Mean field and annual variation of surface flow in the East China Sea as revealed by combining satellite altimeter and drifter data

An analysis of a surface flow field in the East China Sea (ECS) obtained by combining satellite altimeter and drifter data provides comprehensive information of its mean field and annual variation. The Taiwan Warm Current, the onshore Kuroshio intrusion, the Kuroshio, the northeastward Kuroshio branch, the northward current west of Kyushu, the Cheju Warm Current, and the Tsushima Warm Current are regarded as persistent currents. Annual variation is a dominant signal in the ECS and is a major component of the seasonal variation. The contribution of the annual variation to the seasonal variation is estimated as 50% over the ECS, and exceeds 70% for the Kuroshio northeast of Taiwan, the Taiwan Warm Current, the Taiwan Strait Warm Current, the onshore Kuroshio intrusion, the northeastward Kuroshio branch, the northward current west of Kyushu, and the Tsushima Warm Current. The annual variation is characterized by propagation of vorticity anomalies from northeast of the Taiwan Strait and in the Tsushima Strait. The vorticity anomaly from northeast of the Taiwan Strait propagates downstream in the northeastward Kuroshio branch along isobaths of about 100 m on the shelf edge. In contrast, the vorticity anomaly in the Tsushima Strait propagates upstream of the Tsushima Warm Current along isobaths of 100–200 m. The propagation of the vorticity anomalies causes the annual variation with a phase difference in the northeastward Kuroshio branch and the northward current west of Kyushu, and the behavior of the vorticity anomalies can be well explained as a first approximation by a dispersion relation of a quasigeostrophic topographic Rossby wave propagating in a mean flow. Generation of the vorticity anomalies from northeast of the Taiwan Strait and in the Tsushima Strait appears to be closely related to the annual variation of the Kuroshio’s current speed northeast of Taiwan and of volume transport through the Tsushima Strait, respectively.