Origin and formation of the Ryukyu Current revealed by HYCOM reanalysis

Abstract

The origin of the Ryukyu Current (RC) and the formation of its subsurface velocity core were investigated using a 23-year (1993–2015) global Hybrid Coordinate Ocean Model (HYCOM) dataset. The volume transport of the RC comes from the Kuroshio eastward branch (KEB) east of Taiwan and part of the North Pacific Subtropical Gyre (p-NPSG). From the surface to 2 000 m depth, the KEB (p-NPSG) transport contributes 41.5% (58.5%) to the mean total RC transport. The KEB originally forms the subsurface velocity core of the RC east of Taiwan due to blockage of the subsurface Kuroshio by the Ilan Ridge (sill depth: 700 m). Above 700 m, the Kuroshio can enter the East China Sea (ECS) over the Ilan Ridge, meanwhile, the blocked Kuroshio below 700 m turns to the right and flows along the Ryukyu Islands. With the RC flowing northeastward, the p-NPSG contribution strengthens the subsurface maximum structure of the RC owing to the blockage of the Ryukyu Ridge. In the surface layer, the p-NPSG cannot form a stable northeastward current due to frequent disturbance by mesoscale eddies and water exchange through the gaps (with net volume transport into ECS) between the Ryukyu Islands.