A Numerical Study of Barotropic Instability Associated with the Soya Warm Current in the Sea of Okhotsk

Abstract

Numerical model experiments have been carried out in an attempt to elucidate the. generation mechanism of the eddy or wave street in the Soya Warm Current. We use a barotropic model incorporating bottom topography. Under realistic conditions, the model produces waves whose properties are within range of the Observed ones. The generation mechanism of the waves proposed in this experiment is as follows. When the current is driven by the mean water level difference between the Japan Sea and the Sea of Okhotsk, the flow separates from the cape in the Soya Strait. In the region of the flow separation, strong positive vorticity is produced by the frictional torques occurring as the water flows through the strait and by vortex stretching as the water is pushed out to deeper depths. This strong vorticity, or horizontal shear, induces barotropic instability, in which the waves are generated. In brief, the system of the Soya Warm Current accompanied by an eddy street is interpreted as a good geophysical example of a two-dimensional jet in a rotating system. In this experiment, intensity of the mean current and bottom friction is crucial for the wave generation. When we treat sea-ice floes. which occur in this region, as a passive tracer for the ocean velocity in the model, we successfully simulate the street of backward wave breaking or cyclonic eddies observed in the ice floe distributions. We also show that a tracer pattern reflects more than the instanteous potential velocity field rather than the instantaneous velocity field.