4

Sea-Level Variability and Change off the
California, Oregon, and Washington Coasts

The waters of the world’s oceans are subject to a variety of forces that create regional and local variations in sea level. Winds and currents move water laterally in the ocean, creating anomalous spatial patterns of sea level that can persist for a decade or longer. The high winds and low atmospheric pressures associated with El Niños and other climate patterns can significantly elevate sea level along the west coast of the United States for intervals of several months, as well as generate damaging high waves and storm surges. Melting of glaciers and ice sheets adds new water to the oceans and the associated gravitational and deformational effects distribute it nonuniformly, raising sea level in some areas and lowering it in other areas. Geologic processes (e.g., tectonics, compaction) and human activities (e.g., withdrawal of groundwater) also raise or lower the coastal land surface, increasing variability in relative (or local) sea-level rise.

This chapter evaluates the current contributions of ocean circulation, short-term climate patterns and storms, modern land ice change, and vertical land motion to sea-level rise in California, Oregon, and Washington. The discussion draws largely from published studies on the variability of sea level in this region, although the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report also summarizes research results on ocean circulation and short-period climate changes in the northeast Pacific Ocean. This chapter concludes with the results of the committee’s analysis of tide gage records along the west coast of the United States.

CHANGES IN OCEAN CIRCULATION

Satellite altimetry data provide unambiguous evidence of significant regional differences in sea-level change in the oceans (Bindoff et al., 2007; Milne et al., 2009; Appendix B). Spatial variability in the North Pacific Ocean is associated with climate patterns— primarily the El Niño-Southern Oscillation (ENSO) but also the Pacific Decadal Oscillation (PDO; Box 4.1)—which affect ocean surface heating, surface air pressure, and wind patterns, and thus change ocean circulation (e.g., Mantua and Hare, 2002; Bond et al., 2003; Cummins and Freeland, 2007). Changes in ocean circulation change sea levels on seasonal to multidecadal timescales by redistributing mass and altering temperature and salinity in the upper ocean.

Estimates from the IPCC Fourth Assessment Report

Satellite altimetry records assessed by the IPCC showed that sea level fell about 0–6 mm yr-1 from 1993 to 2003 along the U.S. west coast and rose by 6 mm yr-1 to ~12 mm yr-1 in the tropical western Pacific Ocean (Bindoff et al., 2007). Temperature data from the upper 700 m of the ocean showed a similar sea-level pattern for the same period, indicating that regional sea level is influenced by changes in the thermal structure of the upper ocean, which are associated with changes in ocean circulation and surface heating. The IPCC (2007) suggested that the largest fraction of this short-term variation was caused by ENSO. Over longer periods, however, the thermosteric sea-level pattern along the U.S. west coast was different, showing a rise



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4 Sea-Level Variability and Change off the California, Oregon, and Washington Coasts T he waters of the world's oceans are subject to a CHANGES IN OCEAN CIRCULATION variety of forces that create regional and local variations in sea level. Winds and currents Satellite altimetry data provide unambiguous move water laterally in the ocean, creating anoma- evidence of significant regional differences in sea- lous spatial patterns of sea level that can persist for a level change in the oceans (Bindoff et al., 2007; decade or longer. The high winds and low atmospheric Milne et al., 2009; Appendix B). Spatial variability pressures associated with El Nios and other climate in the North Pacific Ocean is associated with climate patterns can significantly elevate sea level along the patterns-- primarily the El Nio-Southern Oscilla- west coast of the United States for intervals of several tion (ENSO) but also the Pacific Decadal Oscillation months, as well as generate damaging high waves and (PDO; Box 4.1)--which affect ocean surface heating, storm surges. Melting of glaciers and ice sheets adds surface air pressure, and wind patterns, and thus change new water to the oceans and the associated gravitational ocean circulation (e.g., Mantua and Hare, 2002; Bond and deformational effects distribute it nonuniformly, et al., 2003; Cummins and Freeland, 2007). Changes raising sea level in some areas and lowering it in other in ocean circulation change sea levels on seasonal to areas. Geologic processes (e.g., tectonics, compaction) multidecadal timescales by redistributing mass and and human activities (e.g., withdrawal of groundwater) altering temperature and salinity in the upper ocean. also raise or lower the coastal land surface, increasing variability in relative (or local) sea-level rise. Estimates from the IPCC Fourth Assessment Report This chapter evaluates the current contributions of ocean circulation, short-term climate patterns Satellite altimetry records assessed by the IPCC and storms, modern land ice change, and vertical showed that sea level fell about 06 mm yr-1 from 1993 land motion to sea-level rise in California, Oregon, to 2003 along the U.S. west coast and rose by 6 mm yr-1 and Washington. The discussion draws largely from to ~12 mm yr-1 in the tropical western Pacific Ocean published studies on the variability of sea level in this (Bindoff et al., 2007). Temperature data from the upper region, although the Intergovernmental Panel on Cli- 700 m of the ocean showed a similar sea-level pattern mate Change (IPCC) Fourth Assessment Report also for the same period, indicating that regional sea level summarizes research results on ocean circulation and is influenced by changes in the thermal structure of short-period climate changes in the northeast Pacific the upper ocean, which are associated with changes Ocean. This chapter concludes with the results of the in ocean circulation and surface heating. The IPCC committee's analysis of tide gage records along the west (2007) suggested that the largest fraction of this short- coast of the United States. term variation was caused by ENSO. Over longer periods, however, the thermosteric sea-level pattern along the U.S. west coast was different, showing a rise 55

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56 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON BOX 4.1 Pacific Ocean Climate Patterns ENSO. The El Nio-Southern Oscillation is a quasi-periodic climate PDO. The Pacific Decadal Oscillation is often described as a long-lived pattern that occurs across the tropical Pacific Ocean about every 2 to (i.e., decadal) El Nio-like pattern of Pacific climate variability. Like 7 years. It is characterized by variations in the sea-surface temperature ENSO, the PDO has warm and cool phases, as defined by patterns of of the tropical eastern Pacific Ocean. In the warm El Nio phase, warm ocean temperatures in the northeast and tropical Pacific Ocean (Figure). ocean temperatures in the tropical eastern Pacific are accompanied by high air surface pressures in the tropical western Pacific (Figure). In the cool La Nia phase, the pattern is reversed. The reversal in surface air pressure between the eastern and western tropical Pacific is known as the Southern Oscillation. FIGURE(Top) Sea-surface temperature anomalies (shad- ing) and sea-level pressure (contours) associated with the FIGUREThe Pacific Decadal Oscillation. (Top) Typical warm phase of ENSO (i.e., El Nio) for the 19001992 winter patterns of sea surface temperature (colors), sea-level period. Positive contours are dashed and negative con- pressure (contours), and surface wind stress (arrows) during tours are solid. (Bottom) Multivariate ENSO index for positive (warm) and negative (cool) phases of PDO. Tem- 19502009. The index is based on variables observed over perature anomalies are in degrees Celsius. (Bottom) History the tropical Pacific, including sea-level pressure, surface of the PDO index (the principal component of monthly sea wind, sea surface temperature, surface air temperature, surface temperature anomalies in the North Pacific Ocean and cloudiness. Positive (red) index values indicate El Nio poleward of 20N) from 1900 to 2010. SOURCE: Figure events and negative (blue) values indicate La Nia events. obtained with permission granted by Nate Mantua at the SOURCE: Figure and details on how the index is computed University of Washington's Joint Institute for the Study of are given in . Atmosphere and Ocean.

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 57 FIGURE 4.1 Trend of thermosteric sea level (mm yr-1) for 19932009 (left) and 19612008 (right), based on an updated version of data from Ishii and Kimoto (2009). SOURCE: Courtesy of Masayoshi Ishii, Japan Meteorological Research Institute. in sea level of about 00.8 mm yr-1 from 1955 to 2003, off the west coast of the United States during El Nio rather than a fall (Bindoff et al., 2007). This difference events and falls during La Nia events. El Nios dif- suggests that the spatial pattern of sea level varies on fer in magnitude and large-scale form (Barnard et al., decadal and longer timescales. 2011) but commonly produce an active winter storm season in the northeast Pacific. The associated winds Recent Advances and ocean circulation changes may elevate sea level by 1030 cm for several months along the west coast Changes in wind-driven ocean circulation can play (Chelton and Davis, 1982; Flick, 1998; Bromirski et an important role in determining patterns of sea-level al., 2003; Allan and Komar, 2006; Komar et al., 2011). change in the northeast Pacific Ocean on seasonal to In fact, the highest sea levels recorded along the west decadal and longer timescales (e.g., Timmermann et al., coast were usually associated with El Nio events 2010; Bromirski et al., 2011; Merrifield, 2011; Sturges (e.g., Figure 4.2). For example, on January 27, 1983, and Douglas, 2011). Recent studies show a decrease during one of the largest El Nios in half a century, in the rate of sea-level rise along the west coast of the seven tide gages along the west coast (San Diego, Los United States since 1993, which is consistent with Angeles, Monterey, Crescent City, Charleston, Astoria, IPCC (2007) findings, but no statistically significant and Seattle) recorded their highest water levels.1 Peak trends appear in tide gage records (Bromirski et al., sea level was 24 cm above predicted in San Diego 2011), satellite altimetry data, or in situ temperature (104 years of record), 31 cm above predicted in Los observations since 1980. For example, thermosteric sea- Angeles (87 years of record), and 76 cm above predicted level calculations show falling sea level off the U.S. west in Seattle (112 years of record). coast from 1993 to 2009 (Figure 4.1, left) and rising sea Large El Nio and La Nia events also can be seen level from 1961 to 2008 (Figure 4.1, right). Bromirski in satellite altimetry data. The top panels of Figure 4.3 et al. (2011) suggested that the flat sea-level trend since show the sea-level rise observed during the El Nio of 1980 and the decrease since 1993 are associated with 19971998 and the sea-level fall observed during the PDO phase changes. 1999 La Nia. The ENSO signal is strongly seasonal and reaches a peak amplitude in the Northern Hemi- Seasonal and Interannual Variability sphere winter. Figure 4.3c shows the ocean seasonal cycle, which is occasionally magnified by ENSO. Among all the climate modes, ENSO is the dominant cause of sea-level variability in the north- east Pacific Ocean on interannual timescales (e.g., 1See .

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58 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 4.2 San Francisco tide gage record showing relative sea-level increases during major El Nio events. SOURCE: Tide gage data from the Permanent Service for Mean Sea Level. FIGURE 4.3 (a) Sea-level anomaly (SLA), the difference between mean sea level for 19932009 and sea level during the December 1997 El Nio. (b) Same as (a) but for a La Nia event in February 1999. Color scale on right is in cm. (c) Time series of monthly SLA offshore San Diego, San Francisco, and Seattle. The two black arrows correspond to the dates shown in the upper figures. SOURCE: AVISO satellite altimetry data from .

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 59 Decadal and Longer Variability SHORT-TERM SEA-LEVEL RISE, STORM SURGES, AND SURFACE WAVES The low-frequency (decadal and longer) variability in sea level off the U.S. west coast often corresponds Any climate-induced increase in storm frequency to forcing by regional and basin-scale winds associ- and magnitude will induce short-term changes in sea ated with climate patterns such as the PDO and the level. This issue is critical to coastal planners because North Pacific Gyre Mode (e.g., Lagerloef, 1995; Fu storm surges and wind-driven waves are responsible for and Qiu, 2002; Jevrejeva et al., 2006; Cummins and most of the flooding and erosion damage along the west Freeland, 2007; Miller and Douglas, 2007; Di Lorenzo coast of the United States (Armstrong and Flick, 1989; et al., 2008, 2010; Bromirski et al., 2011; Sturges and Domurat and Shak, 1989; Allan and Komar, 2006). Douglas, 2011; Merrifield, 2011). For example, ocean The most severe coastal impacts tend to occur when modeling by Bromirski et al. (2011) found that surface a storm surge coincides with high tides and/or during heating alone produced falling sea level--the opposite periods of anomalously high sea level, such as those to that observed--whereas forcing by winds explained caused by El Nios. For example, the simultaneous the rise in sea level along the U.S. west coast since 1950. occurrence of anomalously high sea level, high waves They suggest that the lack of a significant trend in sea in late January and early March, and high astronomical level observed in tide gages since 1980 reflects forcing tides caused significant damage along the California by winds associated with phase changes of the PDO. coast during the El Nio winter of 1983 (Figure 4.4). Sea level rose when the PDO changed from negative The amplitude of local sea-level rise from storm and (cool) to positive (warm) around 19761977, and it fell wave events can greatly exceed the projected amplitude when the PDO changed from positive to negative at of global and regional sea-level rise, even beyond 2100, the end of the 1990s (see lower figure in Box 4.1). The so understanding their additive effects is crucial for PDO has largely been in a positive phase since 1977, coastal planning. This section describes the contribu- although negative phases have occurred almost a half- tions of these factors to short-term sea-level rise and a-dozen times since the 1990s. the extent to which they may be changing with climate ENSO may also play a significant role in decadal change (Task 2b). and longer sea-level variability (Newman et al., 2003). Indeed, ENSO and the PDO are not independent. Contributions of Tides, Storms, and El Nios to ENSO can influence the PDO (Newman et al., 2003; Local Sea Level Schneider and Cornuelle, 2005), and the PDO can modulate tropical Pacific circulation and ENSO (e.g., High tides along the U.S. west coast occur twice Vimont et al., 2009; Alexander et al., 2010). daily, often of uneven amplitude, caused predominately by the gravitational attraction of the Moon and the Sun Summary on the Earth. The Earth-Moon-Sun orbital geometry also results in heightened high tides twice monthly The spatial variability of sea level in the Pacific (spring tides, near the times of the full and new moon) Ocean is driven primarily by ENSO, which affects sea and every 4.4 years and 18.6 years (Zetler and Flick, level on seasonal to decadal timescales, and is also asso 1985). The largest tidal amplitudes of the year along ciated with phase changes in the PDO, which affects the coasts of California, Oregon, and Washington usu- sea level on decadal and longer timescales. Satellite ally occur in winter and in summer (Zetler and Flick altimetry, tide gage, and ocean temperature measure- 1985). Tides in the highest winter and summer months ments all indicate a long-term increase in sea level off are often more than 20 cm higher than tides in the the U.S. west coast, with large amplitude seasonal to spring and fall months.2 The peaks in the 4.4-year and multidecadal variability. The measurements show no 18.6-year cycles produce monthly high tides that are statistically significant sea-level trend since 1980, con- about 15 cm and 8 cm, respectively, higher than they sistent with the PDO phase changes. are in the intervening years (Flick, 2000). Flick et al. 2 See data compiled at .

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60 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON (a) (b) (c) (d) (e) FIGURE 4.4(a) Hourly sea-level pressure (SLP; mb), (b) sea-level anomaly (cm) above tide-predicted levels, (c) predicted and (d) observed sea level (cm) relative to a mean sea-level datum, and (e) significant wave height (Hs, the average height of the highest one- third of waves [m]) from a buoy sensor near San Francisco during the El Nio winter of 1983. SOURCE: Adapted from Flick (1998). (2003) reported increases in the range from high to low from long-term average levels, typically with greater astronomical tide over multiple decades at some, but pressure drops in Washington and Oregon than in not all, U.S. west coast tide gages. California. The drop in atmospheric surface pressure Storm surges are created when high winds, the raises sea level by approximately 1 cm for every 1 mb Coriolis force, and low barometric pressures from decrease in atmospheric pressure. The resulting in- coastal storms force sea water onto the shore. During crease in sea level is usually regional, according to the the most severe winter storms, surface atmospheric regional scale of winter cyclones, and typically lasts pressure along the west coast drops by 20 mb or more only a few days at most (Flick, 1998). Woodworth and

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 61 Blackman (2004) investigated high-water levels from can exceed 10 m (Figure 4.5; Ruggiero et al., 2010; tide gages around the world since 1975 and found that Seymour, 2011), although they are usually smaller as the magnitude of sea-level extremes has risen in many they approach the shoreline. Significant wave heights at locations, including some parts of the U.S. west coast, the shoreline vary considerably depending on incident and that these extremes closely followed increases in wave direction and nearshore bathymetry. the median sea level. Wave swells generated by storms propagate long Strong ocean winds also produce surface gravity or distances (e.g., from the central North Pacific to the wind waves. The most extreme such waves are of two U.S. west coast) over several days. Swells generated types: sustained intervals of large waves (measured by far from the west coast tend to peak at relatively long the significant wave height, the average height of the periods (12 seconds or more), whereas more locally largest one-third of the waves) and rogue waves, which generated wave swells tend to peak at periods of 10 sec- have individual crests that are much larger than the onds or less. The largest swells are generated by winter significant wave height. Sustained intervals of large cyclones that produce high winds with a long fetch (the waves occur during strong storms. These storm waves total distance that wind blows over the sea surface dur- can propagate over a long distance to the shoreline. ing the storm) directed toward the west coast. A broad, Rogue waves are produced by interactions among waves deep low-pressure system over the North Pacific favors and perhaps currents, and they have the greatest impact these conditions (Figure 4.6; Bromirski et al., 2005). when they arise during a sustained interval of large Synoptic timescale patterns like this tend to occur waves. By definition, they are expected but relatively during El Nio winters, but not exclusively (Seymour uncommon events (Baschek and Imai, 2011). et al., 1984; Bromirski et al., 2005; Allan and Komar, El Nios can significantly elevate sea level along 2006). Larger than normal waves have occurred during the west coast during winter months (see "Changes in El Nio winters along the California coast and some Ocean Circulation" above), especially along the Cali- parts of the Oregon and Washington coasts (Bromirski fornia coast because the North Pacific storm track is et al., 2005; Allan and Komar, 2006). La Nias have displaced toward the equator during El Nio events been shown to produce smaller than normal winter (Seager et al., 2010). The wind and pressure patterns wave heights at some California locations, but not that elevate sea level above climatological normals along everywhere along the west coast (Allan and Komar, the west coast also may occur in winters when El Nio 2006). Overall, the occurrence of large storms and high is not present. Winters with high sea-level anomalies waves is clustered in time, with particular years and have usually had a few large North Pacific storms with groups of years having many large storms, and other strong westerly, southwesterly, or northwesterly winds years having few or no large storms. offshore, which generate storm surges and high waves Peaks in wind waves are generally much higher along the coast of California and sometimes the coasts than sea-level anomalies (Seymour et al., 1984; of Oregon and Washington. Seymour, 1998; Storlazzi and Griggs, 1998; Ruggiero The path and propagation speed of storms controls et al., 2010). High breakers induce a change in mean the wind direction and barometric pressure, which, in water level at the beach (set-up), which can be about turn, affects the generation of wind waves and high water 20 percent of the breaking wave height (Dean and (e.g., O'Reilly and Guza, 1991). The highest winds, and Dalrymple, 1991). High wave events sometimes, but hence waves, along the west coast of the United States not always, coincide with high sea levels (Cayan et al., nearly always occur during strong winter extra-tropical 2008; Ruggiero et al., 2010). cyclones (Wang and Swail, 2001; Bromirski et al., 2003; Caires et al., 2004; Ruggiero et al., 2010; Barnard et al., Changes in Storminess and Extreme Wave Heights 2011; Seymour, 2011). Tropical cyclones rarely travel as far north as California, although two cases have Evidence of changes in storminess (wind intensity) been recorded historically (Hurd, 1939; Chenoweth in the North Pacific Ocean is mixed. Bromirski et al. and Landsea, 2004). Significant wave heights recorded (2003) examined nontidal sea-level fluctuations from by offshore coastal buoys during extra-tropical events 1858 to 2000 in the San Francisco tide gage record

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62 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 4.5 (a) Number of storm events per month off Oregon and Washington between 1976 and 2007, when the significant wave height (SWH) exceeded a threshold of 8.1 m at two deep-water wave buoys. (b) Days when the threshold of 8.1 m was ex- ceeded (dots), annual maxima (circles), and the five largest storms per year (asterisks) for 19762007, illustrating the seasonality of the extreme wave climate. The 100-year significant wave height is shown by the solid horizontal line and its associated uncertainty is the dashed horizontal lines. SOURCE: Ruggiero et al. (2010). FIGURE 4.6 Atmospheric circulation during periods of high waves along the central California coast exhibits broad-scale low pres- sure over the North Pacific. This map shows anomalies of 700 hPa height in meters during the 15 winter months (November through March) from 1981 to 2003 when wave energy offshore San Francisco was greatest. The region of anomalously low 700 hPa indicates a low-pressure trough and increased storminess in the central and eastern North Pacific. Significant negative and positive anomalies are blue and red, respectively. SOURCE: Adapted from Bromirski et al. (2005).

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 63 and found significant decadal variability. Although the wave buoy records (Ruggiero et al., 2010), and satellite record showed an increase in storminess from 1950 altimeter observations (Young et al., 2011a). to 2000, the storm intensity in recent decades did not A study of North Pacific wind variability on 2- to significantly exceed that in the decades prior to 1950 10-day timescales from the National Centers for Envi (Bromirski et al., 2003). On the other hand, the IPCC ronmental Prediction (NCEP) Reanalysis (Kalnay et Fourth Assessment Report cited several studies that al., 1996) indicated that wind speed trends are vari- reported increases in the strength of the winter westerly able, owing to the occurrence of relatively infrequent wind circulation across the North Pacific during the large events. From the 1950s through the 1990s, wave past few decades (Trenberth et al., 2007). model reanalyses over the North Pacific (Graham Lowe et al. (2010) described climate change effects and Diaz, 2001; Caires et al., 2004) indicate a trend on storm intensity as inconclusive, with no consensus toward increasing wave height. From a series of buoy among different model simulations on local changes in observations beginning in the late 1970s, Storlazzi and storm frequency. A simulation of San Francisco sea- Wingfield (2005), Allan and Komar (2006), Ruggiero level anomalies forced by 21st century climate change et al. (2010), and Seymour (2011) found that the largest simulations (Cayan et al., 2008) found considerable waves along the coast from California to Washington inter annual and decadal variability, driven partly by state were larger in the period after 1990 than in the storm characteristics, superimposed on an assumed period before (Figure 4.7). This change was associated long-term rise in mean sea level. Several climate models with a deepening of the winter low pressure system over discussed in the IPCC Fourth Assessment Report the North Pacific Basin and partly to the incidence of project that the mid-latitude storm tracks in both some relatively strong El Nio years since 1995. the southern and northern hemispheres will migrate Increases in wind speed and wave heights in the poleward over the 21st century (Meehl et al., 2007). A northeastern Pacific Ocean have been reported recently, subsequent projection by Salath (2006) also showed a northward shift in the North Pacific winter storm track over the next several decades. The storm tracks and Pacific wind fields in some global climate model projections suggest that future wave heights might diminish somewhat over the open ocean and along the coast from southern and central California to Oregon (Salath, 2006; Cayan et al., 2009). If frequency or intensity of storminess changes as a result of climate change, the frequency of high sea- level extremes also would likely change. Even if the storminess regime does not change, sea-level rise will increase the exposure of the coast to storm-driven surge and high waves, magnifying their impact on the coast. Analyses of marine weather reports discussed in the IPCC Fourth Assessment Report showed an increase in significant wave height of 810 cm per decade over the central and eastern North Pacific from 1950 to 2002 (Trenberth et al., 2007). Gulev and Grigorieva (2006) attributed these increases to longer period, l onger distance sources of swell as well as to more locally FIGURE 4.7Increases in the annual maximum wave height (green; m), average of the five largest wave events per year generated wind waves. The tendency for an increase (blue), winter average height (red), and annual average height in wave energy over the eastern North Pacific is also (black) from northeast Pacific wave buoy sensors. Open circles indicated by wave hindcasts (Graham and Diaz, 2001), represent years with too much missing data (i.e., winter months buoy observations (e.g., Allan and Komar, 2006), some missing more than 60 percent of data). SOURCE: Ruggiero et al. (2010), after Allan and Komar (2006).

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64 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON but the interpretation of these changes is controversial. natural variability of the Pacific atmosphere-ocean Analyses of global ocean winds from ship observations system. Some global climate models predict that the (Tokinaga and Xie, 2011), satellite microwave sensors North Pacific storm track will shift northward as global (Wentz et al., 2007), and satellite altimeters (Young climate warms during the next several decades, which et al., 2011a) indicate that wind speeds have risen would generate extreme wave heights and storm surges over the global oceans, although the trends found by along the Oregon and Washington coasts. However, a Young et al. (2011a) are greater than those derived from northward shift in the North Pacific storm track has Tokinaga and Xie (2011) and Wentz et al. (2007) by not yet been confirmed. approximately a factor of two (Wentz and Ricciardulli, All climate models project ample winter storm 2011; Young et al., 2011b). The Young et al. (2011a) activity in the North Pacific in future decades, sug- analysis also found that wind speeds within the highest gesting that periods of anomalously high sea level and 1 percent of events have risen over much of the extra- high waves will continue to occur along the west coast. tropical oceans over the past two decades, including an Storm-generated bursts of high sea levels and waves increase of about 1 percent per year in the northeast are expected to vary from year to year and decade to Pacific, and that this increase is accompanied by in- decade. Over the next few decades, these anomalies creases in the extreme wave heights. The latter occurs will likely eclipse the secular rise in sea level (few to in particular in the northeast Pacific Ocean, which is several mm per year). Short-period fluctuations of sea consistent with increasing extreme wave heights (by as level may sometimes exceed 20 cm, and storm-driven much as 2 m over the record period) during big storms wave heights of 1 m or even higher amplitudes than are recorded in near coastal deep-water buoy records from seen in the historical record could easily occur. These northern California to Washington (Allan and Komar, variations will have greatest impact when they occur on 2006; Menndez et al., 2008; Ruggiero et al., 2010). days with high tides. However, further analysis by Gemmrich et al. (2011) suggests that much of this change is spurious, caused SEA-LEVEL FINGERPRINTS OF MODERN by changes in buoy hardware and data processing. All LAND ICE CHANGE of these estimates were made from records that are only a few decades long, and thus partly reflect changes in As glaciers and ice sheets melt and lose mass and wind forcing associated with natural climate variabil- the melt water is transferred from the continents to the ity such as the Pacific Decadal Oscillation and other ocean, the solid earth deforms and the gravitational interannual-interdecadal fluctuations. However, the field of the planet is perturbed. The addition of new global extra-tropical pattern of extreme wave increase water to the ocean basins and the associated gravita- found by Young et al. (2011a) is atypically widespread tional and deformational effects create regional patterns for most decadal natural variability, and thus might of sea level change. Both modern melting and deglacia indicate a longer trend. As yet there is no good explana- tion of the ancient ice sheets affect sea-level change tion for why such a trend would occur. along the west coast of the United States. Melting of the ancient ice sheets caused the solid earth to rebound Summary (glacial isostatic adjustment), resulting in significant vertical land motions in the vicinity of the California, Periods of anomalously high sea levels and wave Oregon, and Washington coasts. In contrast, modern heights along the west coast of the United States ex- melting affects land motions at the ice masses, which hibit considerable variability on synoptic, interannual, are far from the U.S. west coast, but the gravitational and decadal timescales, in association with ENSO and effect influences the height of the sea surface in the other climate patterns. Some evidence suggests that northeast Pacific Ocean. This section describes the ef- wave heights have increased along the west coast from fects of modern land ice melt on sea-level rise off the northern California to Washington during the past coasts of California, Oregon, and Washington. The few decades. However, it is likely that much of this effects of ancient ice melt are discussed in the follow- increase is associated with interannual- to decadal-scale ing section (see "Glacial Isostatic Adjustment" below).

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 65 Modern melting of land ice affects sea level along Alaska, Greenland, and Antarctica. The figure shows the west coast of the United States in two ways. First, that melting of Alaska glaciers creates a strong north- the large mass of glaciers and ice sheets generates an south gradient in relative sea-level change along the additional gravitational pull that draws ocean water west coast. The gradient from uniform melting of the closer, raising relative sea level near the ice masses. As Greenland Ice Sheet is much smaller (Figure 4.9B). the ice melts, the amount of ice mass on land declines, Uniform melting of either the Antarctic Ice Sheet decreasing its gravitational pull on the ocean water. or the West Antarctic Ice Sheet leads to a uniform The loss of mass also results in uplift of the land mass change in relative sea level along the entire west coast under the ice. The combination of these effects causes (Figure 4.9C). relative sea level to fall in the vicinity of the ice mass. To estimate the effect of fingerprinting from these The fall extends, at decreasing rates, in the region three ice masses on relative sea level, it is necessary only within a few thousand km of the melting ice. Second, to multiply the global sea-level equivalent of the mass ice melt enters the ocean, raising global mean sea level. loss from each source by the appropriate scale factor Because of gravitational and deformational effects, (colored contours) indicated in the figure and then add however, the distribution of new ice melt is nonuniform the contributions from all three sources. Scale factors over the globe. Relative sea level falls near the shrink- greater than 0 indicate that the sea-level fingerprint ing ice mass and rises everywhere else. This effect is increases relative sea-level rise at that location, and scale shown schematically in Figure 4.8. The combined ef- factors greater than 1 indicate that the rise is higher fect of new water mass entering the ocean and altered than the global sea-level equivalent value. Scale factors gravitational attraction results in a spatial pattern of less than 0 mean that the effect of mass loss from a sea-level rise that is unique for each ice sheet or glacier source causes the relative sea level to fall. Scale factors (Mitrovica et al., 2001; Tamisiea et al., 2003). As a for other ice sources (e.g., European Alps, northeastern consequence, these sea-surface geometries have come Canadian Arctic, Patagonia) are not available at the to be known as sea-level fingerprints. resolution shown in Figure 4.9, but these sources are Only a few studies have attempted to map the sea- likely too small and/or too distant to affect the gradient level fingerprints of melting land ice along the west in sea-level change along the U.S. west coast. coast of the United States (e.g., Tamisiea et al., 2003, The scale factors and ice loss rates used to calculate 2005). Figure 4.9A shows the sea-level fingerprints the adjusted rates of relative sea-level rise are given in of the three largest sources of land ice that are most Table 4.1. Modeling or estimating individual regional likely to have significant effects on west coast sea level: land ice losses is beyond the scope of this study, so ice volume t1 ice volume t2 mean sea level t2 mean sea level t1 FIGURE 4.8 Schematic view of the changing sea level caused by a shrinking land ice mass. Relative sea level at time t1 exceeds the mean sea level near the ice mass and is less than the mean at some distance beyond the mass. As the land ice mass decreases (time t2), the local gravitational attraction decreases and the land in the vicinity of the ice rises, causing the relative sea level to fall, even though the mean sea level increases. SOURCE: Adapted from Tamisiea et al. (2003).

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72 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON Time Inter- and Pre-seismic During earthquake (tsunami) Uplift Subsidence Subsidence Uplift Overlying plate Oceanic plate Seismogenic zone Pre-seismic subsidence / relative sea-level rise 2 Rapid co-seismic subsidence / relative sea-level rise Uplift Altitude (m) Subsidence Centuries long inter-seismic uplift / relative sea-level fall Rapid post-seismic uplift / relative sea-level fall 0 0 1000 2000 Time (year) FIGURE 4.11(Top) Deformation associated with a subduction-zone thrust fault on a coastline during an earthquake cycle. (Bottom) Idealized seismic cycle for a subduction zone, showing a long period of uplift, followed by small-scale subsidence and then a sudden drop in land elevation during a great earthquake. SOURCE: Modified from Horton and Sawai (2010). TABLE 4.4 Vertical Land Motion Rates Predicted by the CAS3D-2 Model for 20102030 Location Latitude Longitude Rate of Vertical Land Motion (mm yr-1) Anacortes, WA 48.56 -122.64 -0.87 Seattle, WA 47.85 -122.73 -0.59 Long Beach, WA 46.58 -123.83 1.87 Pacific City, OR 45.38 -123.94 1.69 Waldport, OR 44.42 -124.02 1.66 Coos Bay, OR 43.36 -124.30 2.33 Eureka, CA 40.87 -124.15 2.98 SOURCE: Rates provided by Kelin Wang, Geological Survey of Canada, using the CAS3D-2 model (He et al., 2003; Wang, 2007). The model deformation history includes a coseismic rupture of the entire Cascadia subduction fault, representing the 1700 M 9 great earthquake, followed by locking of the fault, modeled using the conventional backslip approach (Savage, 1983). A mantle wedge viscosity of 10 Pa s was used, consistent with the results of postglacial rebound analyses at northern Cascadia and values adopted at other subduction zones. be experiencing subsidence. Comparisons of the model San Andreas Fault Zone projections with GPS data are discussed below (see "Current Rates of Vertical Land Motion Along the Unlike the Cascadia Subduction Zone, vertical U.S. West Coast"). Model projections further forward land motions along the San Andreas Fault Zone cannot in time are given in Chapter 5. be characterized by a single tectonic model. The San Andreas Fault Zone comprises multiple sub-parallel

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 73 faults, each with limited extent and unique seismo (e.g., composition, porosity), the water content, and tectonic character. Although crustal displacement is the loading history (Brain et al., 2011). For example, primarily horizontal (Figure 4.12), local vertical mo- deposits with a high sand fraction undergo little com- tions result from rock uplift associated with restraining paction, whereas peat may compact as much as 90 per- bends (e.g., Anderson, 1990) and active contractional cent by volume ( Jelgersma, 1961). processes associated with the Transverse ranges and the Early studies of wetlands in North America (Kaye Ventura and Los Angeles basins (Namson and Davis, and Barghoorn, 1964) and Europe ( Jelgersma, 1961) 1991; Donnellan et al., 1993; Yeats, 1993; Shaw and illustrated the importance of sediment compaction Suppe, 1994, 1996; Yeats and Huftile, 1995; Dong to relative sea-level rise. However, only a few studies et al., 1998; Orme, 1998; Argus et al., 1999, 2005; have quantified compaction rates of coastal sediments. Hager et al., 1999; Shaw and Shearer, 1999; Argus and Trnqvist et al. (2008) analyzed wetland sediments Gordon, 2001; Bawden et al., 2001). A comprehensive from the Mississippi Delta and found compaction rates analysis of tectonically induced vertical land motions of 5 mm yr-1 on millennial timescales and more than for the San Andreas Fault Zone has not been done. 10 mm yr-1 in some areas on decadal to century time scales. These high rates of compaction were thought Sediment Compaction to contribute significantly to the high rates of relative sea-level rise (10 mm yr-1 over the past century) in the Compaction may rearrange the mineral matrix of Mississippi Delta. Horton and Shennan (2009) found sediment, reducing its volume (Kaye and Barghoorn, compaction rates of 0.4 0.3 mm yr-1 during the past 1964; Allen, 2000; Brain et al., 2011). The amount of 4,000 years in eastern England, with higher values in compaction depends on a number of factors, including large estuaries and considerable local variability depend- the mechanical and chemical properties of the sediment ing on sediment types and drainage histories. Galloway FIGURE 4.12 Faults (black lines) and GPS-defined horizontal velocities (red arrows) for sites in the western United States relative to stable North America. Circles are error ellipses at the 95 percent confidence level. SOURCE: Bennett et al. (1999).

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74 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON et al. (2001) found that compaction of organic soils in the Sacramento Bay Delta (27 cm yr-1), combined with reclamation and agriculture, has resulted in islands sinking below sea level (see also "California Bay Delta Case Study" in Chapter 6). Comprehensive studies of compaction rates for the types of geomorphic environments that domi- nate the U.S. west coast (see "Geographic Variation Along the U.S. West Coast" in Chapter 1) are not available. Most of these environments, particularly the peat- and mud-rich estuaries and tidal marshes, will subside as a result of compaction. Groundwater and Petroleum-Related Drawdown and Recharge Withdrawal of groundwater and petroleum can lower large areas of the land surface. Subsurface fluid extraction depressurizes underground reservoirs, alter- ing the arrangement of in situ stresses within the res- ervoir and surrounding rock or sediment (Donaldson et al., 1995). The elastic compaction can be recovered if the fluid level rises again (e.g., Schmidt and Brg- mann, 2003), but the inelastic compaction becomes permanent, resulting in subsidence (Sun et al., 1999). Some of the best documented examples of subsidence due to groundwater withdrawal along the U.S. west coast are in California (Figure 4.13). Intense cultiva- tion in the Santa Clara Valley during the first half of the 1900s caused the land surface to subside up to 4 m in San Jose and 0.62.4 m near the southern end of San Francisco Bay, putting 44 km2 below the high-tide level (Galloway et al., 2001). In the San Joaquin Valley, one of the world's most productive agricultural regions, the land surface dropped 0.39 m over 75 years, mainly due to groundwater pumping and compaction. Since FIGURE 4.13 Areas in Washington, Oregon, and California where significant subsidence has been attributed to groundwater 1969, groundwater recharge and the supplemental use withdrawal (blue). The impact of groundwater withdrawal has of surface water for irrigation has slowed land subsid- been greater in California than in Oregon or Washington. ence in both valleys. SOURCE: Modified from Galloway et al. (2001). In some cases, subsidence is partly offset by ground- water recharge. For example, long-term subsidence in the Santa Ana Basin (Los Angeles area) is ~12 mm surface subsidence related to petroleum withdrawal yr-1, but groundwater recharge produces seasonal verti- has been documented in a number of areas, including cal oscillations of up to 55 mm (Bawden et al., 2001). the California San Joaquin Valley, Las Vegas, New Petroleum production requires the withdrawal of Orleans, and Houston. The best documented example subsurface liquid hydrocarbons and also significant is the Wilmington oil field in Long Beach, California, quantities of groundwater (Yuill et al., 2009). Ground which subsided up to 9 m over 27 years (Mayuga and

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 75 Allen, 1969; Nagel, 2001). However, use of secondary land motion estimate with well-defined and conserva- recovery techniques, such as pumping seawater into tive error estimates (see Appendix A). The vertical the reservoirs to increase oil production, can stabilize land motion rates are shown in Figure 4.14. Most of compaction and halt subsidence. Large active oil fields the coastal CGPS vertical land motion rates fall within along the coastal west U.S. coast are located mainly in 3 mm yr-1 (Figure 4.14b). The average rates with ob- the area between Santa Barbara and the Los Angeles vious outliers removed (Figure 4.14c, d) are similar to Basin. longer-term estimates from leveling data for Cascadia (Burgette et al., 2009) and the San Andreas region Current Rates of Vertical Land Motion Along the (Appendix D). Annual rates of vertical land motion U.S. West Coast are generally positive in Washington and Oregon and generally negative in California (Figure 4.14a). This Observations of vertical land motion in coastal spatial pattern suggests that the tectonic boundary at California, Oregon, and Washington are given in the Mendocino Triple Junction is a fundamental and, Table 4.5. The values in the table represent the total most likely temporally stationary, boundary for verti- vertical land motion, which is often caused by a com- cal land motion. Uplift in Washington and Oregon bination of processes. For example, in the Los Angeles is consistent with the buildup of interseismic strain Basin, subsidence due to hydrocarbon and groundwater in the Cascadia Subduction Zone as described by withdrawal, together with faulting, raised or lowered the CAS3D-2 model (He et al., 2003; Wang, 2007), the surface elevation by upwards of 10 mm yr-1 from rather than the subsidence predicted by GIA models. 1992 to 2000, with seasonal oscillations as high as Subsidence in California is consistent with glacial iso- 55 mm yr-1 (Box 4.3). static adjustment; most GIA models predict subsidence The spatial distribution of published data on verti- south of the Mendocino Triple Junction (gray band cal land motions is not optimal for assessing sea-level in Figure 4.14b; see also Sella et al., 2007; Mazzotti rise along the west coast. Consequently, the commit- et al., 2008; Argus and Peltier, 2010). As noted above tee characterized the spatial variability of vertical land (Box 4.3, Table 4.5), however, large vertical land motion motion using the Scripps Orbit and Permanent Array signals associated with local tectonics and/or subsurface Center velocity model and continuous GPS (CGPS) fluid movements can locally overwhelm the regional velocity data taken within ~15 km of the coast. The tectonic signal. This effect appears to be most prevalent CGPS data provide an accurate, self-consistent vertical toward southern California, although the paucity of TABLE 4.5 Current Rates and Causes of Vertical Land Motion Along the U.S. West Coast Rate of Vertical Land Source Location Method Period (yr) Motion (mm yr-1) Cascadia Subduction Zone Mazzotti et al. (2008) Cascadia Subduction Zone GPS 19932003 1.13.5 Burgette et al. (2009) Cascadia Subduction Zone Leveling 19252006 -0.283.29 San Andreas Fault Zone Cooke and Marshall (2006) and Palos Verdes Fault Geodesy and modeling HoloceneQuaternary -0.50.4 Wills et al. (2006) Santa Monica Fault 0.51.0 Los Angeles Basin interior faults 0.265.0 Brgmann et al. (2006) San Andreas System InSAR 19922000 -2.01.5 California Aquifers and Oil Fields Bawden et al. (2001) Santa Ana Aquifer, long term InSAR 19972000 -12 Santa Ana Aquifer, seasonal 55 Argus et al. (2005) Santa Ana Aquifer, seasonal InSAR and GPS 19921999 -6235 Long Beach Oil Field 5 Huntington Beach Oil Field -8 Wilmington Oil Field -69

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76 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON BOX 4.3 Spatial Variability of Vertical Land Motion and Relative Sea-Level Change in Los Angeles Vertical land motions in the Los Angeles Basin vary on small spatial scales because of subsidence from groundwater and hydrocarbon withdrawal and active thrust faulting (Bawden et al., 2001; Lanari et al, 2004; Argus et al., 2005). Brooks et al. (2007) used InSAR to create a vertical land motion map of the Los Angeles Basin. The figure shows the rapid spatial change in land elevation at sub-15 km scales in this area. Brooks et al. (2007) also used land motion rates to adjust local tide gage records to produce a profile of relative sea-level change along the coast. Vertical land motion differs on the west and east side of the Palos Verdes Peninsula. To the west, relative sea level was nearly constant from 1992 to 2000, with most values less than zero. To the east, approaching the Long Beach/Wilmington oil field, relative sea-level rates varied from -1.7 to 1.3 mm yr-1 and by as much as ~3 mm yr-1 over distances as short as ~5 km. The Brooks et al. (2007) results show the danger of assuming that a tide gage is representative of relative sea level for a region undergoing uplift or subsidence. Interpretation of the Los Angeles Harbor tide gage alone would miss the spatial variability in sea level to the east and assume the wrong sign of relative sea-level change to the west. A B FIGURE Land motion (line-of-sight, 23 degrees inclined from vertical) from 1992 to 2000 in the Los Angeles Basin deter- mined from InSAR (colors coded in mm yr-1) and GPS (red circles), showing variability due to tectonics and hydrocarbon and groundwater fluctuations. Tide gages are shown as yellow squares. SOURCE: Brooks et al. (2007).

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 77 -128 -126 -124 -122 -120 -118 -116 50 50 A B 48 48 GPS data 46 CAS 3d-2 46 model Cascadia GIA models 44 44 42 latitude 42 40 40 MTJ 38 38 San Andreas 36 36 7. 5mm/yr 34 0. 0 34 -7. 5 32 32 -10 -5 0 5 longitude VLM (mm/yr) Cascadia San Andreas 10 10 C D A 8 8 6 6 number number 4 4 2 2 0 0 -10 -5 0 5 10 -10 -5 0 5 10 VLM (mm/yr) VLM (mm/yr) FIGURE 4.14 Continuous GPS vertical land motion (VLM) rates. (A) Map of the west coast of the United States showing major tec- tonic boundaries and locations of GPS stations color-coded for vertical land motion rates. Squares are stations within ~15 km of the coast. Circles are other stations, which are shown to demonstrate the overall spatial variability of vertical land motion in the western United States. MTJ = Mendocino Triple Junction. (B) Vertical land motion versus latitude for the coastal GPS stations (squares in panel A), compared with predictions of current uplift from the CAS3D-2 model (green diamonds; from Table 4.5) and the ensemble of GIA models (gray shading; from Table 4.3). GPS errors are 1 standard deviation. (C) Histogram and normal density function for the Cas- cadia coastal stations in panels A and B. (D) Histogram and normal density function for the San Andreas coastal stations in panels A and B. In both areas, obvious outliers have been removed. SOURCE: GPS data from the Scripps Orbit and Permanent Array Center, .

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78 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON data adequate to sense the km-scale variations in verti- level rise between coastal California and northernmost cal land motion precludes complete characterization of California, Oregon, and Washington, consistent with a these strong local signals along the entire west coast. major tectonic influence (Table 4.6). Only a few other tide-gage-based estimates of sea-level change along the Summary U.S. west coast have been published (e.g., Tebaldi et al., 2012), and most are based on the Douglas (1991) The west coast of the United States is undergoing data (e.g., Peltier, 2001) or consider records from only active vertical deformation due to a combination of a few gages (e.g., Nakada and Inoue, 2005; Bromirski tectonics, sediment compaction, fluid withdrawal and et al., 2011; Table 4.6). recharge, and glacial isostatic adjustment. Assessing The committee obtained records from 28 tide their relative contribution to the observed vertical land gages along the California, Oregon, and Washington motion is complicated by a shortage of data and by coasts archived at the Permanent Service for Mean Sea the wide spatial and temporal variability of the various Level. Of these, 12 are currently operating, contain no processes. Continuous GPS measurements over the long gaps, and have been recording sea level for at least past two decades, in concert with 20th century leveling 60 years, and thus were considered suitable for deter- studies, show that the coast north of Cape Mendocino mining long-term trends in sea-level rise. For each is rising on the order of ~1.53.0 mm yr-1, likely as a gage, the rate of relative sea-level rise was determined result of building interseismic strain along the Cascadia by fitting a straight line through the monthly mean Subduction Zone. In contrast, the California coast data plotted as a function of time (see Appendix A south of Cape Mendocino is subsiding at a mean rate for details). The committee's estimated rates of rela- of ~1 mm yr-1 or more, although GPS-measured verti- tive sea-level change at the 12 tide gages are given in cal land motions vary widely (-3.70.6 mm yr-1). The Table 4.6 and shown geographically in Figure 4.15. boundary between uplift and subsidence takes place Most of the gages north of Cape Mendocino (Crescent at the Mendocino Triple Junction, highlighting the City to Neah Bay) indicate that relative sea level is importance of regional tectonics in relative sea-level falling, which is consistent with uplift associated with rise. Subsidence south of Cape Mendocino is consistent the buildup of interseismic strain along the Cascadia with models of glacial isostatic adjustment. However, Subduction Zone, whereas most of the gages south of more detailed analysis of potential reference frame bias Cape Mendocino show that relative sea level is ris- and sensitivity tests of GIA models have to be carried ing, which is consistent with land subsidence. Some out to determine whether GIA is responsible for the gages (e.g., Friday Harbor, Seattle) deviate from these regional subsidence. Local tectonics, sediment compac- regional sea-level trends, likely as a result of local tion, and fluid withdrawal and recharge can cause much tectonic, compaction, or fluid withdrawal or recharge higher rates of subsidence or uplift than the regional effects. The average rate of relative sea-level rise is mean, especially in California, but at spatial scales too 0.03 1.49 mm yr-1 north of Cape Mendocino and small (as little as 1 km) to have a significant impact on 1.38 0.64 mm yr-1 south of Cape Mendocino for the sea-level change in the region. past 610 decades. The change in relative sea level is what coastal WEST COAST TIDE GAGE RECORDS residents experience and state and local managers factor into planning. To compare west coast sea-level The sea level along the west coast of the United trends with the global sea-level trend, it is necessary to States reflects contributions from both the global sea adjust the relative rates of sea-level rise for changes in level and the local and regional processes discussed atmospheric pressure and vertical land motions, both above. Tide gage data can be used to estimate rates of which affect the local water level (see Appendix A). of relative sea-level change, but only a few such esti- Figure 4.16 illustrates the effect of these corrections mates have been made for the west coast of the United on the sea-level trend for 108 years of monthly tidal States. Douglas (1991) compared tide gage records for data for Seattle, Washington. The slope of the blue 19301980 and found large differences in rates of sea- straight line gives the rate of relative sea-level rise, in

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 79 TABLE 4.6 Rates of Relative Sea-Level Rise Estimated from U.S. West Coast Tide Gages Source Tide Gage Period Rate of Sea-Level Rise (mm yr-1) Convergent margin Douglas (1991) Friday Harbor, WA 19301980 0.6 This report 19342008 1.04 Douglas (1991) Neah Bay, WA 19301980 -1.6 This report 19342008 -1.77 Douglas (1991) Seattle, WA 19301980 2.5 Bromirski et al. (2011) 19301980 2.47 This report 19002008 2.01 Douglas (1991) Astoria, OR 19301980 -0.4 This report 19252008 -0.38 Douglas (1991) Crescent City, CA 19301980 -0.9 This report 19332008 -0.73 Transform margin Douglas (1991) San Francisco, CA 19301980 1.8 Bromirski et al. (2011) 19301980 1.91 This report 19002008 1.92 This report Alameda, CA 19392008 0.70 This report Port San Luis, CA 19452008 0.68 This report Santa Monica, CA 19332008 1.41 Douglas (1991) Los Angeles, CA 19301980 0.2 This report 19232008 0.84 Douglas (1991) La Jolla, CA 19301980 1.8 This report 19242008 2.08 Douglas (1991) San Diego, CA 19301980 1.7 Bromirski et al. (2011) 19301980 1.80 This report 19062008 2.04 this case, 2.01 mm yr-1. The green line in Figure 4.16 land motion does not vary between the gage and the shows that the atmospheric correction is small. The GPS station. It is likely that rates of vertical land mo- atmospheric adjusted sea-level rise (using data from tion near at least some of the tide gages have varied over the National Ocean and Atmospheric Administration's the past century because of earthquakes, groundwater Earth System Research Laboratory) is 2.10 mm yr-1, extraction and recharge, or other processes, but the about 4 percent higher than the relative sea-level rise. absence of detailed geologic histories for each gage Ideally, vertical land motions would be corrected precluded a more sophisticated approach. The vertical using GPS data collected at the tide gage. However, land motion correction to the sea-level record was often none of the tide gage stations analyzed in this report relatively large, changing rates in one case by almost include GPS instruments. Consequently, the commit- 150 percent (see Table A.2 in Appendix A). For five of tee followed the practice of using data from the closest the gages analyzed, correcting for vertical land motion CGPS station, as long as it is within 15 km of the gage changed the sign of sea-level change. (e.g., Mazzotti et al., 2007; Wppelmann et al., 2007). The rate of sea-level rise at the tide gage, ad- Data from all CGPS stations within a 15 km radius of justed for vertical land motion and atmospheric the gage also were analyzed to assess the spatial vari- pressure, is the slope of the red line in Figure 4.16, ability of vertical land motions near the tide gages. which is 2.3 mm yr-1 for Seattle, about 15 percent The CGPS data were obtained from the Scripps higher than the rate of relative sea-level rise. Along Orbit and Permanent Array Center. Although GPS the coast, the mean adjusted rates of sea-level rise are records extend back only a few decades, the commit- 1.59 0.80 mm yr-1 north of Cape Mendocino and tee assumed that current motions are representative of 1.02 1.73 mm yr-1 south of Cape Mendocino, both motions over the entire history of the tide gage and that of which are lower than global mean sea-level rise.

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80 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON -128 -124 -120 -116 Friday Harbor Neah Bay 48 Seattle Astoria 44 Crescent City Cape Mendocino 40 San Francisco Alameda 36 Port San Luis mm/yr 3 0 Santa Monica Los Angeles -3 La Jolla San Diego 32 FIGURE 4.15 Rates of relative sea-level change estimated from long tide gage records (63108 years) analyzed in this report.

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SEA-LEVEL VARIABILITY AND CHANGE OFF THE CALIFORNIA, OREGON, AND WASHINGTON COASTS 81 FIGURE 4.16 Monthly sea level for Seattle, Washington, from 1900 to 2008. Straight-line fits to the data show the relative sea- level rise (blue line), the sea-level rise adjusted for atmospheric pressure (green line), and the sea-level rise adjusted for vertical land motion and atmospheric pressure (red line). CONCLUSIONS elevate coastal sea level by 1030 cm for several winter months. Cool climate phases have less influence on Sea level at any given place and time depends on local sea level than warm climate phases. Changes the global sea level and the net contribution of atmo- between warm and cool climate phases, which occur spheric, oceanographic, geologic, and anthropogenic on seasonal to multidecadal timescales, cause large- processes operating in the area. Processes that raise amplitude variations in the relative sea-level trend. relative sea level in the northeastern Pacific Ocean Modern melting of glaciers and ice sheets adds include warm phases of climate oscillations (El Nios, new water to the ocean basins and produces gravita- positive phase of the PDO) and land subsidence due tional and deformational effects that create regional to glacial isostatic adjustment, sediment compaction, patterns of relative sea-level change. The glaciated and the withdrawal of groundwater or hydrocarbons. land masses that most effect sea level along the west Processes that lower relative sea level include cool coast of the United States are Alaska, which is close, phases of climate oscillations (La Nias, negative phase and Greenland and Antarctica, which are large. The of the PDO), gravitational and deformational effects gravitational and deformational effects reduce the of modern melting of glaciated land masses, and land contribution of melting of these three ice sources uplift due to tectonics or fluid recharge. to relative sea-level rise for 19922008 by about The highest sea levels recorded along the west coast 42 percent along the north coast (Neah Bay, Wash- are usually associated with El Nio events, which can ington), 24 percent along the central coast (Eureka,

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82 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON alifornia), and 14 percent along the south coast C Mendocino show relative sea-level rise, consistent with (Santa Barbara, California). land subsidence. When adjusted for vertical land mo- Vertical land motions along the west coast of the tions and for atmospheric pressure effects, the rates of United States are caused by a complex combination of relative sea-level rise along the U.S. west coast are lower tectonics, glacial isostatic adjustment, sediment com- than the rate of global mean sea-level rise. paction, and fluid withdrawal and recharge. The area Although rates of sea-level rise are relatively low straddles two tectonic regimes: (1) the Cascadia Sub- along the west coast of the United States, the combi- duction Zone, where the buildup of interseismic strain nation of sea-level rise and winter storms increases the is causing coastal uplift north of Cape Mendocino, potential for significant coastal damage. Historically, California; and (2) the San Andreas Fault Zone, where most coastal damage has occurred when storm surges the lateral motion of the lithospheric plates produces and large waves coincided with high astronomical tides relatively little vertical land motion south of Cape and El Nios--a combination that can raise short-term Mendocino. Glacial isostatic adjustment is producing sea level above sea levels projected for 2100. All climate uplift in northernmost Washington, which had been models project ample winter storm activity, but a clear covered by the former Laurentide Ice Sheet, and sub- consensus has not yet emerged on whether storm fre- sidence in areas peripheral to the center of the former quency or intensity will change in the northeast Pacific. ice mass, including the rest of Washington, Oregon, Several climate models predict a northward shift in the and California. Land levels in some areas also are ris- North Pacific storm track over the 21st century, and ing or sinking because of local tectonics, compaction some observational studies report that a northward of wetland sediments, and/or fluid withdrawal or re- shift has been detected. However, most observational charge. Continuous GPS measurements over the past records are not long enough to determine whether a two decades and leveling studies over the past eight shift has begun. or nine decades shows that the coast north of Cape Several observational studies have reported that Mendocino is rising at rates of 1.53.0 mm yr-1 and the high waves have been getting higher and that winds coast south of Cape Mendocino is subsiding at a mean have been getting stronger in the northeastern Pacific rate of about 1 mm yr-1, although with considerable over the past few decades. The magnitude and cause of spatial variability (-3.70.6 mm yr-1). these changes are under investigation; at least part of Tide gage records along the west coast of the the observed increase likely reflects natural climate vari- United States indicate that relative sea-level change ability. But even if storminess does not increase in the is variable along the coast. Most gages north of Cape future, sea-level rise will magnify the adverse impact of Mendocino show relative sea-level fall for the past storm surges and high waves on the coast of California, 610 decades, consistent with coastal uplift along the Oregon, and Washington. Cascadia Subduction Zone. Most gages south of Cape