Changes in the Speed of the Kuroshio in Response to Asian Monsoon Wind Stress Variability

Abstract Seasonal variations in wind stress impact the velocity of the upper layer of the Kuroshio over the East China Sea (ECS–Kuroshio). The dynamics of this process, which involves vorticity adjustments in the baroclinic jet that is induced by local Ekman pumping over the jet, can be attributed to any of the wind stress components acting upon the jet: 1) wind stress relative to the sea surface current; 2) the along-stream wind stress component, which causes nonlinear Ekman pumping due to the effective Coriolis parameter; and 3) the wind stress curl over the jet. This study examines the contribution of these wind stress components to seasonal variability in the current velocity of the upper layer of the ECS–Kuroshio mainly using a satellite-based surface geostrophic-velocity time series and wind stress data. The results showed that the magnitudes of the first and second wind stress components are comparable, but the third component is negligible. The rate of change in the surface speed of the ECS–Kuroshio in response to seasonal variability in the along-stream wind stress was approximately 1.5–2.1 m s −1 (N m −2 ) −1 . This rate of change is approximately 3–4 times as large as that obtained using a numerical model in a previous study, which did not consider the first and third components of wind stress. Consequently, our results showed that the response of the ECS–Kuroshio speed to seasonal wind stress variability is influenced not only by the effect of nonlinear Ekman pumping but also by the effect of wind stress relative to the surface current. Significance Statement The Kuroshio is driven by the westerlies and trades over the North Pacific for interannual–interdecadal variabilities, while driven by the Northeast Asian monsoon over the Kuroshio for seasonal variability. The former mechanism is known as a remote process through baroclinic Rossby waves, while the latter mechanism is uncertain. This study aims to clarify a local mechanism driving seasonal variability. Following the vorticity response to nonlinear Ekman pumping over the Kuroshio, we analyzed observational data and concluded that seasonal variability in the Kuroshio speed is influenced by the wind stress relative to the sea surface current and the along-stream wind stress component, both being almost equivalent in amplitude. This implies that seasonal-variability modulations in the Northeast Asian monsoon can drive the interannual–interdecadal Kuroshio speed variabilities.