Spectral Descriptions of Submesoscale Surface Circulation in a Coastal Region

Abstract

Submesoscale coastal surface currents at hourly and O(1) km resolutions, obtained from an array of high-frequency radars in a coastal region off the east coast of Korea over a period of 1 year (2013), are described in the frequency and wavenumber domains. The low-frequency surface currents exhibit more consistent variability with the regional geostrophic currents in summer than those in winter because of the relatively weak wind conditions in summer. The clockwise near-inertial surface currents show onshore phase propagation and decreasing amplitudes. The kinetic energy spectra of the surface currents in the wavenumber domain (k) become steeper, increasing from a slope of k-5/3 at a length scale of approximately 10 km to slopes between k-2 and k-3 at a length scale of 2 km. These kinetic energy spectra exhibit anisotropy and weak seasonality with an injection scale of O(10) km, consistent with dominant length scales of the regional submesoscale eddies and the zero-crossing wavenumber in the estimated kinetic energy fluxes. We suggest possible mechanisms to explain the above findings. Submesoscale processes in this region are primarily initiated by surface frontogenesis caused by regional mesoscale currents, including low-frequency circulations and topographically linked shear currents, then maintained by baroclinic instability in the mixed layer with moderate seasonality.