Abstract
Based on three climatologically observed temperature and salinity datasets (i.e., GDEM-V3, SCSPOD14 and WOA13), this paper reports a low-salinity (~34.32) water mass in the subsurface-to-intermediate layer around the Xisha Islands in the South China Sea. This water mass mainly subducts from the surface layer into the intermediate layer, characterized by a relatively low potential vorticity tongue extending from the bottom of mixed layer to the thermocline, and accompanied by a thermocline ventilation in spring (especially in April). The potential dynamics are the joint effects of negative wind stress curl, and an anticyclonic eddy triggered by the inherent topographic effect of the Xisha Islands, reflecting that downward vertical motion dominates the subduction. Despite lacking of the homogenous temperature and density, the low-salinity water mass is to some extent similar to the classic mode water and can be regarded as a deformed mode water in the South China Sea.
Highlights
The South China Sea (SCS) is the largest semi-enclosed marginal deep sea located west of the North Pacific (NP) (Fig. 1), and its circulation patterns are associated with East Asian monsoon and Kuroshio intrusion
Subduction of a low-salinity (~34.32) water mass is found in spring near the Xisha Islands (XS) in the SCS based on climatologically observed temperature and salinity datasets, and with a spatial scale of at least 200 km
The relatively homogeneous low-salinity water mass mainly subducts from the surface layer into the intermediate layer, accompanied by a thermocline ventilation, and a relatively low potential vorticity (PV) tongue vertically extending from the bottom of the mixed layer to the thermocline
Summary
The South China Sea (SCS) is the largest semi-enclosed marginal deep sea located west of the North Pacific (NP) (Fig. 1), and its circulation patterns are associated with East Asian monsoon and Kuroshio intrusion. The additional eddy-induced subduction can be regarded as the rectified transfer of a water mass from the mixed layer into the thermocline by an eddy-induced “bolus” velocity[19]. Observations presented that subduction caused by an anticyclonic eddy (AE) is comparable in magnitude to that by the mean flow[18]. These suggest that eddies play a significant role in the total subduction. In our study region (Fig. 1), our goals are to explore a subduction of low-salinity water mass which has been rarely reported before, and to preliminarily analyze the potential dynamic mechanisms for its formation, by using climatologically observed datasets. The present study will provide clearer insight into the circulation dynamics, ecological effect and climate change in the SCS
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