Interannual to Decadal Variability of Ocean Heat Content in the Japan Sea: Role of the Tsushima Warm Current and Its Relation to the Kuroshio Extension Variability

Abstract Using a long-term high-resolution ocean reanalysis dataset, interannual-to-decadal variability of ocean heat content (OHC) in the Japan Sea is investigated with a focus on the role of the Tsushima Warm Current (TWC) and its relation to the Kuroshio Extension (KE) variability. A lagged covariance between detrended OHC and each term in the OHC heat budget suggests that the TWC heat transport is the main factor for the interannual OHC variability, and the surface heat flux responds passively to OHC changes. It is further suggested that the TWC plays an important role in determining the time scale of the decadal OHC variability. We also find a close relationship between the sea level variability at the Japanese coast and the TWC volume transport, which is explained by the propagation of Kuroshio-induced coastal trapped waves (CTWs) shown by previous studies and resulting changes in the sea level difference across the Tsushima Strait. In addition, considering the relationship between the Kuroshio-induced CTWs and the wind-forced KE variability indicated by previous studies, we propose a mechanism that the interannual-to-decadal OHC variability in the Japan Sea is forced by the KE variability through adjustments of the sea level at the Japanese coast and the TWC transport. Significance Statement The Japan Sea exhibits a prominent warming trend as well as significant interannual-to-decadal variability, but their mechanisms are still unclear. The key issue for such long-term trend and variations is to clarify the role of the Tsushima Warm Current (TWC). As a first step toward this goal, interannual-to-decadal variability of ocean heat content (OHC) in the Japan Sea is investigated with a focus on the role of the TWC. By analyzing a long-term high-resolution ocean reanalysis dataset, we propose a mechanism that the OHC in the Japan Sea is forced by the Kuroshio Extension variability through adjustments of the sea level at the Japanese coast and the TWC transport. This mechanism would be applicable to the long-term trend as well.