Numerical Study of Formation of Dichothermal Water in the Bering Sea

Abstract

A one-dimensional numerical model with a level-2.5 turbulent closure scheme to provide vertical mixing coefficients has been used to investigate the process by which the dichothermal water is formed in the Bering Sea, the density of which is about 26.6 sigma-theta. The water column to be simulated is assumed to move along a predetermined path. That is, the present model is of the Lagrangian-type. Surface boundary conditions are given using the climatologies of heat, freshwater and momentum fluxes. In order to obtain a plausible moving speed of the water column along the path, pre-liminary experiments were done using the surface fluxes in the central part of the Bering Sea for the initial temperature and salinity profiles at the entrance of the Sea. As a result, it was found that the temperature minimum layer, i.e., the dichothermal water with temperature similar to the climatology at the exit of the Bering Sea, was formed after about two years of integration. Based on the result, the movement speed of the water column along the path was set as 4.5 cm/s in the standard run. It was found that this model could plausibly reproduce the subsurface temperature minimum layer. That is, the dichothermal water was formed in the winter mixed layer process in the Bering Sea. The existence of the subsurface halocline (pycnocline) prohibited the deeper penetration of the winter mixed layer, and therefore water with a temperature colder than that under the mixed layer was formed in the mixed layer due to wintertime surface cooling. In the warming season this water remains as the subsurface temperature minimum layer between the upper seasonal thermocline and the lower halocline.