Physical Oceanographic Setting

The North Pacific is the location of one of the major storm tracks in the Northern Hemisphere. Simulation models suggest that the southern side of the Arctic front will be the region of greatest alteration due to global climate change. The storm track responds to two global teleconnection patterns: 1) the West Pacific oscillation that influences the location of storm generation; and, 2) the Pacific-North American (PNA) pattern that influences the track of storms across the subarctic Pacific. The PNA pattern is often considered the major mode of planetary variability of the atmosphere. Any systematic shifts that occur will be modulated by the large natural variability that exists on time scales from seasonal to millennia. This variability has a profound impact on circulation, mixed layer depths and the extent of ice coverage, all of which influence the rich biological resources of the subarctic Pacific and Bering Sea.

Figure 1 shows the climatological mean circulation patterns of the subarctic Pacific based on geostrophic flow (e.g., Reed, 1984; Reed et al., 1993), and direct current measurements (Stabeno and Reed, 1994; Schumacher and Kendall, 1995; Schumacher and Stabeno, in press). The values of velocity given are estimates of typical flow. In the swifter currents, peak speeds can be substantially larger than the values given.

Oceanic conditions in the Bering Sea are also influenced by the extent of ice cover (Fig. 2). During extreme conditions, ice covers the entire eastern shelf, however interannual variability of coverage can be as great as 40% (Niebauer, 1988). The buoyancy flux from melting ice initiates both baroclinic transport along the marginal ice zone and stratification.

Evidence of decadal-scale variability in climate conditions and regime shifts is prevalent in the North Pacific and Bering Sea. The climate of the Subarctic Pacific changed during the late 1970s. The Aleutian Low intensified (Trenberth and Hurrell, 1994) and coastal sea surface temperatures rose rapidly by several degrees (Rogers and Ruggerone, 1993; Royer, 1989; Graham, 1995). The most recent shift occurred in the late 1970s.

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