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 (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.

风、地形和台风对北部湾环流季节变化的影响：一项高分辨率三维区域模拟的研究

中文摘要：本研究探讨了北部湾（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9年（2009–2017）的高分辨率数值模拟。利用历史水文数据、海平面数据、高频雷达数据和卫星产品对其进行评估。我们的结果揭示了一个夏季反气旋表面环流和一个冬季气旋表面环流，琼州海峡环流季节性反转（夏季向东，冬季向西）。在冬季北部湾西部发展出的强劲沿岸流(～0.4米秒?1）对海水流出北部湾起主要作用。EOF分析表明，非地转流（风驱动流）主要驱动GoT海表环流的季节性变化。然而，沿岸流是由风和密度（地转流）驱动的。我们的结果进一步表明台风的强烈影响。它们可以通过逆转夏季环流和加强冬季环流来显著调节GoT的季节环流。最后，结果表明，台风在经过的几天内，可以产生比月气候通量高4倍的总水通量。