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Spatial distribution and seasonal and interannual variability are determined for parameters of the upper mixed layer and seasonal thermocline in the western deep basin of the Bering Sea in summer and fall of 2002-2013 and compared with chlorophyll- a concentration at the sea surface. The mixed layer is formed by solar heating of the sea surface and wind mixing. Its thickness doesn’t exceed 10-15 m in summer but increases up to 25-40 m in fall. The thermocline becomes deeper from summer to fall, too, in particular its lower bound that sinks from 37 to 48 m, on average; however, its thickness decreases from 20-25 to 16-22 m and temperature and density gradients increase from 0.23-0.25 to 0.37-0.45 °C/m and from 0.032 to 0.062 units/m, respectively, that means that the thermocline becomes steadier. Tendency to cooling was observed for both layers in the last decade, the process was more visible in autumn, with a shift at the upper bound of thermocline (from the mean temperature of 9.8 °C in 2002-2006 to the mean temperature of 8.4 °C in 2007-2013) and gradual decreasing at its lower bound (from 3.3 °C in 2003 to 2.1 °C in 2013), and not so obvious in summer. Year-to-year fluctuations of salinity in summer/autumn were within the limits 32.77-33.03/32.60-32.96 psu for the upper bound of the thermocline and 32.98-33.14/33.06-33.13 psu for its lower bound and had slight negative tendency. In both seasons, the mixed layer depth has significant negative correlation with the temperature at the upper bound of the thermocline (R2 = 0.6) and the temperature at the lower bound of the thermocline correlates significantly with salinity at its both bounds (R2 > 0.4). Spring bloom of chlorophyll- a has no relationship neither with temperature in the mixed layer in April and May, nor with the rate of warming between these months. Strong fall bloom of chlorophyll- a develops if the fall cooling begins in conditions of high temperature at the sea surface (> 6 °C) that negatively depends on the mixed layer thickness, so slow cooling and delay of the fall mixing are favorable for chlorophyll- a blooming in autumn.

Water circulation in the western Bering Sea is considered for summer and fall seasons on the data of oceanographic surveys conducted in 2002-2012 by Pacific Fisheries Research Center (TINRO) under BASIS program. In general, cyclonic circulation was observed over the deepwater basin, without prominent changes form summer to fall. However, the cyclonic gyre was wider until 2006 than later when it was limited by the Commander Basin in both seasons, while the northward flow from the Near Strait became stronger and the westward flow from the Aleutian Basin along Koryak shore and the Bering Slope Current became weaker. The former inflow formed the hydrodynamic front along the border of Russian EEZ that prevented eastward spreading of the cold subsurface water. This change was connected with reconstruction of atmosphere circulation: change of the Aleutian Low orientation in spring. It extended latitudinally from Asia to North America till 2006 that favored to eastern and northeastern winds over the Bering Sea, then moved to southwest-northeast position with shift to south of its western trough, so northern and northeastern winds prevailed. After reconstruction of air and water circulation, feeding migration of immature chum, sockeye and chinook salmons to the western Bering Sea became less intensive and their abundance decreased in 2, 5, and 9 times, respectively, with corresponding changes of their distribution patterns. About 70 % of immature salmons fed in the waters of the Aleutian Basin in 2002-2006 but < 50 % in 2007-2010; their concentration density in this area decreased in 2-3 times though it did not changed significantly in the Kamchatka Basin. After five years of abnormal orientation, the Aleutian Low returned to latitudinal position in 2012, just as the water circulation changed back: the northward flow from the Near Strait weakened, the eastward stream appeared along Aleutian Islands, the front at the border of Russian EEZ relaxed and the cold subsurface water penetrated to the Aleutian Basin again. As the result, the feeding salmon abundance in the EEZ of Russia grew and reached the level of 2002-2006.