Deep countercurrent beneath the Kuroshio in the Okinawa Trough

Abstract

The Okinawa Trough is geographically separated into the northern and southern parts by the Kerama Gap, and the northern part is composed of the northern, central and southern subbasins formed by bights of the continental slope. Previous observations have indicated that a deep countercurrent is present beneath the Kuroshio on the continental slope in the northern Okinawa Trough. However, its persistence over time and its spatial structure over the entire basin have not been clarified to date. The present study examines moored current meter data on the continental slope in the southern and central subbasins of the northern Okinawa Trough for November 2004 to November 2006. Deep flows on the continental slope have a clear seasonality in the southern subbasin; eddy motions due to Kuroshio meanders are organized into a persistent countercurrent below the Kuroshio in the winter‐spring period. During this time, high‐frequency Kuroshio meanders with periods near 10 d are likely to diminish in the southern subbasin while low‐frequency Kuroshio meanders with periods of 1–3 months tend to dominate in the northern subbasin. In addition, a high‐resolution ocean general circulation model is used to explore the deep flow field over the entire Okinawa Trough. The model indicates that the deep flow field is stable in the southern Okinawa Trough over the year, whereas it is unstable in the northern Okinawa Trough particularly in the winter‐spring period. This results in a persistent countercurrent driven by cyclonic eddies on the continental slope in the northern Okinawa Trough.