Interannual variability of the Kuroshio transport in response to the wind stress field over the North Pacific: Its relation to the path variation south of Japan

Abstract

We have investigated the interannual variability in the Kuroshio transport, focusing on the baroclinic response of the upper ocean to the long‐term variation of the wind stress field over the North Pacific from 1961 to 1987. First, temporal change of the Kuroshio transport at the western boundary from 17° to 31°N has been estimated on the assumptions that the Kuroshio is a baroclinic flow confined in the upper layer and that the effect of interior wind stress curl on the volume transport at the western boundary is transferred by nondispersive baroclinic Rossby wave. Peak values in the estimated transport appear in its beginning region (17°–21°N) when or just before the Kuroshio settles into a meandering state south of Japan in 1975, 1981, and 1986. These peaks are due to negative wind stress curl anomalies which are induced by the intensified trade winds in the central and eastern tropical North Pacific on and just after an El Niño event and in the western tropical North Pacific months to years before the next El Niño event. Such peaks are not found in the barotropic Sverdrup transport. In downstream regions, i.e., in the East China Sea and south of Japan (north of 23°N), on the contrary, the estimated transports (through baroclinic and barotropic responses) do not have such good correlation to the path variation. Temporal change in thermocline depths estimated from bathythermograph data clearly shows the westward propagations of positive anomalies by nondispersive baroclinic Rossby waves at latitudes of the beginning of the Kuroshio (south of 21°N), which is in phase with peaks of the Kuroshio baroclinic transport at the western boundary. On the other hand, there are only the local propagations of the anomalies in the downstream regions, as previous studies reported. However, it is emphasized that temporal change of the geostrophic transport in the East China Sea (∼28°N) is coincident with that of the baroclinic transport at the beginning of the Kuroshio, not with that at the same latitude. A numerical experiment with a nonlinear reduced‐gravity model confirms that temporal changes of the transport in the East China Sea and south of Japan are due to the advective (nonlinear) effect of the Kuroshio itself, while the linear baroclinic response to the wind stress field is dominant in its beginning region.