Local wind effect on the Kuroshio path state off the southeastern coast of Kyushu

Abstract

Small meanders of the Kuroshio southeast of Kyushu, Japan, are conventionally known to form in winter or early spring, but rarely in other seasons. This study examines the seasonal nature of the small meander formation as it relates to the seasonal monsoon winds over the East China Sea, based on observational data analyses, theoretical considerations and numerical experiments. Positional data of the Kuroshio path indicate that the small meander experiences phase locking with the seasonal cycle, as is conventionally thought, although its intensity is modulated on decadal time scales. Monthly mean data of the surface geostrophic current and wind stress fields indicate that the southwestward wind blowing against the Kuroshio in the autumn and early winter causes an inshore-ward shift of the Kuroshio in the northern Okinawa Trough. This effect acts to strengthen the convoluted pattern of the Kuroshio path around Kyushu, and to develop the small meander southeast of the same island. Assuming that the Kuroshio is a surface geostrophic jet with a double-exponential velocity profile, the observed evidence is well explained by a combination of Ekman layer dynamics and quasi-geostrophic dynamics. More specifically, Ekman pumping due to nonlinear Ekman divergence over the jet decreases (increases) the surface velocity of the offshore (inshore) side of the jet, resulting in the inshore-ward shift of the jet. A two-layer shallow water model with idealized topography shows that the response of the modeled Kuroshio to the local wind stress is consistent with observational and theoretical results.