Seasonal variability of the mixed layer depth in the East Sea

Abstract

We constructed a new profile-based 0.5°-resolution monthly mean climatology of the mixed layer depth (MLD) in the East Sea (ES) using most of the available temperature and salinity observations collected from 1931 to 2005. The MLD is defined by a depth at which the temperature is changed from the surface reference value by 0.2oC. The temperature-based MLD shows no significant difference (mostly less than 20 m) from the density-based values over most of the East Sea except in the regions near the Russian coast and the Japanese coast, where barrier layers are formed during a period from late fall through early spring, suggesting that the temperature-based MLD can be a good proxy for the MLD in the East Sea. The range of seasonal variation in the MLD is from about 20 m near the Sub-polar front (SPF, 38oN ~41oN) to about 200 m near the winter convection region (132oE~135oE, 41oN~43oN). The weaker seasonal variation near the SPF results from year-round strong stratification sustained largely by the subduction of relatively cold water underneath the East Korean Warm Current (EKWC) and the complex dynamic process of frontogenesis including lateral dynamics or wind-induced friction. The spatial distribution of the MLD is also changed significantly due to both atmospheric and oceanic processes including eddies and winter convection. For example, a deep mixed layer is developed in the Ulleung Basin during winter due to the Ulleung warm eddy. There was a abrupt and localized deepening along the near-coastal areas in the northern part of the ES in winter and early spring. Barrier layers were clearly identified mainly along the Russian shelf and the Japanese coast during winter, caused by presence of the low-salinity water.