A salinity front in the southern East China Sea separating the Chinese coastal and Taiwan Strait waters from Kuroshio waters

While the current in the entire cross section of Taiwan Strait has been considered for a long time to flow northward in summer, in this paper the observational results are presented to indicate that only the western three-quarters of the northernmost part of the Taiwan Strait is occupied by the Chinese coastal and Taiwan Strait waters, and the remaining part by the lower temperature and higher salinity Kuroshio subsurface waters which have circumvented the northern tip of Taiwan. Chinese coastal and Taiwan Strait waters are separated from Kuroshio waters by a salinity front. In fact, the waters are vertically well-mixed near the front where the surface waters show a clear horizontal salinity maximum but a weak temperature minimum. These surface waters are in sharp contrast to the bottom waters which show a horizontal salinity minimum but a strong temperature maximum. Subsurface waters near the front are distinctively warmer and lighter but less saline and less oxygenated than the waters on either side. The warm, fresh, light, low-oxygen characteristics of these subsurface waters are attributed to the downwelling of waters from the east which have upwelled earlier near the shelf-break northeast of Taiwan. These waters can be traced along the isobath of about 75-m to at least 30°N in the East China Sea (ECS). Similarly, the water column-integrated nutrients, chlorophyll a and primary productivity also show a horizontal minimum near this depth. Such an unusual subsurface warm temperature ridge but low salinity trough correctly delineates the seaward boundaries of the coastal and Taiwan Strait waters in the ECS. The salinity maximum at the surface also seems to define the seaward boundaries of the fresh (S<33) coastal waters.