Frontal zones in the Bering

The Bering Sea is a northward extension of the Pacific Ocean between Siberia and Alaska. Separated from the Pacific Ocean by the Aleutian Islands, it is connected with the Arctic Ocean via the Bering Strait. The general circulation in the Bering

Sea is complicated and variable due to the interaction of wind, water inflow through the Aleutian Islands, tides, bottom relief and other factors (Fig. 7). The main feature of the circulation is a general cyclonic motion in the deep basin, the so-called Bering Sea Gyre. It is composed by the North Aleutian Slope Flow (the eastward motion of water incoming through the Aleutian Islands via Near Strait, Amchitka, Buldir and Amukta Passes), the Bering Slope Current flowing northwestwards and the Kamchatka Current flowing southwards.

Bering Sea Circulation
Fig. 7. Composite surface circulation in the Bering Sea derived from diagnostic calculations. (Adapted from Terziev 1999.)

Dodimead et al. (1963) were probably the first to indicate the internal front which separates coastal and middle shelf water masses in Bristol Bay, and the front along the shelf break. Major oceanographic studies in the Bering Sea by PROBES (Processes and Resources of the Bering Sea Shelf), OCSEAP (Outer Continental Environmental Shelf Assessment Program) and other programs focused on the southeastern shelf area, this is why this region is the best documented (see list of references in Kostianoy et al. [2004]). Kinder and Coachman (1978), Schumacher et al. (1979), Coachman et al. (1980), Kinder and Schumacher (1981) described frontal systems in the eastern Bering Sea. Iverson et al. (1979) gave a description of fronts in this region based on results of PROBES, which became a classical one for the southeastern Bering Sea. Information about the fronts in other parts of the sea can be found in several publications, but a synoptic view of the whole frontal system of the Bering Sea based on CTD observations and/or satellite imagery, and its variability is absent (see also Belkin and Cornillon [2003]).

A systematization of the frontal zones and fronts in the Bering Sea is given below based on information accumulated from publications, satellite imagery and SST data, as well as from CTD observations and numerical modeling performed during the ISHTAR Program.

1. The Bering Slope Front is a basic frontal feature extending some 1,000 km along the continental slope of the Bering Sea year-round. The highest contrasts across the front are observed from January to June. It is associated with the Bering Slope Current, concentrated in the upper 300 m, which has velocities of 5-15 cm/s and a transport of 3-5 Sv. It is more correct to call this front a frontal zone, because of its spatial structure, comprising meanders and eddies, and permitting intense cross-shelf water exchange. The Bering Slope Frontal Zone is responsible for the "Green Belt" associated to it. The concept of the Bering Sea Green Belt (a highly productive habitat along the edge of the continental shelf) is based upon compelling observations of a variety of physical and biological features acquired from many sources over many years (Springer et al. 1996).

2. The Inner, Middle and Shelf Break Fronts in the eastern Bering Sea. The balance between tidal and wind mixing and buoyancy flux (freshwater discharge, ice melting, insolation) results in three distinct hydrographic and biological domains. In the coastal domain (z < 50 m, 200 km large) tidal and wind mixing usually overlap, resulting in a weakly stratified or mixed water column. In the middle shelf domain (50 < z < 100 m, 150 km large) the overlap between the near surface and bottom mixed layers is limited, and during weak wind mixing in summer, a two-layered vertical structure is formed. The outer shelf domain (100 < z < 180 m, 120 km large) has separate mixed upper and bottom layers. The domains are delimited by the Inner (10 km wide), Middle (50-75 km wide), and Shelf Break (50 km wide) Fronts, which follow the 50, 100, and 200 m isobaths, respectively.

3. Coastal fronts. A system of three hydrological zones exists over the western shelf, which is much narrower than the eastern one. These hydrological zones, coastal, transitional and oceanic, are easily distinguished by thermohaline characteristics. The coastal zone has low-salinity (<31.0 psu) surface water and a strong pycnocline. The transitional zone is characterized by a weak two-layered structure. The oceanic zone is identified by a three-layered vertical structure with relatively warm bottom temperatures, indicating the presence of slope waters. The location of these zones, and the coastal fronts separating them, are not as stationary as at the eastern shelf, because of the Kamchatka Current, a strong boundary current which determines the water dynamics at the western shelf and slope. The Kamchatka Current itself has a front on its eastern periphery, which is more pronounced southward of the Kamchatka Strait.

4. Freshwater fronts, originating from river discharges into coastal areas of the ocean, are divided in two types. According to Fedorov (1986), if the river has an estuary, the latter can have its own estuarine front independently of the front which encloses the lens of freshened waters in the open sea, and which he referred to as the river discharge front. In large estuaries, tidal mixing can be so intensive that river discharge fronts may not be present at all beyond the estuary. In the Bering Sea, July is a month of maximum freshening of the surface layer in the bays and sounds, caused by maximal river discharges in June-July. Main river runoffs in the Bering Sea are from the Yukon (45% of the total discharge), Anadyr (20%), Kuskokwim (12%), Nushagak (3%) and Kvichak Rivers. Freshwater plumes and related coastal fronts (river discharge fronts of the Anadyr and Yukon Rivers) are perfectly visible on the IR satellite images due to a strong SST contrast. Estuarine fronts are perfectly identified by sharp salinity and thermal contrasts related to general freshening in the Gulf of Anadyr, Norton Sound, Bristol and Karaginskiy Bays.

Fig. 8. Cartoon of the ecohydrodynamics of the Bering Sea. The figures in the circles are the total (ammonium, urea and nitrate) integrated nitrogen uptake rates (mg-at N m-2 h-1) during the summer 1987.

5. The Date Line Front. The thermohaline characteristics of the Anadyr Stream (33 psu) contrast with the Alaskan Coastal Current transporting low-salinity (<32.1 psu), seasonally warm coastal water northwards along the western coast of Alaska and through the eastern channel of the Bering Strait. These waters are separated by the Date Line Front passing from St. Lawrence Island to the Bering Strait, and farther northward (Fig. 8). Experimental evidence of the Date Line Front is found in the remote sensing images of the northern Bering Sea (Nihoul 1986) and in several field surveys (Coachman et al. 1975).

6. Upwelling fronts. The strongest upwellings/upsloping areas are located along the Siberian coast (around Capes Chukotskiy and Dezhnev) and the eastern coast of St. Lawrence Island, where vertical velocities as high as 4 m day-1 can be found. CTD casts in the Anadyr Strait made during the Akademik Korolev cruise (ISHTAR Programme) in August 1988 demonstrated coastal upwelling with a front located in the middle of the strait (Nihoul et al. 1993). SST contrasts across the surface front reached 4oC. The depth of the front was 30 m. This upwelling is maintained by the Anadyr Stream together with favorable winds, and seems to disappear very rarely, for short period of time, presumably when it is blocked by strong southward blowing winds and/or reversal currents in the Bering Strait. Upwellings in the centers of bays due to a cyclonic circulation is a most common feature of the Bering Sea coastal dynamics (Terziev 1999). Such an upwelling in the Bristol Bay was reported by Kinder and Schumacher (1981). It is attributed to a cyclonic flow that approximately follows the 50 m isobath and lift the isopycnals and isotherms. The same feature at the entrance of Norton Sound was reported by Nihoul et al. (1993).

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