Mean current – eddy interaction in the Japan Sea, as derived from satellite altimetry

Eddy kinetic energy (EKE) derived from satellite altimetry measurements is an acknowledged characteristics of mesoscale processes in the ocean (Wunsch, Stammer, 1998; Ducet et al., 2000; Zhai et al., 2008). Globally, EKE is concentrated around mean currents where it is 1-2 orders of magnitude larger than in other areas, being generated by baroclinic instability and flow interactions with bathymetry (Ducet et al., 2000). It was also shown that in the Subtropics EKE has the seasonal maximum in late summer, while in the Subarctics the seasonal maximum occurs in winter, implying barotropic instability due to the strengthening of the wind currents (Zhai et al., 2008). To analyze EKE in the Japan Sea, SSALTO/DUAC gridded sea level anomalies available from 1993 onwards are decomposed to empirical modes. Interacting (non-orthogonal) modes covering 60% of the total variance capture the seasonal EKE intensification in summer and fall and weakening in winter, with the extremes in October to November and in March to April, respectively. The same seasonal variation is characteristic of the mean current strength controlled by meridional density gradient generated by warm water inflow from the south through the Korea Strait. Although mean EKE in the subarctic area of the Japan Sea is an order of magnitude smaller than in the subtropical area, local spatial maxima of the leading EKE mode are linked to the zones of the mean currents and bathymetric features in the entire Sea. The same seasonal variation of EKE and mean currents, although an effect of their strength variation on EKE is removed, implies a strong impact of barotropic instability. Another mode accounts for EKE variability in the subarctic area off the Russian coast, with the annual, quasi-biennial, and 4-5–year variability. As the spatial maxima are not linked to hydrographic or bathymetric features, wind forcing of EKE off the Russian coast is suggested.