The ocean-atmosphere interaction over a summer upwelling system in the South China Sea

Abstract

Abstract The ocean-atmosphere interaction over a summer upwelling system in the South China Sea (SCS) is investigated. A strong positive correlation between sea surface temperature (SST) and wind stress exists near the southeast coast of Vietnam, where strong SST fronts occur during summertime. This air-sea coupling relationship indicates that the ocean drives the atmosphere and receives feedback at the oceanic mesoscale, in contrast with the case for large scales. The coupling process off the southeast coast of Vietnam is initiated by coastal upwelling, which is driven by wind-induced offshore Ekman transport in early summer. Cold upwelled water reduces the wind stress aloft and results in a positive wind stress curl. The Ekman pumping induced by the wind stress curl is on average 18% stronger than that of the offshore Ekman transport and occupies a larger extent. As the upwelling season terminates by the end of summer, the air-sea coupling also diminishes. Our results reveal that the SST-induced wind stress curl anomalies have a positive feedback on SST change and that wind stress curl anomalies can further reduce the SST magnitude by as much as 0.5 °C. The factors influencing the strength and interannual variability in ocean-atmosphere interaction are also explored. The wind stress directional steadiness (WSDS) plays an important role in the air-sea coupling over the SCS SST frontal regions. The coupling between the wind stress and SST can fully develop under high WSDS. Furthermore, the interannual variabilities in air-sea coupling are largely impacted by the El Nino events by changing the regional wind. The influence of a strong El Nino event on the summer upwelling system is characterized by a warmer SST and declining winds, which are associated with weakened SST gradients and mesoscale air-sea coupling. The present study reveals that the interannual climate signals have a significant impact on the mesoscale ocean-atmosphere interaction in a typical upwelling system. It provides an example for investigation of seasonal and interannual variability of mesoscale air-sea coupling in other coastal regions.