Observational Study on the Variability of Mixed Layer Depth in the Bering Sea and the Chukchi Sea in the Summer of 2019

Based on the high-resolution CTD data from 58 stations in the Bering Sea and the Chukchi Sea in the summer of 2019, the mixed layer depth (MLD) was obtained according to the density difference threshold method. It was verified that the MLD could be estimated more accurately by using a criterion of 0.125 kg/m3 in this region. The MLD in the Bering Sea basin was larger than that in the Bering Sea shelf, and both of them were smaller than that in the Bering Sea slope. The MLD increased northward both in the Chukchi Sea shelf and the Chukchi Sea slope. The farther northward, the greater the difference between the MLD calculated from temperature (MLDt) and the MLD calculated from density (MLDd) was, and the more important the role of salinity was in determining the MLD. The larger MLD (refer to MLDd specifically) in the Bering Sea slope might be due to the enhancement of mixing caused by the Bering Slope Current (BSC) and eddies. The horizontal advection of the Bering Sea Anadyr Water and the Alaska Coastal Water in the Bering Sea shelf led to the shallower MLD in the central transition zone. The northward increase of the MLD in the Chukchi Sea might be related to the low-salinity seawater resulting from the melting of sea ice in summer. The spatial variation of MLD was more closely related to the surface momentum flux than the sea surface buoyancy flux, and the wave had little effect.

Role of physical processes in formation of spring phytoplankton bloom in the Bering Sea

The main part of the annual primary production in the Arctic and Subarctic zones of the World Ocean is formed during the spring phytoplankton bloom. The timing of the bloom depends on combination of physical factors. Oscillating control hypothesis, proposed in [Hunt et al., 2002] for the Eastern Bering Sea, describes annual peculiarities of ecosystem development related to conditions of the spring phytoplankton bloom. We review propositions of this hypothesis on the reasons of phytoplankton bloom and its connection with physical processes for four local regions of the Bering Sea shelf. The regions include western, northern and south-eastern parts of the shelf. The analysis is based on ocean color and microwave remotely sensed data as well as on atmospheric reanalysis. The results allow for hypothesis improvement. An early phytoplankton bloom may be present in the surface layer in April or May along the eastern Bering Sea shelf even in situations of early sea ice retreat (e. g. February-March) or absence of ice during winter. However, such combinations were not observed in the western Bering Sea shelf region. In 1998–2018, early ice retreat in the western shelf region was always accompanied by relatively late phytoplankton bloom. The temporal lag between sea ice retreat and phytoplankton bloom may be substantial in some years along the southernmost position of the ice edge. On the other hand, the spring bloom in the northern part of the shelf usually follows the ice retreat. In case of early ice retreat, the timing of the bloom is determined not only by wind conditions, but also by heat balance at the surface of the sea. The results are proposed to be used in further analysis of ecosystem dynamics of the western Bering Sea shelf.