water from the Bering Sea basin Shelf edge. Advection is small 

 over the whole central shelf with water depths between 50 and 

 100 m (Coachman, 1986), so this water mass, intermediate in 

 salinity and with long residence times, is most strongly 

 conditioned by climatic factors such as brine rejection due to 

 freezing and degree days of frost (see discussion in Coachman, 

 1986). Furthermore, Bering Shelf water in the area immediately 

 south of St. Lawrence Island is directly influenced by special 

 freezing conditions associated with the large polynya always 

 found on its south side (Schumacher et al., 1983). In the area 

 of this polynya, the freezing over-winter of the equivalent of 8 

 to 10 m of ice causes a substantial increase in salinities of the 

 shallow watercolumns. Thus increased in density, some of this 

 water moves away to the southwest (in the direction of 

 deepening) into the central Gulf of Anadyr, where it can be 

 distinguished as a separate water mass we call Gulf of Anadyr 

 water (Fig. 6, IV); this is in fact the "cold center" water of 

 Barnes and Thompson because its temperatures over the summer 

 normally remain close to freezing (<- 1 .5°C) and are the coldest 

 observed anywhere on the whole Bering Sea Shelf. 



Most of the year. Bering Shelf water (Fig. 6, II) moves 

 north around both ends of St. Lawrence Island and then 

 occupies the middle area between Alaskan Coastal and Anadyr 

 Current waters. In late summer, however, some years Alaskan 

 Coastal water expands to nearly fill Shpanberg Strait, as in late 

 August 1988, and northward transport of Bering Shelf at these 

 times is predominately through Anadyr Strait (Fig. 6). North 

 of Bering Strait, Anadyr Current water becomes so blended 

 with the shelf water it loses identity. Across the Chukchi Sea 

 we continue to identify this water mixture as Bering Shelf 

 water , because salinities are little altered by the admixture of 

 Anadyr water, and the name connotes its basic origin. 



In the Chukchi Sea occurs another water mass, Siberian 

 Coastal water (Fig. 6. V), identified by values of salinity 

 greater than any entering the sea through Bering Strait 

 contemporaneously. For example, in August 1988, the 

 maximum observed S in Anadyr Current water was <33.0, 

 while salinities ofSiberian Coastal water were up to 33.6. This 

 water mass is associated with the Siberian Coastal Current. 



Though lateral mixing is in general quite small in this 

 regime of strong advection, and relatively discrete boundaries 

 obtain between the water masses (transitions between two 

 water masses are typically complete in <10 km), there is some 

 lateral interaction. This almost always takes the form of 

 layering, the slightly heavier water mass on one side encroaches 

 under the neighboring water, which fonns a lighter surface 

 water layer; or, frequently, the lighter water mass is driven by 

 wind over the heavier. 



Layering varies seasonally. In winter and spring there is 

 practically none; all the shallow watercolumns are well mixed. 

 Nomially only in deeper areas like the central Gulf of Anadyr 

 does a layered structure survive the winter cooling and freezing. 

 Layered water columns appear with the advent of freshwater 

 accumulation and some seasonal warming, usually late June 

 and July, and is most widespread at the end of summer before 



fall cooling begins. The extent and degree of layering observed 

 in August 1988 (Fig. 7) is typical for late season. We make the 

 following points and interpretations: 



/. Strong layering is typical of the boundary between 

 Alaskan Coastal and Bering Shelf water, particularly in the 

 Chirikov basin (cf Fig. 6). 



2. Layering is minimal in very shallow near-shore waters, 

 (e.g., in eastern Kotzebue Sound). 



3. Both the Gulf of Anadyr and Siberian Coastal are 

 basically layered water masses. In the Gulf of Anadyr depths 

 in the central part deepen to 100 m (Fig. 1 ). Here the very cold 

 water of the "cold center," with slightly enhanced salinities, 

 resides beneath Bering Shelf water; water columns are 

 sufficiently deep that the layering survives rigorous winter 

 cooling and freezing. During the summer along the Siberian 

 coast in the Chukchi Sea, runoff and ice melt create a very light, 

 low salinity surface layer over the high salinity water at bottom; 

 both layers are part of the Siberian Coastal Current. 



4. Minimum stratification, even in late summer, is always 

 observed directly downstream from Anadyr and Bering Straits, 

 a consequence of turbulent energy generated in these 

 constrictions. 



We can now positively show that all waters of the ecosystem 

 derive from a single source, the water of the Bering Slope 

 Current of the northern Bering Sea. During the Third Joint US- 

 USSR Bering & Chukchi Seas Expedition, samples from all the 

 water masses were analyzed for "O heavy oxygen isotope. 

 Two factors make these measurements diagnostic for water 

 mass analysis: 



70' 



SALINITY LAYERING 

 S130m) - SlOml. %o 

 AUGUST 1988 



°I80 



175° 



170° 



165° 



160° 



Fig. 7. 



Salinity layering (S at 30 m minus S near-surface) in August 1988. 

 The distribution is typical: strong layering in the central Gulf of 

 Anadyr (deeper water), along the boundary between Bering Shelf and 

 Alaskan Coastal in the Chirikov basin, and in the Siberian Coastal 

 Current. Very little layering in shallow water near Alaska, and in two 

 plumes extending downstream from Anadyr and Bering Straits. 



20 



