Time-evolving state of the Japan Sea is estimated using an ocean general circulation model and satellite altimeter measurements by approximate RTS smoother. The approximation is established by seeking the time-asymptotic error covariance and also by reducing the controlled state to the barotropic and first baroclinic dynamic modes at coarse horizontal grids. The expected error variances of the external and internal displacements are decreased by about 49 and 68%, respectively from the simulation to the smoothed estimation.
The smoothed estimate explains more than 50% variance of the observed sea level variation. Large-scale, cyclonic propagation of barotropic signal is found at the eastern basin with a time scale of one to two months. Mescoscale variabilities asscociated with the Tsushima Warm Current are largely detected. The smoothed result is validated by comparing to independent in-situ temperature observations including two peaks in the seasonal variation at subsurface layers near the Japanese coast. It is explained that the first maximum in September reflects normal accumulation of surface heating, and the second one in November or December is associated with separation of the Tsushima Warm Current from the coast. |
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