Overview of Physical Oceanographic Features

The following description of the physical and biological features of the three study regions provides a setting for subsequent discussions of the linkages between climate variability and ecosystem response.

Physical Oceanographic Setting

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 Domain

The swiftest flow exists in the Kuroshio Current, but peak speeds (>100 cm s-1) in the Alaskan Stream and the Kamchatka Current (Stabeno and Reed, 1991, 1994) are at least half those off Japan. The Kuroshio Current extension retains appreciable speeds, and the mixture of this water with the Oyashio Current is the broad slow Subarctic Current. The Subarctic Current is affected more by wind drift than any other flow in this region. It has geostrophic speeds that are usually ~ 5 cm s-1, but winds blow in the same direction and augment speeds, especially during winter (McNally et al., 1983). The Subarctic Current diverges off the west coast of North America near Vancouver Island. The southward flow is the California Current, while the remainder turns northward into the Gulf of Alaska. As this flow leaves the head of the Gulf, it deepens, narrows, and intensifies. This westward flow, known as the Alaskan Stream, continues along the Aleutian Islands, with the majority (above 2000 m) entering the Bering Sea through Near Strait (170°E) (Figure 2). Although flow of Alaskan Stream water through both Amchitka and Amukta Pass is relatively small in volume, it strongly influences water properties and circulation in the eastern Bering Sea (Schumacher and Stabeno, 1994; Reed and Stabeno, 1994; Reed, 1995).

The Bering Slope Current (BSC; Kinder et al., 1975, 1986; Schumacher and Reed, 1992) has a transport of 3-5 x 106 m3 s-1, and contains Alaskan Stream water that flows through Amchitka and Amukta Passes and then eastward along the northern side of the Aleutian Islands. The BSC influences slope water properties (Schumacher and Stabeno, 1994; Reed and Stabeno, 1994) which flux onto the outer continental shelf. A subsurface temperature maximum is a characteristic of the southern Bering Sea. It is >4°C when the Alaskan Stream flows northward through Amukta Pass (Schumacher and Reed, 1992). Eddies are a common feature in the southeastern Bering Sea; some are formed by flow through Amukta Pass (Schumacher and Stabeno, 1994).

The northwestward flow of the BSC (3-15 cm s-1) along the eastern shelf break is concentrated in the upper 300 m (Schumacher and Reed, 1992: Muench and Schumacher, 1985). Although wind energy approximately doubles in winter, kinetic energy of the current fluctuations and the vector mean currents do not; only a small fraction of the current fluctuations measured here can be accounted for by the wind. Small (5-30 km) eddies with strong (20-30 cm s-1) rotational speeds are features of the current regime. Estimates of salt fluxes indicate some significant shoreward transport (Schumacher and Reed, 1992).

The BSC separates from the slope near 58°N; it then flows across the basin. This flow is the main source of the Kamchatka Current (Stabeno and Reed, 1994: Khen, 1989). Much of the remaining BSC flow likely recirculates over the basin (Overland et al., 1994). The Kamchatka Current, which originates near 175°E and exhibits both strong speed (40-100 cm s-1) and numerous meanders and eddies, dominates circulation off the western shelf of the Bering Sea (Stabeno et al., 1994: Cokelet et al., in press). This current exits the basin through Kamchatka Strait.

The Kamchatka Current forms the western boundary and the Bering Slope Current (BSC) the eastern boundary of the cyclonic gyre in the Bering Sea (Reed et al., 1993). This gyre is mainly an extension of the Alaskan Stream, with the majority of volume transport entering through the deeper western passes of the Aleutian Islands (Near Strait and Amchitka Pass) and exiting via the Kamchatka Current (Stabeno and Reed, 1994). Occasionally, the Alaskan Stream does not flow into the Bering Sea through Near Strait (Stabeno and Reed, 1992) which results in a reduction of transport (by ~50%) in the Kamchatka Current (Verkhunov and Tkachenko, 1992). After a disrupted or weak inflow that started in late 1990, normal flow resumed in early 1992 (Reed and Stabeno, 1993). A numerical study (Overland et al., 1994) suggests that flow instabilities, both in the Alaskan Stream and within the basin, contribute to substantial interannual variability in the circulation.

A climatology of the wind forcing shows that eastward and northward-propagating storm systems dominate the surface stress at short periods (<1 month), which serves principally to mix the upper ocean (Bond et al., 1994). At longer periods (> months), the wind-driven transports account for roughly one-half of the observed transport within the Kamchatka Current. The interannual variations in the Sverdrup transports are ~25% of the mean.

Coastal Currents of the Gulf of Alaska

A separate coastal current, the Alaska Coastal Current, exists inshore of the Alaskan Stream, extending from regions south of Prince William Sound to Unimak Pass. This is one of the most vigorous coastal currents in the world with speeds from 25 to 100 cm s-1 (Stabeno et al., 1995). The transport results from the addition of freshwater along the entire coastline forced by alongshore winds (Schumacher and Reed, 1980; Royer, 1981; Reed and Schumacher, 1981). The observed mean transport in Shelikof Strait is 0.85 x 106 m3 s-1 ; wind forced pulses exceed 3.0 x 106 m3 s-1 (Schumacher et al., 1989: Stabeno et al., 1995). Estimates of volume transport computed from water property observations collected between 1985 and 1992 have a mean of 0.66 x 106 m3 s-1 (Reed and Bograd, 1995). This current plays a central role in survival and recruitment processes of walleye pollock in Shelikof Strait through both transport and generation of eddies (Schumacher and Kendall, 1995) (Figure 3).

The Alaska Coastal Current flows through Unimak Pass (~0.3 x 106 m3 s-1). Some turns eastward resulting in a current along the coastline (2-5 cm s-1) with the remainder flowing toward the northwest along the 100-m isobath (Reed and Stabeno 1994). The coastal flow (< 3 cm s-1) follows the 50-m isobath turning northward along near 58°N. The coastal current has a seasonal pattern in strength, with flow increasing in winter concomitant with increased storm activity (Schumacher and Kinder, 1983).

Coastal Currents of the Bering Sea

Over much of the middle shelf of the Bering Sea, weak and/or statistically insignificant mean currents exist. Climatology of water properties and Lagrangian measurements reveal a convoluted flow eastward across this domain (Stabeno and Reed, 1994). This advective feature has not yet been incorporated in the salt, heat and nutrient flux models.

Over the outer-shelf domain of the Bering Sea, flow (5-10 cm s-1) containing water from both Unimak Pass and the slope follows the 100-m isobath toward the northwest. Near the shelf break, the inshore edge of the Bering Slope Current results in stronger (10-20 cm s-1) currents. Like the coastal flow, much of this flow is baroclinic. While pulses of cross-shelf flow (5-10 cm s-1) occur, their inherent variability precludes establishing a mean from existing moored current data. Propagation of eddies onto the shelf provides one possible mechanism for generation of pulses of cross-shelf current.

The northward mean transport through Bering Strait is driven by the surface height difference between the Pacific and the Arctic Ocean and modified by the wind. Strong (5 x 106 m3 s-1) wind-driven reversals occur mainly during winter. During summer, maximum northward transport exceeds 3 x 106 m3 s-1. An annual mean of 0.8 x 106 m3 s-1 has been estimated (Coachman, 1993). Transport through Shpanberg Strait provides about one-third of the northward transport through Bering Strait.

It has been hypothesized that the northward mean flow over the shelf undergoes westward intensification as water column depth decreases from south to north (Kinder et al., 1986). Flow through Anadyr Strait (15-40 cm s-1) provides about two-thirds of the transport through Bering Strait (Coachman, 1993). The current in Anadyr Strait is more stable in strength and location than that observed in Shpanberg Strait. Some of the flow continues eastward along the south coast of St. Lawrence Island and then turns northward, joining the coastal flow through Shpanberg Strait.

The suggested coastal flow from the Gulf of Anadyr westward past Cape Navarin (Overland and Roach, 1987) is supported by water property distributions. At the mouth of the Anadyr River, low salinities (~10 psu) occur during summer (Favorite et al., 1976). Dilute (<31.0 psu) surface waters, whose origin lies in the Gulf of Anadyr (Verkhunov and Tkachenko, 1992), exist over the shelf between Capes Navarin and Olyutorski. Satellite-tracked buoys (drogue at 40 m) transited here at speeds of ~40 cm s-1 (Stabeno and Reed, 1994). Buoy observations from the Gulf of Olyutorski show the strong (30-50 cm s-1) flow of the Kamchatka Current over the slope, while more moderate (15-25 cm s-1) speeds occur over the adjacent shelf. These observation suggest a coastal current over the western shelf.

In both the Gulf of Olyutorski and Karaginski Bay, quasi-stationary eddies exist, and similar features also occur in embayments along the Kamchatka coast (Stabeno et al., 1994; S. Gladyshev, unpublished manuscript). Satellite-tracked buoy trajectories substantiate such a feature in the bay centered at 54°N. Generation of these features likely results from meanders in the inshore flow interacting with topography and/or formation of saline lenses due to brine expulsion (Verkhunov, 1994).

Sea Ice

Oceanic conditions in the Bering Sea are influenced by the extent of ice cover (Figure 4). 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 (~0.3 x 106 m3 s-1; Muench and Schumacher, 1985) and stratification. The ensuing ice edge bloom of phytoplankton accounts for between 10% and 65% of the total annual primary production (Niebauer et al., 1990). The nutrient-rich slope waters combine with summer solar radiation to create one of the world's most productive ecosystems (Walsh et al., 1989). Annual primary production varies from >200 gC m-2 over the southeastern shelf to >800 gC m-2 north of St. Lawrence Island. Over the western shelf, ice cover extends southwestward to Cape Kamchatka and seaward over the slope (Khen, 1989).

Ice production and cold bottom water exert an important influence on distributions of biota over both the western (Radchenko and Sobolevskiy, 1993) and eastern (Ohtani and Azumaya, 1995: Wyllie-Echeverria, 1995) shelves. The production of dense water has a marked impact on the halocline of the Arctic Ocean (Cavalieri and Martin, 1994), with water from the Anadyr and Anadyr Strait polynyas providing a substantial fraction of the total dense water. From 9 to 25 m of ice formation occurs depending on the location and duration of a given season (Cavalieri and Martin, 1994), but the average thickness of ice over most of the eastern shelf is only ~0.5 m (Coachman, 1986).