On the origin of the Tsushima Warm Current

Abstract

The origin of the Tsushima Warm Current (referred to as TWC) was investigated by surface drifter experiments and conductivity, temperature, and depth (CTD) surveys in the northeastern East China Sea (ECS) at three different times: July 1991, and April and November 1992. Trajectories of 10 satellite‐tracked drifters provide direct information for the first time on the surface flow patterns in each season, and CTD observations allow identification of warm and saline TWC waters. The results of the experiments argue against two historical concepts of TWC origins, i.e., (1) a northward flow transporting warm and saline water through the deep trough southwest of Kyushu toward the Korea Strait after separation from the Kuroshio and (2) a northeastward continuation of the Taiwan Current (TC) on the shelf of ECS after passing through the Taiwan Strait. A persistent northward current was found to exist both on the shelf west of the trough and on the western flank of the trough. The northward flow seems to bifurcate around the northwestern corner of the trough, splitting into a northward continuing flow on the shelf of 100–150 m and an eastward flow along the northern wall of the trough. The northward flow on the shelf, which might be the shore fringe of the Kuroshio, corresponds to the origin of TWC entering the Korea Strait. The eastward flow on the northern slope turned back to the south along the west coast of Kyushu and eventually joined the Kuroshio. This structure was accompanied by an anticyclonic eddy in the northern trough. The second concept, that of TWC originating from TC, contradicts the observed differences in physical properties between TWC and TC waters. The saline water in the Taiwan Strait flows out intermittently only during late winter‐early spring, and its salinity during other seasons is lower than that of the TWC water in the Korea Strait. Experiments also indicated a seasonal shift of drifter paths in the northeastern ECS and coexistence of cyclonic and anticyclonic eddies in the trough during the cold season.