Spectral descriptions of coastal submesoscale surface currents and passive tracers in an observational view (session speaker invited)

Abstract

Submesoscale coastal surface currents and chlorophyll concentrations at hourly and O(1)-km resolutions, obtained from an array of high-frequency radars and geostationary ocean color imagery in a coastal region off the east coast of Korea over a period of one 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 and shallow mixed layer depths in summer. The clockwise nearinertial surface currents show onshore phase propagation and decreasing amplitudes associated with enhanced friction and reduced polarization due to the effects of the coastal boundary. The kinetic energy (KE) spectra of the surface currents in the wavenumber domain (k) become steeper at a scale of approximately 10 km from a slope of k-5/3 to slopes between k-2 and k-3 at a length scale of 2 km. Moreover, the KE spectra of the chlorophyll exhibit anisotropy associated with bathymetric effects and regional circulation, and their decay slopes change from k-5/3 to k-1 at O(10) km scales and from k-1 to k-3 at O(1) km scales, which is consistent with the two-dimensional quasi-geostrophic turbulence theory. The spectral decay slopes of these KE spectra show weak seasonality, which can be interpreted with the baroclinic instability in the weak seasonal mixed layer and the persistent and non-seasonal regional circulations.