Journal of Oceanography, 2004 (in press)

A generation mechanism for mesoscale eddies in the Kuril Basin
of the Okhotsk Sea : baroclinic instability caused by the enhanced tidal mixing

Kay I. Ohshima, Yasushi Fukamachi, Takeshi Mutoh, and Masaaki Wakatsuchi
Mesoscale eddies (particularly anticyclonic ones) are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol' Strait, which has a diameter of ~100 km, typical surface velocity of 0.2--0.3 ms-1, and less dense core extending to a depth of ~1200 m. Based on an idea that the generation for the mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of ~ 70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions.
In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances gradually increasing. Typical eddy scale and its westward propagation speed are ~100 km and ~0.6 km day-1, respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the β-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of ~1200 m, can be reproduced.