On the mechanism of seasonal variation of the Tsushima Warm Current

Observations show that the Tsushima Warm Current (TSWC) varies considerably on a seasonal time scale, with strong northward transport into the Japan/East Sea (JES) in summer and weak transport in winter. It has been shown in the previous studies that the mean TSWC is forced primarily by wind stress over the open ocean, while the local wind stress is less important. Nevertheless, the forcing mechanisms for the seasonal variability of the TSWC remain to be examined. A three-dimensional ocean model is used to investigate the contributions from both local and the open-ocean forcing to the seasonal variability of TSWC. The observed seasonal cycle is simulated well in the model, and then three additional sensitivity runs were conducted to isolate the local and open-ocean forcing. It was shown that the open ocean forcing and the local wind stress both contribute to the seasonal cycle of the TSWC. The impact from the open ocean forcing is facilitated through the Kuroshio Current (KC) interaction with Honshu-Kyushu Islands. The KC transport is stronger in summer than in winter, thus a similar seasonal variability takes place in the TSWC through the mechanism discussed by Yang (2007). On the other hand, the local wind stress varies with the East Asian Monsoon, which results in that the southward wind in winter tends to reduce the TSWC transport while the northward wind in summer is favorable for a stronger transport. The model experiments showed that the contribution from the open ocean is considerably more important than the contribution from the local wind stress forcing.