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.