NPS model contributions to the Steller Sea Lion synthesis paper - Part I
W. Maslowski (NPS) and S. R. Okkonen (UAF)
Use of limited domain models of ocean circulation allows employing higher resolution (as compared to global ocean general circulation models (GCMs)) and focused studies of critical processes and circulation in a region. Such an approach provides means for proper representation of the complex air-sea-bottom interactions and their influence on exchanges between the North Pacific Ocean and the Bering Sea, which occurs through the straits and passes of the Steller Sea lion populated Aleutian-Komandorskii Island arc. A pan-Arctic coupled sea ice – ocean model has been developed at the Naval Postgraduate School (NPS) to improve understanding of the circulation and exchanges between the sub-arctic and arctic basins (Maslowski et al., 2004, Maslowski and Lipscomb, 2003, Maslowski and Walczowski, 2002). The model domain extends from about 30oN in the North Pacific, through the Bering Sea, Arctic Ocean, into the North Atlantic to about 45oN. The numerical grid is configured at 1/12o (or ~9 km) and 45 levels using rotated spherical coordinates. The model has been integrated for over seven decades, including a 48-year spinup and a 23-year interannual run forced with realistic daily-averaged 1979-1993 reanalyzed data and 1994-2001 operational products from the European Centre of Medium-range Weather Forecast (ECMWF). Additional information about the model setup, boundary conditions, and model results is provided by Maslowski et al. (2004) and Maslowski and Lipscomb (2003). Model output from the interannual run is used to investigate interannual-to-interdecadal variations in transport through and properties within the passes of the central and eastern Aleutian Islands.
One of the important and successfully modeled features of ocean circulation in the northern North Pacific is the Alaskan Stream, its interannual variability, and effects on the mass and property transport through the Aleutian Island passes. A comparison of transport estimates of the Alaskan Stream (Onishi, 2001; Reed and Stabeno, 1999; Roden, 1995; Warren and Owens, 1988; Reed, 1984; Favorite, 1974; Thomson, 1972) with those through the eastern and central passes (Schumacher et al, 1982; Reed, 1990; Reed and Stabeno, 1997; Stabeno et al., 1999) suggests that even small variations in the magnitude and position of the Alaskan Stream could have significant consequences on the dynamics and hydrographic conditions within and to the north of the passes. Based on available data from direct current meter measurements, the Alaskan Stream is considered to be a stable western boundary current with relatively small seasonal but large interannual variability (Reed and Schumacher, 1984; Onishi and Ohtani, 1999). Similarly, NPS model results suggest the mean total (i.e. from the surface to bottom) volume transport is over 40 Sv with 6-10 Sv variability over the 23-year mean annual cycle compared to more than 30 Sv of interannual variability during 1979-2001 (Maslowski et al., 2004 – submitted). Analyses of model output suggest that the dominant mechanism of interannual variability in volume transport is related to anticyclonic mesoscale eddies (100-250 km diameter) propagating westward along the Alaskan Stream with mean speed of a few km per day. Similar eddies have been observed from satellites (Crawford et al., 2000; Okkonen, 1992, 1996) and in field observations (Reed et al., 1980; Musgrave et al., 1992). Model simulated eddies along the Alaskan Stream have significant influence on both the circulation and water mass properties across the eastern and central Aleutian Island passes.
An example of such an influence is given in Figure X, where depth-averaged (0-100 m) velocity snapshots and salinity difference across the Amukta Pass between eddy and no eddy conditions in 1984 are shown. In March (Figure Xa) no eddy is present in the Alaskan Stream and the dominant flow in the region to the south of Amukta Pass is westward and parallel to the pass. Two months later (Figure Xb) when a mesoscale eddy enters the region, the flow of the Alaskan Stream is significantly modified down to well over 1000 m, with a strong northward velocity component into Amukta Pass and a strong southward component some 200 km to the east. Such a circulation has several implications both on the transport of Alaskan Stream and on the flow through and conditions in Amukta Pass. We focus here on those with impact on oceanographic conditions across Amukta Pass. Other aspects are discussed in detail by Maslowski et al. (2004, submitted).
The salinity difference between the model sections marked “Cross Slope” when the eddy (Figure Xb) and no eddy (Figure Xa) is present in the region is shown in Figure Xc. One of several observations that can be made from these results is that the eddy-related upwelling of salty water along the southern slope affects water column down to about 1000 m. Next conclusion is that more than 0.1 ppt salinity increase extends all the way to the surface within the Amukta Pass region when the eddy is present. This is a result of the upwelling and possibly increased mixing associated with the presence of an eddy and the stronger flow across the pass. Given a high correlation between salinity and nutrient content at depths, the increased salinity in the upper ocean over the pass can represent nutrient input for enhanced and/or prolonged primary productivity. Since modeled eddies along the Alaskan Stream occur throughout a year, their contribution to high surface nutrient concentrations within the Aleutian Island passes could be especially significant during otherwise low primary productivity seasons. This effect would be most important during years with mesoscale eddies frequently propagating along the Alaskan Stream. On the other hand, when no eddies pass along lower salinities and no extra nutrient inputs to the upper ocean occur. The eddy-driven upwelling forms a layer of increased salinity (by up to 0.4 ppt and probably higher nutrient concentrations) in the Bering Sea, by spilling over the sill and spreading northward and downward below 100 m. Finally, to balance salinity increase over the slopes, there is a corresponding freshening of the upper water column away and on each side of this region. The strongest salinity decrease (up to 2.0 ppt) is simulated on the North Pacific side and it is concentrated in the upper 400 m. Much smaller magnitude freshening (0.3 ppt) is modeled in the upper 100 m on the Bering Sea side. It is also worth to note that in the end an overall net increase of salinity in the upper water column is experienced within the region adjacent to Amukta Pass after the eddy moves further to the west. In summary, strong evidence exists for mesoscale eddy activity along the Alaskan Stream and for their contribution to circulation and hydrographic variability across Aleutian Island passes. However, long-term observations and more modeling studies of the Aleutian Island passes are needed to fully understand impacts of eddies, tides, and wind forcing on the biological environment not only related to primary productivity but also to higher trophic levels including Steller Sea Lions.
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