the 22nd Pacific−Asian Marginal Seas meeting (PAMS)

Variability in strong boundary currents, such as the Kuroshio, often induces coastal sea level and velocity anomalies in remote regions. This study proposes a new mechanism that highlights the critical role of bottom Ekman pumping. A semi-analytical model is developed to demonstrate that bottom Ekman pumping/suction induced by boundary currents generates coastal trapped waves (CTWs), which propagate along the coast and leave sea level and velocity anomalies along their path. The characteristics of these anomalies are primarily determined by the projection of the bottom Ekman pumping/suction onto the first-mode CTWs, as well as by their propagation speed and decay timescale. To demonstrate the relevance of this mechanism, a numerical simulation is performed in which idealized bottom Ekman pumping/suction, associated with a slight northward shift of the Kuroshio axis south of Japan, is imposed as external forcing. Coastal responses characterized by positive sea level and clockwise circulation anomalies are excited along the Japanese coastline, consistent with observations in both pattern and magnitude. Although the overall response pattern agrees with the semi-analytical model, an energetics analysis reveals that variable bottom topography induces substantial wave conversion. CTWs generated south of Japan propagate into the East China Sea and are converted into surface gravity/Kelvin waves over the continental slope. These surface waves subsequently enter the Japan Sea and are partially reconverted into CTWs over steep topography, generating additional anomalies along the northern coast of Japan. These findings underscore a previously unrecognized pathway through which large-scale boundary currents influence coastal environments.

Coastal Trapped Wave,Bottom Ekman Pumping,Boundaty Current,Kuroshio,Japan Sea,Coastal Sea Level