the past, ocean circulation was unlike today's and that it could switch rapidly from its present state to a radically different one. When the ocean was in this alternate state, Earth's climate was not at all like today's. This idea dates back to a paper written by Henry Stommel (1961), an academic scientist driven primarily by his own curiosity and supported by ONR and NSF. The paper had little impact for more than 20 years. Now regarded as seminal, it illustrates the need to sustain basic science so that future generations will have a knowledge base from which to develop their policy decisions. The authors of the following sections were asked to discuss the dominant issues of their disciplines and to lay out the grand themes, providing a scientific underpinning to discussion of the new partnerships. Ten years is probably the outer limit of an attempt to suggest what the major science themes will be. A decadal report written in 1960 would almost surely have missed the revolution in plate tectonics and thus would have been hopelessly wrong in its discussion of some dominant scientific themes in 1970. On the other hand, such a report could have captured accurately the methodologies of work at sea and the human resource requirements. Of course, the central questions of the field did not change either—although an intellectual revolution in the way they could be discussed occurred. The decision to organize this chapter according to traditional oceanographic disciplines was not arbitrary (the coastal ocean is a special case, discussed below). Anyone who reads each section will perceive exciting and important scientific problems that cut across many or even all disciplines. Examples are the growing importance of paleoceanographic studies that involve geology, geophysics, chemistry, biology, and physical oceanography because of their climate implications. Likewise, the study of ridge crests cuts across geology and geophysics, biology and chemistry, and even slightly, physical oceanography. Nonetheless, the board believes that there is a danger in declaring such interdisciplinary studies as the likely focus of future marine science efforts. Without denigrating the science done on such problems, interdisciplinary studies clearly build on the foundations of chemistry, physics, geology, geophysics, and biology. These, in turn, depend directly on their nonmarine counterparts of physics, mathematics, numerical methods, and other fields that provide the intellectual fertilization of marine studies. The history of ocean sciences suggests that one cannot have good interdisciplinary science without good disciplinary foundations, and it is essential that the traditional
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Oceanography in the Next Decade: Building New Partnerships oceanographic disciplines retain their identity and vitality. One needs to encourage scientists working on interdisciplinary problems, but they must first be expert in one or more of the basic disciplines. Just how such fostering should take place is the subject of debate, and the reader will detect a degree of disagreement as to how we should move forward. Working out various combinations of scientists and institutions is a major challenge for our academic institutions in the next decade. The board makes no specific recommendation except to note that the strength of the U.S. scientific community is its ability to tolerate and encourage great diversity in its institutions. Because the following sections were written by a number of different authors, they differ in style and content. The sections are not meant to be all inclusive but instead to provide a flavor of the excitement of each discipline of oceanography. The treatment here of coastal oceanography is anomalous because it deals with a geographic region—that is, shorelines, estuaries, bays, and the continental shelf—and not a discipline. The large percentage of the U.S. and world population that lives in the coastal zone, and the multiple human uses and impacts on the coastal ocean, place this area of oceanography much more conspicuously and immediately in the public policy arena. Unlike the participants in deep-water marine science, states, cities, and private enterprise are all prominent players in understanding and using the coastal ocean. The interplay of the basic sciences of fluid flow, chemistry, biology, shoreline physics, and geology with public policy concerns leads to a near-term urgency that cuts across scientific disciplines. However, it is important to recognize, as this report does, that the foundations of understanding must rest firmly on the underlying basic sciences. DIRECTIONS FOR PHYSICAL OCEANOGRAPHY Summary The great volume of water in the ocean exerts a powerful influence on the Earth's climate by absorbing, storing, transporting, and releasing heat, water, and trace gases. The goal of physical oceanography is to develop a quantitative understanding of the ocean's physical processes, including circulation, mixing, waves, and fluxes of energy, momentum, and chemical substances within the ocean and across its boundaries. Addressing such problems will require sustained large-scale observations of the world ocean
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Oceanography in the Next Decade: Building New Partnerships aided by advances in measurement and computational technology. Designing and deploying a global ocean observing system are among the most important and difficult tasks for physical oceanography and climate studies for the next decade. Such a system would incorporate existing measurement programs as well as observations that are not yet routine. Several topics will dominate physical oceanographic research in the coming decade. Research in modeling, ocean mixing, thermohaline circulation, and water mass formation processes will be important. To achieve their scientific objectives and to make more complete ocean observations, physical oceanographers must use both proven methods and new technologies, including acoustic techniques; measurements made from volunteer observing ships; satellite observations and data relay; and measurements of the distributions of trace chemicals. Introduction The ocean consists of nearly 1.4 billion cubic kilometers of salty water, about 97 percent of the free water on Earth. In comparison, the atmosphere holds only about 0.001 percent. This volume of water exerts a powerful influence on Earth's climate by transporting heat, water, and other climate-relevant properties around the globe and by exchanging these properties, as well as greenhouse gases (e.g., carbon dioxide, methane, and chlorofluorocarbons), with the atmosphere. Net ocean absorption of greenhouse gases and some greenhouse-induced heat from the atmosphere can delay greenhouse warming of the atmosphere. Predicting future climate conditions depends on learning what controls ocean circulation and water mass formation, and whether the system is predictable, even in principle. Physical oceanography, like many fields of science, consists of theory, observations, and numerical models. Physical oceanographic theories use the equations of fluid dynamics, modified to account for Earth's rotation and shape (e.g., O'Brien, 1985). A goal of physical oceanography is to develop a quantitative understanding of the ocean circulation, including fluxes—of energy, momentum, and chemical substances—within the ocean and across its boundaries. Physical oceanographers must contribute to the increasing societal emphasis on measuring, predicting, and planning for changes in global climate by improving understanding of the physical factors that maintain the overall physical, chemical, and biological characteristics of the ocean. Advances in measurement and com-
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Oceanography in the Next Decade: Building New Partnerships putational technology will continue to contribute to advances in physical oceanography. Studies of climate change put the skills of oceanographers to a severe test. The time scales are long: interannual, decadal, and beyond. Physical processes are three dimensional and involve interaction of the ocean with the atmosphere. Winds transfer momentum and promote mixing and evaporation. Atmospheric temperature influences the density of ocean surface layers through effects on seawater temperature and salinity (through ice formation and melting), which in turn modify the atmosphere. Development of the physical state of the ocean is difficult to model because it involves the complex interaction of processes that operate on vastly different time and space scales. Nonetheless, progress is being made. Techniques that will permit better and more frequent observations are being developed, and advances in numerical modeling will soon permit representation of the major components of ocean circulation. Ocean observations reflect the state of the ocean and hence the forces acting on it. Because observations are made in a corrosive, turbulent environment with high pressures at depth, they are difficult and expensive to obtain. Because of the size and variability of the ocean, measurements are always incomplete in space and time. Yet understanding the ocean depends on adequate measurements, and to make them we need to use technologies that permit a view of the global ocean. Technologies based on acoustics, space-based remote sensing, and underway automatic measurements could all be applied to global-scale observations. Predictions of the ocean can be carried out only when the initial and boundary conditions are provided from observations with an accuracy and precision consistent with the physics present. Because oceanic observations are so expensive, models and theories must be used to help determine the most cost-effective measurements and measurement systems. Global Ocean Observing System Physical oceanographic observations and modeling are becoming global, but the resources required to deploy and sustain large-scale observations of the world ocean are enormous. The exact configuration of a global ocean observing system is unknown, but it would probably include existing observations from satellites, moored open ocean sensors, volunteer observing ships, and the global sea-level network, as well as other observations that are
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Oceanography in the Next Decade: Building New Partnerships not yet defined or collected routinely. The scientific and technological results from several ongoing large-scale research programs—the Tropical Ocean-Global Atmosphere program, the World Ocean Circulation Experiment (WOCE), and the Joint Global Ocean Flux Study—should be used to design an operational observation system that is effective, affordable, and consistent with our knowledge of the scales of ocean biology, chemistry, and physics. It would be the largest field enterprise ever undertaken by the oceanographic community, and it must have an international and multidisciplinary scope well beyond previous experience. The design and implementation of a global ocean observing system (GOOS) must involve ocean scientists substantially because the design is extremely important to the science itself and depends on firm scientific understanding. Designing and deploying a GOOS is one of the most important and difficult tasks for physical oceanography and climate studies in the next decade. The United States should take a major leadership role in both the research and the operations. Because of the present paucity of ocean data, numerical models are important in the development of a GOOS. Models will be used to interpret available data for testing possible system designs and, ultimately, to interpret the data from such a system. Major Research Topics for the Coming Decade Several topics will dominate physical oceanographic research in the coming decade. The list is incomplete; the topics mentioned received some emphasis during the Ocean Studies Board workshops as representing key research issues and include the following: research in modeling; ocean mixing, including interior mixing and the surface mixed layer; thermohaline circulation; and heat and freshwater fluxes. Ocean Modeling The central focus of numerical modeling of the ocean has been, and continues to be, directed toward fluid dynamics, but the models have importance far beyond physical oceanography. For example, communities of organisms in the upper ocean live in a delicate balance, depending on the stability of the water column, its mixing rates, and its large-scale vertical and horizontal fluid movements. Our limited ability to predict the movements of the upper ocean limits understanding of basic biological processes.
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Oceanography in the Next Decade: Building New Partnerships The transfer of gases between atmosphere and ocean is central to the carbon cycle; this transfer relies on many scales of circulation and mixing. Combined atmosphere-ocean simulations at interannual time scales require more accurate ocean models. The present generation of coupled atmosphere-ocean models exhibits unacceptable drifts (Manabe and Stouffer, 1988). Climate forecast models must be free of even small systemic errors that accumulate over long simulated periods, hiding the signals that are sought. To understand and mimic the paleoceanographic record, a major test of global models, one must be able to carry model integrations over time scales corresponding to thousands or tens of thousands of years. It is not clear that ocean and atmosphere behavior is predictable on scales of decades or longer. The limits of predictability are being explored as a research topic. The global ocean is so large and its circulation occurs over such a variety of space (tens to thousands of kilometers) and time (days to centuries) scales that ocean circulation modeling has always overwhelmed even the largest supercomputers. This situation will probably remain for some decades to come. Thus it is a major intellectual challenge to design models of ocean circulation with time and space increments small enough to model processes adequately, given foreseeable limitations in computing resources. Prominent features and processes that must be incorporated more accurately into physical oceanographic models (in a manner consistent with observations) include the effects of complex bottom topography on deep-water masses, deep vertical and horizontal mixing, eddies and fronts in the upper ocean, the interaction of water flow and diffusion of a variety of properties, boundary effects at the seafloor and surface, and the dynamics of shallow and deep boundary currents. Ocean Mixing Interior Mixing Large-scale ocean circulation is coupled with, and partially controlled by, small-scale mixing processes. Understanding the places, rates, and mechanisms by which the ocean mixes heat, salt, and momentum is crucial to understanding the circulation of the largest scales and essential to any capability to predict future oceanic states. It is intimidating to realize that to understand the dynamics of large-scale circulation and convective water mass formation, we must also understand the physics acting on the smallest scales (centimeters and millimeters). Heat-
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Oceanography in the Next Decade: Building New Partnerships ing, cooling, flow, and mixing processes act together to determine physical properties of the ocean. Changes in any one of these processes can affect the global climate system. Significant progress in the observation of ocean mixing processes and in the interpretation of these observations has been made, but understanding remains inadequate. The capability to compare direct mixing measurements (through microstructure and purposeful tracer releases) with natural mixing (estimated indirectly from natural tracer distributions) is rapidly accelerating our understanding of mixing processes (Watson and Ledwell, 1988). The results of such comparisons will direct future research. For example, if deep-sea observations confirm that mixing rates are lower than predicted, attention will focus on mixing processes in the benthic boundary layer and continental slopes. If tracer studies indicate significantly more mixing than is seen by direct measurement, double diffusive and other mechanisms, will be explored. With new observational techniques and a clear measurement strategy, significant progress can be expected in the coming decade in the study of ocean mixing. Surface Mixed Layer The primary production that supports the entire marine food web occurs in the upper sunlit portion of the ocean (the euphotic zone), where photosynthesis occurs. Our growing concern for climate variation makes understanding the uptake of carbon dioxide related to photosynthesis of particular importance. Fortunately, several developments over the last few years in biological oceanography, marine chemistry, and ocean physics promise advances in the study of the biological-chemical cycles in the euphotic zone. Exploitation of new techniques could significantly improve our ability to predict various aspects of global environmental change, including the ocean's role in sequestering carbon dioxide. The topmost layer of the ocean is called the ''mixed layer" because the waters are mixed by wind, waves, and currents. This layer is often nearly homogeneous in temperature and chemical characteristics, and is bounded by the sea surface and a layer of denser water. The transfer of gases between atmosphere and ocean depends primarily on mixed-layer processes. Understanding the physics of the ocean surface mixed layer, and its coupling with the ocean interior and the atmosphere, is essential if the combined biogeochemical systems of ocean and atmosphere are to be represented correctly in ocean models. Mixed-layer studies are among the endeavors of physical oceanography in which strong
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Oceanography in the Next Decade: Building New Partnerships Processes The lateral boundaries and shelf-slope topography that characterize continental margins substantially determine the nature of coastal currents. For example, on a rotating planet, nearly steady currents are constrained from crossing isobaths (lines of constant depth). As a result, flow in the coastal ocean tends to parallel the coast, and exchange between waters over the continental shelf and the adjacent deep ocean is inhibited. Thus in many cases, distinct shelf water masses form, and the shelf represents a partially closed chemical and biological system. Fronts often mark the boundaries between these coastal and oceanic systems, and these fronts have their own important biological and atmospheric effects. Wind-driven currents over continental shelves tend to be particularly energetic because the coastline interrupts water transport in the turbulent layer in the upper ocean. This interruption leads to a connection between surface winds and currents deeper in the water column. The resulting currents flowing alongshore below the turbulent surface layer dominate variability in most places over the continental shelves. Wind-driven currents are understood well enough that models are able to predict the speed and direction of coastal currents, as shown by the close agreement between observed and predicted currents shown in Figure 3-4. Of broader importance to coastal ecosystems is the related onshore-offshore circulation, including the coastal upwelling of cold, nutrient-rich subsurface waters. Their temperature leads to the unusually cool, stable atmospheric conditions that characterize the U.S. West Coast during spring and summer. The upwelled nutrients fuel marine plant growth, leading to high biomass throughout the food web and some of the world's greatest fisheries, including those off the West Coast and off the coast of Peru. Upwelling can also intensify the transfer of organic materials from the surface to the seafloor in such areas. For example, off Peru, as much as one-half of the carbon fixed by phytoplankton production induced by upwelling may be deposited on the bottom. Upwelling in the coastal ocean can also be caused by factors other than wind. For example, upwelling of nutrient-rich water along the inshore edge of the Gulf Stream does much to stimulate productivity off the southeastern coast of the United States, as determined by chlorophyll measurements (Figure 3-5). Whatever its cause, upwelling contributes to the well-known high biological productivity of the coastal ocean (Figure 3-6). Estuaries and coastal embayments, on
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Oceanography in the Next Decade: Building New Partnerships FIGURE 3-4 Observed (solid line) and modeled (dashed line) alongshore currents from over the continental shelf off Oregon, summer 1978. Positive velocity denotes northward flow. After Battisti and Hickey (1984). the other hand, owe their high biological productivity to nutrient inputs from the land and density-driven internal circulation that serves to retain and enhance the recycling of these nutrients. Sea ice is important in controlling air-sea fluxes in coastal regions when it forms there. Ice cover decreases heat, moisture, and gas fluxes and modifies momentum fluxes. During the formation of ice, salt is excluded, creating saltier adjacent water. These dense water masses can sink, impacting an entire basin through thermohaline circulation (see ''Directions for Physical Oceanography"). Freshwater generated by ice melting stabilizes the water column, thus helping to prompt the spring phytoplankton bloom. Tidal currents are sometimes enhanced over the continental shelves by physical resonances taking place in bays, such as in the well-known Gulf of Maine-Bay of Fundy example. Strong tidal currents intensify near-bottom mixing that can extend to the sea surface in shallow regions such as Georges Bank. This mixing and the resulting circulation enhance nutrient availability in the upper ocean, cause high primary productivity, enrich fisheries, and increase the transfer of organic material to underlying sediments. Energetic tidal currents can reinforce the many physical processes (including waves and wind-driven currents) that increase sediment resuspension and transport as well as the transport of chemicals that adhere to the particles. Continental shelves are the transition zone between the land and the ocean and are thus particularly important in processes involving sediment and chemical fluxes. Freshwater outflows propel currents with distinct properties. Sediments from the land are
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Oceanography in the Next Decade: Building New Partnerships FIGURE 3-5 Upper plate: Schematic of a Gulf Stream frontal eddy (shingle) off the U.S. southeast coast south of Cape Hatteras. Upwelling of nutrient-rich-waters occurs in the cold core of the eddy. Lower plate: Cruise track, surface temperature, chlorophyll a, and salinity across a Gulf Stream frontal eddy off the southwest U.S. coast on April 20–22, 1979. The dashed line in the temperature frame indicates Gulf Stream surface thermal front as determined by satellite-derived frontal analysis on April 20. Note that highest chlorophyll values (>4 mg/m3) are located within upwelled cold core of the eddy. Figures from Lee et al. (1991) and Yoder et al. (1981).
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Oceanography in the Next Decade: Building New Partnerships
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Oceanography in the Next Decade: Building New Partnerships FIGURE 3-6 World ocean primary production according to Koblents-Mishke and coworkers on an equal area projection. Productivity categories are, from low to high, <36, 36–54, 54–90, 90–180, >180 gC/m2/yr. Note that most of the areas of high productivity are located on the ocean margins. From Berger (1988). often deposited on the continental shelves, although they are sometimes transported to the slopes and deep ocean later. Sedimentary conditions on the shelf are far from static: numerous physical and biological processes can lead to reworking of the sediments and to their eventual transport to other locations. New evidence suggests that the shelf can be a source of particulates that accumulate within estuaries together with sediments delivered to the estuaries by rivers and shoreline erosion. Over geological time scales, the fates of sediments can vary widely with sea level; shelf processes can differ markedly, depending on how much of the shelf (or slope) is exposed above the sea surface. Coastal waters also receive chemicals and particulates weathered from continental rocks and transported to the ocean by rivers, groundwater, and winds. When these chemicals reach the coastal ocean, they are transformed or removed, so that although the properties of the estuarine waters may differ from those of the open ocean, shelf waters closely resemble open ocean water.
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Oceanography in the Next Decade: Building New Partnerships Physical processes on the scale of millimeters to kilometers have a major impact on the behavioral responses, feeding rates, interactions, and distributions of plankton, fish, and benthic invertebrates in the coastal ocean. For example, coastal fronts, island wakes, tidal flows, and vertical circulation cells are only a few of the many types of physical phenomena that can aggregate organisms or alter their behavior. Moreover, turbulence and small eddies on the scale of millimeters to meters partially determine the encounter rates of herbivores feeding on passive phytoplankton and bacteria and of predatory interactions among smaller pelagic organisms. An understanding of the effects of water movements on the behavior and distribution of organisms in the ocean will be one of the most challenging aspects of future research, particularly in coastal areas, where both physical processes and organisms are especially diverse and numerous.
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Oceanography in the Next Decade: Building New Partnerships Future Directions The worldwide coastal ocean exhibits vast geographical diversity, depending on the size and openness of bays and estuaries; the width of the continental shelf; the proximity of strong oceanic currents; the strength of tides, winds, river runoff, and surface heat fluxes; and other characteristics. It is clearly impractical to explore fully the biological, chemical, geological, meteorological, and physical structure and variability of every estuary or shelf region of the United States, let alone of the world. One way to proceed is to identify the most significant physical-meteorological processes that to some extent act on all the world shelves and coastal waters. Each physical process and its effects on the biology, chemistry, and geology of the local area could then be studied in a prototypical environment (not limited to U.S. waters) where the process tends to predominate. The results of such interdisciplinary studies could be used to improve our modeling capabilities, enhancing our ability to model more typical shelves or estuaries where a combination of processes interacts. Although this approach is not a panacea, it can at least define the information needed to gain a desired level of understanding of a given coastal region. Within this broad approach to the coastal ocean, a number of important themes will be common to any detailed study of processes. Air-Sea Interactions The atmosphere is a major driving force of coastal ocean processes, through both its role in driving currents and its direct and indirect controls on biological and chemical processes. For example, wind-driven coastal upwelling can provide nutrients to the euphotic zone, leading to enhanced primary productivity, and atmospherically generated turbulence can increase predator-prey encounters among plankton (Rothschild and Osborn, 1988). Each of these biological processes results in distinct chemical transformations as well. Present knowledge of atmospheric effects on the coastal ocean is limited to the effects of large-scale (500-kilometer) atmospheric features. This knowledge is useful for predicting alongshore currents or estimating the transport of dust particles from land to ocean (eolian deposition). Smaller scales in the wind field seem to be more important in determining cross-shelf currents; yet small-scale coastal winds are poorly observed and understood. Interaction of the atmosphere with the coastal ocean on these important scales of tens to hundreds of kilometers is not well-understood.
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Oceanography in the Next Decade: Building New Partnerships Air-sea fluxes of momentum and heat, for example, are not adequately characterized in present models, which do not take into account small-scale variability, directional offsets between the wind and waves, limited fetch, and limited water depth (which characterize the coastal environment; Geernaert, 1990). In addition, thermal fronts, which occur throughout the coastal ocean, greatly perturb the atmospheric layer directly above the sea surface and sometimes perturb weather systems. Further, the coastal topography helps to generate small-scale disturbances in the surface winds that can affect currents over the shelf. Air-sea fluxes of particles and chemicals, known to be important, must be a significant part of any study. Until we can quantify the air-sea momentum, heat, and chemical fluxes in this complex environment, we cannot understand the coastal ocean system as a whole. Air-sea exchange is complex, but answers to the questions must be found. The atmosphere is the basic driving force of many coastal ocean processes. Ocean fluxes, especially heat fluxes, are critical to properties of the atmosphere. Air-sea exchanges that govern the effects of ocean and atmosphere on each other need to be quantified. Cross-Margin Transport The interaction of currents with bottom topography tends to isolate continental shelves from the rest of the ocean, although the strength of this isolation is significantly modulated by other processes. Even when the isolation is especially strong, shelf waters resemble the open ocean more than they resemble estuaries. It is difficult to identify which processes determine the cross-margin fluxes of water, particulates, chemicals, and organisms within estuaries, between estuaries and the shelf, on the shelf, and at the shelf-ocean boundary. The relative importance of such factors as wind-driven motions, frontal instabilities, turbulent boundary-layer transports, exchanges through submarine canyons, and the sinking of dense waters has not been evaluated. The difficulty is ultimately their episodic nature in terms of both location and time. Each has distinct effects on biological, chemical, and geological processes, so that interest in them is not limited to physical oceanographers. Information on cross-margin transport is critical to all subdisciplines of coastal ocean science. Alongshore gradients of most characteristics tend to be small relative to cross-shelf gradients, and alongshore currents are relatively well understood. It is cross-shelf transport, or its absence, that shapes many distributions,
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Oceanography in the Next Decade: Building New Partnerships such as those of sediments, that are of scientific interest. Estuarine and cross-shelf exchange is also of interest from a societal standpoint, for example, in determining the fate of riverine inputs of excess nutrients or pollutants. Thus it seems likely that estuarine and cross-shelf exchanges will be a central focus of future efforts in coastal ocean science. Carbon Cycles An important and controversial question in oceanography is, What is the role of the coastal ocean in global cycles of carbon, oxygen, nitrogen, and other significant elements? The coastal ocean occupies approximately 20 percent of total ocean area, yet accounts for approximately 50 percent of ocean primary production and approximately 50 percent of global ocean nitrate assimilation by phytoplankton (Walsh, 1991). Describing the mechanisms controlling cycling rates of essential elements has taken on new urgency because of the recently recognized potential for human alteration of global chemical cycles. Biological processes mediate the cycling of many elements and control the fate of numerous materials that enter the ocean. Constructing accurate models of biological controls and predicting their effect on the fate and transformation of dissolved substances and particles in the ocean are severely limited by our lack of understanding of the structure and function of marine ecosystems and their responses to physical and chemical processes. Elucidating these mechanisms is critical to understanding the coastal ocean because of its generally high productivity (and thus its processing capability), its substantial biological variability in space and time, and its role as a conduit between the continents and the deep ocean basins. A major uncertainty in models of global change, including climate change, is the role of biological processes in mediating and controlling geochemical cycling of important elements. Most scientists agree that biological processes play a key role in the ocean carbon cycle and the cycle of nitrogen, oxygen, and related elements. However, the possible role of marine plants as a sink for carbon dioxide from human activities is highly controversial, and no generally acceptable model has been proposed to explain how the transfer of carbon from the ocean surface to the seafloor (the biological pump) should be working significantly faster now than before the Industrial Revolution. This is an important issue to be considered during the next decade. Understanding ocean margin food webs is of particular interest because they can be altered by eutrophication and other human activities.
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Oceanography in the Next Decade: Building New Partnerships In the ocean, the amount of organic material transferred vertically from the surface to the bottom and horizontally from estuaries to shelf waters to the deep ocean is not a simple linear function of primary production; nor are burial rates of organic matter in ocean sediments. The amount of material transported depends on the physical and chemical characteristics of the environment (e.g., rates and mechanisms of nutrient delivery) and on various largely unappreciated characteristics of the species composition and structure of marine food webs in the euphotic zone, deeper in the water column, and in and around the seafloor. Some biogeochemical cycling processes are summarized in Figure 3-7. Particle Dynamics Research in several areas needs to be initiated to improve our basic understanding of particle dynamics. Some of these areas have been mentioned, for example, the cross-shelf transport mechanisms and the use of narrow coastal margins with significant sediment inputs to model transport conditions during past times of lower sea level. Among other research possibilities is the need to test the wide range of theoretical models for sediment transport that evolved in the past two decades. For example, models have been developed to describe the coupling between slowly varying currents and surface gravity waves and to predict resulting sediment transport. FIGURE 3-7 Schematic of some processes relevant to biogeochemical cycling on the inner continental margin; (DOM = dissolved organic material; DIN = dissolved inorganic nitrogen; DON = dissolved organic nitrogen; POM = particulate organic material). From Mantoura et al. (1991).
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Oceanography in the Next Decade: Building New Partnerships However, these models have received little or no testing in the laboratory or through field observations. Muddy sediments are geologically relevant, and some research has been conducted on the transport of low concentrations of fine-grained sediments. Dense concentrations [>10 grams per liter (i.e., fluid muds)] are also observed in the marine environment, and their transport is poorly understood. Carbonate sediments are widespread at low latitudes, but the effects of physical processes on their dispersal have not been thoroughly studied. Differences in the particle shapes and densities of carbonate sediments from more common sources make it difficult to extrapolate existing theory of sediment transport. Theory regarding the formation of sediment layering within the seabed and its dependence on sediment transport and biological activity has evolved rapidly. Additional laboratory and field documentation is needed to link formative mechanisms and preserved strata. The overall importance of the coastal ocean extends far beyond its relatively small areal extent. An environment of remarkably high biological productivity, this transition zone between land and open ocean is of considerable importance for recreation, waste disposal, and mineral exploitation. Such societal issues as pollution (in its many forms), bioremediation, waste disposal, and risk assessment cannot be addressed adequately until we make substantial advances in our basic understanding of the coastal ocean. A holistic approach to the coastal ocean system, blending marine meteorology with biological, chemical, geological, and physical oceanography, should enable us to progress sufficiently so that we will be better prepared to make the technical and policy decisions facing us over the next decades. Four issues of particular importance are air-sea interactions, cross-margin transport, carbon cycles, and particle dynamics. A balanced program would include studies focused on specific processes, long-term measurements, modeling, and instrumentation development. To take best advantage of the results of these studies, strong working relationships with the applied science communities need to be forged. Coastal measurements will be an important part of a global ocean observing system because it is at the coasts that most countries, particularly developing nations, will make most of their measurements. Therefore, it is essential that the design of a GOOS include coastal measurements as a critical element of the system.
Representative terms from entire chapter: