Interannual Sea-Level Variation Around Mainland Japan Forced by Subtropical North Pacific Wind and Its Possible Impact on the Tsugaru Warm Current

The volume transport of the Tsugaru warm current (TWC), defined as the branch current of the western boundary current of the North Pacific subtropical gyre flowing through the Tsugaru Strait, is a critical factor affecting the acidification in the strait as well as the surrounding coastal regions by promoting the enhancement of the vertical mixing of the deep water rich in dissolved inorganic carbon. The in-phase sea-level variation along the coast of mainland Japan with a gap at the Tsugaru Strait permits the discharge of the sea surface water from the Sea of Japan to the Pacific Ocean, which is significantly correlated to the variation in the sea-level difference across the Tsugaru Strait, being possibly related to that in the TWC volume transport. Furthermore, by averaging the wind-driven Sverdrup streamfunction along the eastern coast of mainland Japan—from the southern end of Kyushu (30 ^∘ N) to the separation latitude of the Kuroshio (36 ^∘ N)—and scaling by a factor of the ratio between the depths of the Tsushima Strait and the East China Sea, we obtained the volume transport varying similarly with the sea-level difference across the Tsugaru Strait. As obtained, the interannual wind stress variation in this latitude band to the east of mainland Japan on the North Pacific is suggested to altered the strength of the TWC.