The oceanography of the Northern south China Sea Shelf-Sea (NoSoCS) and its adjacent Waters—overview and Highlights

Abstract

Tropical shelf-Seas constitute a sub-set of shelf-seas that has not been studied extensively. The Northern South China Sea Shelf-sea (NoSoCS) study is a continuation of the longstanding interest in Taiwan on the oceanography of the East Asian shelf-seas, and is an attempt to provide systematic and synoptic observations on a tropical shelf-sea. The two basic hypotheses in the study are: (1) as in the Ocean interior, the behaviors of the shelf-seas vary with latitude; and (2) the behaviors of the NoSoCS are representative of those of the shelf-seas in the tropical zone. The NoSoCS has been sampled bi-annually in alternating seasons since 2009. The results obtained primarily in 2009 to 2012 are presented here. As in the tropical Ocean interior, a distinguishing characteristic of the NoSoCS is its shallow mixed layer, with depths of about 40m in the summer and 70m in the winter. As similar mixed layer depths are found in the adjoining Northern South China Sea (SCS) and they are shallower than the shelf break depth of 120m, the upper nutricline Water in the Northern SCS extends freely into the NoSoCS and forms a layer of Water with significant concentrations of the nutrients immediately below its mixed layer. Any process that can induce vertical mixing on the shelf may make the nutrients in this subsurface Water available for supporting biological activities in the mixed layer in the NoSoCS. Such processes include winter convective overturn over the entire NoSoCS as a result of surface cooling and the strong northeast monsoonal wind, winter formation of bottom Water in parts of the inner and middle shelf, the action of internal waves at the outer shelf and the continental slope, and upwelling maintained by wind and/or topography. The upwelling takes place in the summer off the coasts of Dongshan–Shantou and the northeast coasts of the Hainan Island, but year-round at the Taiwan Bank. Upwelling over the shelf break is not required for bringing the nutrients in the sub-surface Waters in the open SCS to the NoSoCS, and it was not observed. In addition to vertical mixing, terrestrial inputs, especially through the outflow of the Pearl River, which reaches its peak in the summer, also contribute to the spatial and temporal variations in the composition of the NoSoCS. The combined effects of all these processes lead to a unique seasonal pattern in the variations of the surface concentrations of chlorophyll-a in the NoSoCS, with two distinct yearly maxima in the winter and summer.