The present study explores the seasonal variability of the circulation in the Gulf of Tonkin (GoT) and associated water fluxes, and identifies the driving physical processes of this circulation. We use a 9 year (2009–2017)high-resolution numerical simulation. It is evaluated with historical hydrographic data, sea level data, high-frequency radar data and satellite products. Our results reveal a summer anticyclonic surface circulation and a winter cyclonic surface circulation, with a seasonal reversal of the circulation in the Hainan Strait (eastward in summer and westward in winter). In winter, a strong ($\sim$0.4 m s⁻¹) downcoast current developing in the western GoT is responsible for most of the water export out of the GoT. EOF analyses reveal that ageostrophic currents (wind-driven currents) mainly drive the seasonal variability of the surface circulation in the GoT. The downcoast current is however both wind and density (geostrophy) driven. Our results further show the strong influence of typhoons. They can significantly modulate the seasonal circulation in the GoT by reversing the summer circulation and intensifying the winter circulation. Finally, it is shown that during few days of their passage, typhoon can generate a total water flux up to 4 times higher than the monthly climatological flux.

本研究探讨了Tonkin湾（GoT）环流的季节性变化和相关的水通量，并确定了这种环流的驱动物理过程。我们使用了一个为期9年（2009-2017年）的高分辨率数值模拟。使用历史水文数据，海平面数据，高频雷达数据和卫星产品进行评估。我们的结果揭示了夏季的反气旋地表环流和冬季的气旋地表环流，海南海峡的环流有季节性逆转（夏季向东，冬季向西）。在冬季，$〜$在西部GoT中产生的0.4 ms ^-1）下流电流是导致GoT大部分水出口的原因。EOF分析表明，变质流（风驱动流）主要驱动GoT中地表环流的季节性变化。然而，下海岸电流是由风和密度（地球动力学）驱动的。我们的结果进一步显示了台风的强大影响。它们可以通过逆转夏季循环和加剧冬季循环来显着调节GoT中的季节性循环。最后，研究表明，台风经过几天后，总的水通量可能会比月度气候通量高出四倍。