Thermohaline circulation in the deep South China Sea basin inferred from oxygen distributions

Abstract

On the basis of the historical oxygen data available, distributions of dissolved oxygen were analyzed in detail to investigate the thermohaline circulation in the South China Sea (SCS). The analyses confirmed the existence of a layer of minimal oxygen content at intermediate depths of the basin, which shows a low‐oxygen (<1.6 mL L−1) core in the southern basin with water “ages” shorter than 100 years after entering the SCS and which appears isolated from its counterpart of the western Philippine Sea. North to the core along the northern slope, there is a band of oxygen‐rich water in both upper and intermediate layers. Above 700 m the band can be attributed to the intrusion of high‐oxygen water through the Luzon Strait, while vertical flux appears to be responsible between 700 and 1500 m. The oxygen distribution provides additional evidence for a sandwiched vertical structure in transports through the Luzon Strait, with outflows in the intermediate layer and inflows above and below. While the abyssal water of the SCS is being replenished by the deep inflows, an abrupt change of water properties was observed in both upper and intermediate layers across the Luzon Strait, suggesting possible domination of local vertical mixing over horizontal spreading of inflows in the northern SCS. A four‐cell conceptual model was proposed for the thermohaline circulation, which consists of a three‐layer, cross‐basin exchange cell in the Luzon Strait, an active ventilation cell in the upper and intermediate layers along the northern slope, an abyssal replenishment cell in the deep, and a cyclonic recirculation cell in the interior basin.