Improved representation of short-term variations of the Kuroshio path south of Japan with a 4DVAR assimilation system

A Four-Dimensional Variational (4DVAR) data assimilation system with an eddy resolving ocean model in the western North Pacific has been developed in the Meteorological Research Institute. The 4DVAR system employs amplitudes of vertical coupled Temperature and Salinity (T-S) EOF modes as control variables, which compose the vertical correlation matrix of the background errors. This method is based on the same strategy as that used in the present operational 3DVAR assimilation system in Japan Meteorological Agency. In this study, we conducted two assimilation experiments using both the 3DVAR and 4DVAR systems. The common observation dataset including sea surface height anomaly derived from satellite altimeter, in-situ T-S profiles, and satellite sea surface temperature was used in the experiments. We compared two assimilated fields especially focusing on short-term variations around the southern coast of Japan, where variability in the Kuroshio path has a large impact on coastal processes. The 4DVAR results successfully reproduce a warm water intrusion event at the southern coast of Japan induced by a short-term fluctuation of the Kuroshio path. In contrast, the 3DVAR results largely capture the path fluctuation but fail to reproduce the subsequent warm water intrusion. It is also revealed that short-term variability with a timescale of a few 10 days in the 4DVAR results is much enhanced and compares well with sea-level variability at tide gauge stations at the southern coast of Japan.