Tempo-Spatial Variations of the Kuroshio Current in the Tokara Strait Based on Long-Term Ferryboat ADCP Data

Transverse-vertical structure and temporal variability of the Kuroshio current across the Tokara Strait during 2003-2012 measured by a ferryboat acoustic Doppler current profiler with a 2-km horizontal resolution and a two-day interval are presented. The Kuroshio passing through the Tokara Strait exhibits a multicore velocity structure. Its seasonal volume transport variation is biannual for baroclinic components relative to 700 m, peaking in July and December-January. However, the barotropic transport component exhibits an annual cycle with a maximum in December. Empirical orthogonal function analysis of the cross-sectional velocity is performed. The first two empirical orthogonal function modes reveal the north-south shift of the Kuroshio current axis and the change in Kuroshio volume transport, respectively. Temporal variabilities of the leading two modes correspond to those of the Kuroshio Position Index and the sea level difference across the strait, respectively. The third empirical orthogonal function mode, with a relatively smaller horizontal scale, was examined in terms of turbulent mixing. The banded structure captured by this mode is likely induced by flow-topography interaction because islands in the Kuroshio route could cause horizontal and vertical flow separation. Additional analysis based on high-resolution reanalysis data suggested that (1) inertial instability, which is expected in the areas with negative Ertel's potential vorticity, arises to enhance vertical mixing around the islands in the Tokara Strait, and (2) when the Kuroshio directly impinges the islands, flow divergence in the lee of the islands drives upwelling and leads to uplift of isotherms. Plain Language Summary The Kuroshio is a western boundary current in the North Pacific which enters the East China Sea and leaves through the Tokara Strait. It is located between Amamioshima and Yakushima Islands to the south of Kyushu, Japan. The variations in the Kuroshio path position and transport across the Tokara Strait are crucial in monitoring the variability of the Kuroshio downstream. This study presents the first long-term continuous transverse-vertical structure and temporal variability of the Kuroshio across the Tokara Strait, with unprecedented resolution based on observational data. The Kuroshio passing through the Tokara Strait exhibits multiple-core velocity structure, with a maximum transport in July and a secondary transport in December-January. The path fluctuation and transport variation explain 41% and 27%, respectively, of the total current variance in the Kuroshio in the Tokara Strait. Moreover, because the Kuroshio current interacts strongly with the topography around the Tokara Strait, many vortices are generated in this region. The lee of Tokara Islands is characterized by the return current against the incoming current, and the wake water is 0.3-0.5 degrees C colder than the surrounding waters. Additionally, the intensity of island-induced wakes depends strongly on the shape of the Kuroshio path around the Tokara Strait.