An Integral View of the Gulf of Tonkin Seasonal Dynamics

Abstract As part of a U.S. – Vietnam collaborative effort, a numerical model is used to describe the dynamics associated with the Gulf of Tonkin seasonal variability. Unique to this study is the synthesis of historical and recent observations throughout the gulf that are leveraged to validate the model's seasonal component. The dynamics is primarily forced by seasonally reversing monsoons, remote transports through the Qiongzhou Strait and by the western boundary current, and river discharge. Monsoonal winds set up pressure gradients across the gulf that tend to be balanced by geostrophic flows (cyclonic during the winter and anticyclonic during the summer). However, a number of processes modify the wind‐driven dynamics with all of them promoting cyclonic circulation. In the eastern region, the tides influence the seasonal variability via (a) a tidal‐residual flow producing an influx of mass through the Qiongzhou Strait and (b) through modification of the summer baroclinic density field. In the western region, the seasonal variability can be strongly dependent on river buoyancy flows and intrusions of the western boundary current. Southwest summer winds produce a low salinity bulge near the Red River mouth that releases and advects as a southward coastal current during the fall as winds relax and reverse direction. Additionally, the vorticity budget shows that topographic steering constrains the seasonal variability promoting intrusions of the western boundary current during winter monsoon conditions, when the current carries water of shallow origin from the Chinese shelf.