Numerical study of the ocean circulation on the East China Sea shelf and a Kuroshio bottom branch northeast of Taiwan in summer

Abstract

Using the Regional Ocean Model System, the ocean circulation on the East China Sea (ECS) shelf was examined by a fine-resolution model which was nested in a coarse-resolution Pacific Ocean model. The high-resolution simulation shows an accurate volume transport of 2.70 Sv (Sv = 10(6) m(3)s(-1)) through the Tsushima Strait, which is more consistent with the previous 5.5 year observation value (2.64 Sv) than former model results. For the Taiwan Strait it also shows a close volume transport (1.03 Sv) to a recent estimate (1.20 Sv). At the same time the model results reproduced almost all of the known circulation structure on the ECS shelf. In addition, the hindcast of 2009 shows a Kuroshio Bottom Branch Current to the northeast of Taiwan (KBBCNT). The KBBCNT is confirmed by the observational bottom high-salinity water (from 15 August to 2 September 2009) whose distribution is also reproduced by the model results. Tracer and particle experiments were carried out to elucidate the formation of the high-salinity water and the pathway of the KBBCNT. In light of the field observation and numerical experiments, a new pathway of the KBBCNT is proposed: bifurcated from the subsurface water of Kuroshio northeast of Taiwan, it upwells northwestward gradually from 300 to 60 m, then turns to northeast in the region around 27.5 degrees N, 122 degrees E, and finally reaches 31 degrees N off the mouth of the Changjiang River along similar to 60 m isobaths, forming the bottom saline water off the coast of Zhejiang province, China.