Intra-annual variability of the mixed layer parameters in the Okhotsk Sea

Abstract

All available oceanographic data for the deep-water part of the Okhotsk Sea, in total 111,944 stations collected in 1931–2014, are analyzed after removing the duplicate and lowquality ones. The mixed layer depth is determined on the water temperature profle for each station and monthly mean values of temperature and salinity in the mixed layer are calculated for the 0.5о-size grid. In May-October, the mixed layer depth varies within the range 5–25 m, with the highest values in the areas with active water dynamics. On the contrary, the mixed layer is much deeper in December-April when its thickness exceeds 40–60 meters and even reaches 100–120 m in the central Kuril Straits. Two types of temperature distribution change annually within the mixed layer in the Okhotsk Sea: the winter pattern with higher (positive) temperature at Kuril Islands occurs in December-April and the summer pattern is formed in June-September and is distinguished by spots of lower temperature in the dynamically active areas with strong tidal and non-tidal currents (Kuril Straits, Kashevarov Bank, entrance to the Shelikhov Bay, Shantar Islands vicinity, northeastern shelf of Sakhalin Island). Transition patterns of the temperature distribution are observed in May, October, and November. Largescale patterns of salinity distribution within the mixed layer are permanent throughout a year and indicate prevailing currents and other processes in the upper layer of the Okhotsk Sea. The maximum salinity is observed in the southern Okhotsk Sea, whereas the lower salinity values are usual for the coastal waters (except the coasts of Kuril Islands) affected to the river runoff.