Distribution, formation, and seasonal variability of Okhotsk Sea Mode Water

Abstract

Russian historical data and recently completed conductivity‐temperature‐depth surveys are used to examine the formation and spread in the deep Ohkotsk Sea of dense shelf water (DSW) produced in the Okhotsk Sea polynyas. Isopycnal analysis indicates that all of the main polynyas contribute to the ventilation at σθ < 26.80, including the Okhotsk Sea Mode Water (OSMW), which has densities σθ = 26.7–27.0. At densities greater than 26.9 σθ the northwest polynya is the only contributor to OSMW. (Although Shelikhov Bay polynyas produce the densest water with σθ > 27.1, vigorous tidal mixing leads to outflow of water with a density of only about 26.7 σθ). In the western Okhotsk Sea the East Sakhalin Current rapidly transports modified dense shelf water along the eastern Sakhalin slope to the Kuril Basin, where it is subject to further mixing because of the large anticyclonic eddies and tides. Most of the dense water flows off the shelves in spring. Their average flux does not exceed 0.2 Sv in summer and fall. The shelf water transport and water exchange with the North Pacific cause large seasonal variations of temperature at densities of 26.7–27.0 σθ (depths of 150–500 m) in the Kuril Basin, where the average temperature minimum occurs in April–May, and the average temperature maximum occurs in September, with a range of 0.2°–0.7°C. The average seasonal variations of salinity are quite small and do not exceed 0.05 psu. The Soya Water mixed by winter convection, penetrating to depths greater than 200 m, in the southern Kuril Basin also produces freezing water with density greater than 26.7 σθ. Using a simple isopycnal box model and seasonal observations, the OSMW production rate is seen to increase in summer up to 2.2 ± 1.7 Sv, mainly because of increased North Pacific inflow, and drops in winter to 0.2 ± 0.1 Sv. A compensating decrease in temperature in the Kuril Basin implies a DSW volume transport of 1.4 ± 1.1 Sv from February through May. The residence time of the OSMW in the Kuril Basin is 2 ± 1.7 years.