Numerical Experiments on the Seasonal Variations of the Oyashio near the East Coast of Japan

Abstract

Seasonal variations of the Oyashio near the east coast of Japan are examined using a simple layered model with a localized diapycnal transport due to the strong tidal mixing around the Kuril Islands. The modeled southernmost latitude is located at around 40°N in spring and 42°N in autumn. The southwestward Oyashio transport in the upper layers and depth-integrated transport become large (small) during winter?spring (summer?autumn). These results are in agreement with observations. The seasonal variation of the southernmost latitude is mainly caused by the baroclinic velocity anomalies off Tohoku and Hokkaido, Japan, which have been excited by the wind stress changes along the west coast of North America, the Kamchatka Peninsula, and the model northern boundary. This is different from the previous studies that emphasized the importance of direct barotropic response of the ocean to the basin-scale wind stress changes. For comparison, an additional experiment in which the diapycnal transport around the Kuril Islands is not included is performed. In this case, the annual-mean Oyashio does not extend south of the wind-driven gyre boundary and the variation of the southernmost latitude is small, even though the baroclinic velocity anomalies are similar to the ones in the case with the diapycnal transport. This is because the annual-mean Oyashio velocities are larger than the anomalies and cannot be distorted, whereas the annual-mean Oyashio in the case with diapycnal transport is weak in intrusion of the wind-driven gyre boundary and can be easily changed by the velocity anomalies. It is suggested that the reproduction of the seasonal variation of the Oyashio southernmost latitude depends on whether the annual-mean Oyashio intrusion could be reproduced adequately in the numerical model.