In the present study, we identify the baroclinic activity inducing the annual variation of the Kuroshio volume transport. The scenario is summarized as follows.
When the annual variation is concerned, the lower-layer motion remains in areas surrounding the ridge because isostasy is not achieved within such a short timescale. Thus, the flow impinges to the bottom slope. Anomalies of the upper-layer thickness are generated on the eastern side of the ridge through the impinging process expressed by the term on the right-hand side of Eq. (1). Thereafter, anomalies move westward with characteristic velocities composed of the vertically averaged flow and westward propagation of the long baroclinic Rossby-wave. As anomalies of the upper-layer thickness move westward above the ridge, isostasy is accomplished rapidly and locally with respect to these anomalies. As a result, the positive (negative) anomaly of the upper-layer thickness has the information of the positive (negative) anomaly of the volume transport when it reaches the western edge of the ridge. Hence, anomalies of the volume transport are released to the west of the ridge.