Seasonal Variation in Volume Transport of the Kuroshio South of Japan

Abstract

A two-layer numerical model driven by the observed wind stress compiled by K. Kutsuwada and T. Teramoto is used to study the seasonal variation in volume transport of the Kuroshio south of Japan. The authors focus on the observational evidence that the geostrophic volume transport of the Kuroshio is maximum in summer and minimum in winter; however, Sverdrup transport estimated by the observed wind stress indicates a maximum in winter and a minimum in summer. It is shown from a flat-bottom model that the western boundary current has a maximum transport of 90 Sv (Sv ≡ 106 m3 s?1) in late winter and a minimum transport of 40 Sv from summer to early autumn. The time lag of the calculated transport from Sverdrup transport is about one month, which shows that the barotropic response is dominant for the seasonal change in wind stress. However, the vertical velocity difference at the western boundary region reaches a maximum in summer and a minimum in autumn, of which the summer maximum coincides with the observed geostrophic transport. It is shown from a realistic bottom model that the western boundary current has a maximum volume transport (18 Sv) in winter and minimum (7 Sv) in autumn, which is larger than the Sverdrup transport west of the Hawaii Ridge in spring to summer and smaller than in autumn to winter. Development of the baroclinic response of the flow formed by the strong winter wind stress is detected in late spring to autumn. However, annual changes in wind stress west of the Hawaii Ridge and those west of the Izu Ridge have almost similar tendency to those in the whole North Pacific; no significant local change is formed by these wind stresses. From these results, existence of a vertically homogeneous barotropic flow with large volume transport and small velocity is suggested in winter along the eastern continental slope off Nansei Islands, which does not flow out through the Tokara Strait.