Sverdrup Balance and the Cyclonic Gyre in the Sea of Okhotsk

Abstract

It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean?Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s?1 near the western boundary and slow northward flow with an average speed of approximately 2 cm s?1 in the east. Climatological data show that along zonal sections at 50°?53°N isopycnal surfaces gradually rise from the east to west and sharply drop near the western boundary, suggesting the Sverdrup balance. This feature persists throughout the year. The integrated northward baroclinic transport of 3.5 Sv along 53°N is comparable to the Sverdrup transport of 3.7 Sv, calculated from the annual mean wind stress. Sverdrup balance appears to hold roughly in the baroclinic field in 50°?53°N. A flat-bottom numerical model forced by realistic wind stress reproduces well the cyclonic gyre, with the observed baroclinic features. In the south, the anticyclonic circulation in the Kuril Basin cannot be explained by the wind stress curl inside the Okhotsk Sea in this simplified model.