FY 1999

Thermal stratification and mixing on the Bering Sea shelf

Overland, J.E., S.A. Salo, L.H. Kantha, and C.A. Clayson

In Dynamics of the Bering Sea: A Summary of Physical, Chemical, and Biological Characteristics, and a Synopsis of Research on the Bering Sea, University of Alaska Sea Grant, AK-SG-99-03, T.R. Loughlin, and K. Ohtani (eds.), North Pacific Marine Science Organization (PICES), 129–146 (1999)

A classical view of April-August stratification on the Bering Sea shelf is an isothermal inner domain, a two-layer middle domain, and a three-layer outer domain; domain boundaries are roughly 50 and 100 m. Based on a data set from 1974 to 1997, we show considerable spatial, intra-seasonal, and interannual variability in this picture. The primary physical balance is between solar heating and turbulent mixing by tidal currents and wind. The inner domain occasionally shows a warmer (1-3°C) surface layer, which is mixed out during subsequent storms. Greater depths in the middle domain have increased top to bottom temperature differences compared with shoaler regions. Transition zones between domains are deeper to the north of the Pribilof Islands, 50-60 m and 100-110 m, compared with the southeast Bering shelf, 40-50 m and 90-100 m. Such spatial variability is due to changes in turbulent kinetic energy, based on variations in tidal currents and water depth. Year-to-year summer temperature variability in the outer middle domain is 4°C in both the surface and bottom layers; this variability is primarily a result of initial temperatures in early spring, and secondarily of summer variations in solar heating.