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.
a)
b)
Cross Slope
Cross Slope
Amukta Pass
Reference Vector
-1
10 cm s
Amukta Pass
Reference Vector
-1
10 cm s
c)
Figure X. Depth-averaged (0-100 m) velocity (cm/sec) snapshots near the Amukta Pass from the end of (a) March and
(b) May of 1984 showing effects of a mesoscale eddy modeled within the Alaskan Stream on the flow across
the Amukta Pass. The green solid contours represent bathymetry (m). (c) The salinity difference (ppt) along the
section marked "Cross Slope" between eddy (May) and no-eddy (March) conditions.
References
Crawford, W.R., J.Y. Cherniawsky, and M.G.G. Foreman, 2000: Multi-year meanders
and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter. Geophys.
Res. Lett., 27(7):1025-1028.
Favorite, F., 1974: Flow into the Bering Sea through Aleutian Island passes. In: D.W.
Hood and E.J. Kelley (eds.), Oceanography of the Bering Sea with emphasis on
renewable resources. Occasional Publication No. 2, Institute of Marine Science,
University of Alaska, Fairbanks, pp. 59-98.
Maslowski, W., J. L. Clement, S. R. Okkonen, P.J. Stabeno, and W. Walczowski, 2004:
Modeling of ocean circulation and property transport from the Alaska Stream through eastern
Aleutian Island passes: 1979-2001. Fisheries Oceanography, submitted.
Maslowski, W., Marble, D., Walczowski, W., Schauer, U., Clement, J. L., Semtner, A. J., 2004,
On climatological mass, heat and salt transports through the Barents Sea and Fram Strait from a
pan-Arctic coupled ice-ocean model simulation, J. Geophys. Res., in print.
Maslowski, W., W.H. Lipscomb: High-resolution Simulations of Arctic Sea Ice During 19791993, Polar Research, 22, 67-74, 2003.
Maslowski, W., W. Walczowski: On the circulation of the Baltic Sea and its connection to the
Pan-Arctic region – a large scale and high-resolution modeling approach, Boreas Environmental
Research, 7, 319-325, 2002.
Musgrave, D.L., T.J. Weingartner, and T.C. Royer. 1992. Circulation and hydrography in the
northwestern Gulf of Alaska. Deep-Sea Res. 39:1499-1519.
Okkonen, S.R. 1996. The influence of an Alaskan Stream eddy on flow through
Amchitka Pass. J. Geophys. Res., 101:8839-8851.
Okkonen, S.R., 1992: The shedding of an anticyclonic eddy from the Alaskan Stream as
observed by the Geosat altimeter. Geophys. Res. Lett., 19(24), 2397-2400.
Onishi, H., 2001: Spatial and Temporal Variability in a Vertical Section across the
Alaskan Stream and Subarctic Current. J. Oceanogr., 57, pp. 79-91.
Onishi, H., and K. Ohtani, 1999: On seasonal and year to year variation in flow of the
Alaskan Stream in the central North Pacific. J. Oceanogr., 55, 597-608.
Reed, R.K., 1990. A year-long observation of water exchange between the North Pacific
and Bering Sea. Limnol. Oceanogr., 35:1604-1609.
Reed, R.K., 1984: Flow of the Alaskan Stream and its variations. Deep-Sea Res., 31, 369386.
Reed, R.K., R.D. Muench, and J.D. Schumacher, 1980: On baroclinic transport of the
Alaskan Stream near Kodiak Island. Deep-Sea Res., 27, 509-523.
Reed, R.K., and J.D. Schumacher, 1984: Additional current measurements in the Alaskan
Stream near Kodiak Island. J. Phys. Oceanogr., 14, 1239-1246.
Reed, R.K. and P.J. Stabeno. 1997. Long-term measurements of flow near the Aleutian
Islands. J. Mar. Res., 55:565-575.
Reed, R.K. and P.J. Stabeno. 1999. A recent full-depth survey of the Alaskan Stream. J.
Oceanogr., 55, 79-85.
Roden, G.I., 1995: Aleutian Basin of the Bering Sea: Thermohaline, oxygen, nutrient,
and current structure in July 1993. J. Geophys. Res., 100, 13539-13554.
Schumacher, J.D., C.A. Pearson, and J.E. Overland. 1982. On exchange of water between
the Gulf of Alaska and Bering Sea through Unimak Pass. J. Geophys. Res., 87:57855795.
Stabeno, P.J., J.D. Schumacher, and K. Ohtani. 1999. The physical oceanography of the
Bering Sea. In: T.R. Loughlin and K. Ohtani (eds.), Dynamic of the Bering Sea.
University of Alaska Sea Grant Publication AK-SG-99-03, Fairbanks. pp. 1-28.
Thompson, R.E., 1972: On the Alaskan Stream, J. Phys. Oceanogr., 2, 363-371.
Warren, B.A., and W.B. Owens, 1988: Deep currents in the central Subarctic Pacific
Ocean. J. Phys. Oceanogr., 18, 529-551.