Evaluation of a regional ocean reanalysis system for the East Asian Marginal Seas based on the ensemble Kalman filter

This study introduces the East Asian Marginal Seas (EAMS) reanalysis system and evaluates its products from 1982 to 2006. The EAMS reanalysis system consists of an ocean circulation model with 0.25° horizontal grid spacing and a data assimilation module. Temperature profiles taken by ship and Argo floats as well as satellite-borne sea surface temperature (SST) were assimilated into the model by applying the ensemble Kalman filter every 7 days. The reanalyzed oceanic fields were compared with ones by the control run without data assimilation to assess the impact of the data assimilation. The assimilative model significantly improved the horizontal structures of SST, sea surface height (SSH), vertical structure of temperature, and volume transports through the major straits. Root-mean-square error (RMSE) of SST decreased from 1.0 to 0.6°C. Horizontal and vertical distribution of subsurface temperature was corrected close to the observed values. High SSH variability in the south of Japan and in the Kuroshio extension region was partially restored. Overshooting of the Kuroshio Current and the East Korea Warm Current was suppressed to the south by the data assimilation. SSH and surface circulation in the Oyashio region and in the East Sea (also known as the Sea of Japan) recovered, which corrected the sea level difference between the East Sea and the Pacific Ocean and produced realistic transport through the Korea Strait. The reanalysis well resolved transport through the Korea Strait. The correlation coefficient with the transport based on an Acoustic Doppler Current Profiler measurement was 0.70 and the RMSE was 0.37 Sv (106 m3/s), whereas those were 0.59 and 0.43 Sv in the control run, respectively. The EAMS reanalysis dataset may help us to understand circulation variations in the marginal seas and to investigate factors controlling volume and heat transport.