Baroclinic responses of Qiongzhou Strait throughflow to different forcings

The Qiongzhou Strait (QS) throughflow is affected by the seasonal circulation in both the northern South China Sea (SCS) shelf and the Beibu Gulf. The water masses with distinct properties between these two regions signify the baroclinic processes in the QS. In this study, a high-resolution unstructured-grid Finite Volume Community Ocean Model (FVCOM) was used to investigate the features and dynamic mechanisms of baroclinic-induced currents (BCC) in the QS. Different forcings were considered such as tides, river discharges, winds, and remote forcings (low-frequency sea surface level and current variations). The model results indicate that the volume transport of the BCC contributes to 31.26% of the total in the QS, determining the transport direction in summer. The baroclinic response to low-frequency remote forcings is most pronounced during the summer-autumn season, but less than winds during the winter-spring season. This finding underscores the importance of low-frequency remote forcings on the QS throughflow, which has often been overlooked in previous studies. Momentum balance suggests that the BCCs are predominantly driven by both the barotropic and baroclinic pressure gradient forces. Decrease (increase) in either of these two forces will lead to an intensified eastward (westward) flow. The contribution of the horizontal advection is relatively weak but cannot be ignored. This study highlights the baroclinic responses of QS throughflow to different forcings, which is important when investigating the circulation in the northern SCS, especially in the Beibu Gulf.