Thomas C. Royer

Climate Change Impacts on U.S. Coastal and Marine Ecosystems

Increases in concentrations of greenhouse gases projected for the 21st century are expected to lead to increased mean global air and ocean temperatures. The National Assessment of Potential Consequences of Climate Variability and Change (NAST 2001) was based on a series of regional and sector assessments. This paper is a summary of the coastal and marine resources sector review of potential impacts on shorelines, estuaries, coastal wetlands, coral reefs, and ocean margin ecosystems. The assessment considered the impacts of several key drivers of climate change: sea level change; alterations in precipitation patterns and subsequent delivery of freshwater, nutrients, and sediment; increased ocean temperature; alterations in circulation patterns; changes in frequency and intensity of coastal storms; and increased levels of atmospheric CO2. Increasing rates of sea-level rise and intensity and frequency of coastal storms and hurricanes over the next decades will increase threats to shorelines, wetlands, and coastal development. Estuarine productivity will change in response to alteration in the timing and amount of freshwater, nutrients, and sediment delivery. Higher water temperatures and changes in freshwater delivery will alter estuarine stratification, residence time, and eutrophication. Increased ocean temperatures are expected to increase coral bleaching and higher CO2 levels may reduce coral calcification, making it more difficult for corals to recover from other disturbances, and inhibiting poleward shifts. Ocean warming is expected to cause poleward shifts in the ranges of many other organisms, including commercial species, and these shifts may have secondary effects on their predators and prey. Although these potential impacts of climate change and variability will vary from system to system, it is important to recognize that they will be superimposed upon, and in many cases intensify, other ecosystem stresses (pollution, harvesting, habitat destruction, invasive species, land and resource use, extreme natural events), which may lead to more significant consequences.

High-latitude oceanic variability associated with the 18.6-year nodal tide

Ocean temperatures in the upper 250 m in the northern North Pacific (60°N, 149°W) increased by more than 1°C from 1972 to 1986 but are now decreasing. Subsurface temperature anomalies are well correlated (∼0.58) with the air temperature anomalies at Sitka, Alaska; hence the coastal air temperatures can be used as a proxy data set to extend the ocean temperature time series back to 1828. Up to 30% of the low-frequency variance can be accounted for with the 18.6-year nodal signal. Additionally, spectral analysis of these air temperature variations indicates a significant low-frequency peak in the range of the 18.6-year signal. Similar low-frequency signals have been reported for Hudson Bay air temperatures since 1700, for sea surface temperatures in the North Atlantic from 1876 to 1939, and for sea level in the high-latitude southern hemisphere. The water column temperature variations presented here are the first evidence that the upper ocean is responding to this very long period tidal forcing. An enhanced high-latitude response to the 18.6-year forcing is predicted by equilibrium tide theory, and it should be most evident at latitudes poleward of about 50°. These low-frequency ocean-atmosphere variations must be considered in high-latitude assessments of global climate change, since they are of the same magnitude as many of the predicted global changes.

Circulation of Prince William Sound, Alaska

The circulation of Prince William Sound, Alaska, is described using hydrographic (1974–1989), current meter (1977–1979), and acoustic Doppler current profiler (1986–1990) observations from both the sound and the adjacent Gulf of Alaska. Ancillary data include data for winds, freshwater runoff, and satellite-tracked drifters. Prince William Sound is a small inland sea in that it is wide enough to have appreciable horizontal cyclonic circulation. It is also a fjord in that it has basin depths to 700 m but is stilled at 180 m to the open ocean. The general circulation pattern is defined by a portion of the westward flowing Alaska Coastal Current on the Gulf of Alaska shelf that enters Prince William Sound through Hinchinbrook Entrance and transits the sound from east to west before exiting through Montague Strait and rejoining the coastal current. However, there is much variability in this circulation, especially in the transport through Hinchinbrook Entrance. In addition, some of the water entering the sound becomes involved in the cyclonic circulation in the northern sound and so has a longer residence time. The circulation is strongly mediated by seasonal and interannual variations in winds and freshwater runoff as well as by local topography both inside and outside the sound. In winter, the strong cyclonic winds over the Gulf of Alaska cause coastal downwelling and strong flow in the upper layers into Prince William Sound through Hinchinbrook Entrance and out through Montague Strait. In summer, the downwelling ceases, allowing subsurface denser water to rise above the sill and flow into the sound through the bottom layers of Hinchinbrook Entrance. We conclude that the best transport data came from Montague Strait, from which we estimate that ∼40% of the volume of Prince William Sound is flushed in summer (May–September). This estimated volume rises to about 200% in winter (October–April).

On the Effect of Precipitation and Runoff on Coastal Circulation in the Gulf of Alaska

Abstract Surface waters in the Gulf of Alaska undergo a net dilution throughout most of the. year since the regional precipitation exceeds evaporation. Recent hydrographic data give evidence that seasonal dynamic height fluctuations in the upper layers ( 130 cm year−1), runoff, longshore accumulation of fresh water around the gyre, and the low water temperatures. The coastal sea level is in phase and has nearly the same amplitude as the local dynamic height, though not in phase wit...

Circulation and hydrography in the northwestern Gulf of Alaska

Abstract Hydrography and satellite-tracked drifters from the Gulf of Alaska Recirculation Study (GARS) were used to describe the regional circulation from 1986 to 1989 in the northwest Gulf of Alaska. The average baroclinic transport (0/1000 db) from six occupations of a section across the Alaska Stream near the Shumagin Islands (55°N, 160°W) was 7.4 Sv. The seasonal variation in the transport of the Alaska Stream was negligible relative to the seasonal variation of the Sverdrup transport in the Gulf of Alaska as calculated from the wind-stress curl. However, the mean transport agreed with mean annual Sverdrup transport. Anticyclonic mesoscale eddies frequently appeared in the dynamic topography and hydrography along the easternmost sections of the cruise grid (140°W). Drifters released in the Alaska Stream offshore of Kodiak Island usually moved southwestward following the isobaths. However, the trajectories of four drifters from 1988 to 1989 described an anticyclonic meander in the Alaska Stream that propagated southwestward at about 0.022 m s−1. The hydrography confirmed the existence of the meander off Kodiak Island in April 1988. The temperature and salinity characteristics of the anticyclonic meanders and eddies indicated that the water masses at the center of the features were derived from Alaska Current water. Two mechanisms of enhanced vertical mixing in the Gulf of Alaska are suggested by the hydrography. The first one is due to wind mixing of the water column by intense winter storms and subsequent outcropping of the 26.8 δφ isopycnal surface in the center of the Alaska Gyre: a mechanism originally proposed by Van Scoy et al. (Journal of Geophysical Research, 96, 16,801–16,810, 1991). The second one is associated with fine structure in the temperature and salinity profiles centered at a density of 26.8 δϑ, the approximate density of North Pacific Intermediate Water. These mechanisms freshen the water on the 26.8 δgj isopycnal surface. Subsequent lateral mixing on isopycnal surfaces by mesoscale eddy activity may contribute to the low salinity signature of the North Pacific Intermediate Water.

Seasonal variations of waters in the northern Gulf of Alaska

Abstract Oceanographic data from the northern shelf of the Gulf of Alaska from December 1970 to October 1972 give evidence of an annual cycle in the water mass characteristics to depths in excess of 250 m. Minimum sea surface temperatures were less than 2°C in spring (March–April) with summer maxima in excess of 11°C. The vertical temperature structure contains both warm and cold water cores. Seasonal variations of sea surface salinitis, in excess of 7%, decrease rapidly with distance from the coastline. A southwestward surface current of slightly less than 1 km h −1 was observed and calculated geostrophically. Variations in the temperature and salinity of the nearbottom waters on the continental shelf indicate that surface heating and dilution are not the only mechanisms significantly influencing the water masses in the region. The dominant process is the annual change in Ekman transport caused by the shift in the atmosphere from a low in winter to a summer high. Intense winter downwelling flushes the shelf with relatively cold, low salinity waters, whereas weak summer upwelling brings warm, more saline waters on to the shelf.

Interdecadal variability of Northeast Pacific coastal freshwater and its implications on biological productivity

Abstract The coastal freshwater discharge along the northern Gulf of Alaska has been determined using a simple hydrology model for 1931–1999, and through the use of autocorrelative and spectral techniques oscillations were discovered with significant periods of 0.5, 1, 1.2 and 16–20 years. Changes in the freshwater discharge are well correlated with hydrographic properties, namely temperature and salinity, at a coastal site near Seward, Alaska. Changes in the salinity should change the vertical stability, which will affect the mixed layer depth and primary production. Changes in the mixed layer depth concurrent with changes in phytoplankton production may provide a link between zooplankton and freshwater discharge. This is supported by periodicities of 0.5, 1 and 1.2 years that have been found in the zooplankton at Ocean Station P. A positive atmosphere–ocean feedback loop is proposed that could maintain accelerated coastal freshwater discharge at periods similar to those seen in the Pacific Decadal Oscillation (PDO). This could provide a mechanism that links the PDO with coastal freshwater discharge and consequently relates coastal freshwater discharge to salmon production in Alaska, since the latter depends on zooplankton abundance.

Observations of the Alaska Coastal Current

The Alaska Coastal Current is a narrow, intense coastal current hordering the southern coast of Alaska. It is characterized by relatively low salinities and its transport is significantly influenced by the regional freshwater discharge. The flow is also modified hy the winds which cause a downwelling condition here throughout most of the year. In many aspects, this current is similar to the Norwegian Coastal Current, with the important exception that wind-driven reversals have not been observed.

Tub toys orbit the Pacific subarctic gyre

In 1992, a cargo container of childrens bath toys fell overboard in the middle North Pacific Ocean. Subsequently, 29,000 toys were tracked 4,000 kilometers to southeastern Alaska [Ebbesmeyer and Ingraham, 1994]. The spills upcoming fifteenth anniversary has prompted an examination of the reports of toys stranded on shorelines around the Subarctic Gyre, a planetary vortex the size of the United States. Previous articles have reported the drift of sneakers and toys for a year or so only along the southern edge of the Pacific Subarctic Gyre [Ebbesmeyer and Ingraham, 1992, 1994]. However, continuing reports of stranded toys have stimulated curiosity about how long it would take the currents that link the Gyres perimeter between Asia and America to transfer flotsam around the Gyre, that is, its orbital period. These currents (Figure 1) are the North Pacific Drift Current, Alaska Current/ Alaska Coastal Current, Alaskan Stream, Bering Slope Current and East Kamchatka Current, Oyashio Current, and Kuroshio. In the Bering Sea, the North Aleutian Current recurves north from Attu Island eastward along the north side of the Aleutians to merge with the Bering Slope Current.

Circulation in the Gulf of Alaska, 1981

Abstract Upper layer ( The cause for this circulation change are not definite but we suggest that the dislocation involves the interaction of wind stress curl (Sverdrup transport) with the bottom topography. Eastward of 155°W many seamount are located northward of the usual axis of the zonal North Pacific Current east of 155°W. This eastward flowing current could be deflected slightly northward by the wind stress curl. Subsequent shoaling could cause a further deflecction of the flow to the north and west, resulting in the displacement of the subartic gyre.