Responses of the Zhe-Min coastal current adjacent to Pingtan Island to the wintertime monsoon relaxation in 2006 and its mechanism

In conjunction with synchronous remotely sensed winds and sea surface temperature (SST), the spatiotemporal features of the Zhe-Min coastal current (ZMCC), especially responses of the ZMCC adjacent to Pingtan Island (PT) to the wintertime monsoon relaxation in 2006 and corresponding mechanism are investigated based on the field observations. In situ data are acquired from Conductivity-Temperature-Depth (CTD) cruise and Bottom-Mounted Moorings (BMM), which are conducted during a comprehensive survey for the Chinese Offshore Investigation and Assessment Project in winter 2006. It is revealed that the ZMCC is well mixed vertically in winter 2006. The ZMCC (<14°C) recedes during the relaxation of the wintertime monsoon and is accompanied by the enhanced northward shift of the warm, saline Taiwan Strait Mixed Water (TSMW, higher than 14°C and is constituted by the Taiwan Strait Warm Water and the Kuroshio Branch Water). And greatly enhanced southward intrusion of the ZMCC can be detected when the wintertime monsoon restores. Correspondingly, the thermal interface bounded by the ZMCC and the TSMW moves in the northwest/southeast direction, leading to periodic warm/cold reversals of the near-seabed temperature adjacent to the PT. By EOF (Empirical Orthogonal Function) analysis of the large-scale wind fields and wavelet power spectrum analysis of the water level, ocean current and the near-seabed temperature, responses of the ZMCC off the PT to wintertime monsoon relaxation are suggested to be attributed mainly to the southward propagating coastally trapped waves triggered by the impeding atmospheric fronts. As a result, ocean current and near-seabed temperature demonstrate significant quasi-5 d and quasi-10 d subtidal oscillations. By contrast, the onshore/offshore water accumulation resulted from Ekman advection driven by the local winds has minor contributions.