Drastic Fluctuation in Water Exchange Between the Yellow Sea and Bohai Sea Caused by Typhoon Lekima in August 2019: A Numerical Study

Connecting the Bohai Sea (BS) and Yellow Sea (YS), the Bohai Strait is vital in controlling physical-biogeochemical conditions in the East Asian marginal seas. In August 2019, Typhoon Lekima moved northward inland into eastern China and caused drastic water exchange through the Bohai Strait. The specific variation and underlying dynamics involved were investigated with observations and high-resolution numerical simulations. Volume transport featured an intense inflow followed by an intense outflow, with a fluctuation magnitude of 1.88 Sv, which was much greater than the seasonal-interannual variations. The corresponding current patterns presented three phases: strait-wide inflow, strait-wide outflow, and outflow in the south and inflow in the north. The strait-wide inflow was driven by strong east-northeasterly winds through local Ekman dynamics. The strait-wide outflow resulted from the dynamic adjustment of ocean currents accompanying the rapid weakening of easterly winds. The third current pattern was part of a large-scale cyclonic circulation in the BS and North YS and was governed by different factors in the southern and northern straits. Further comparisons indicated that transport caused by winter storms and other typhoons in the year had opposite phase variations to that caused by Lekima. The underlying dynamics mainly involved local Ekman dynamics and/or remote wind forcing via the propagation of coastal trapped waves but could differ greatly from events. Finally, we found that to a large extent, geostrophic transport derived from the sea surface height difference across the Bohai Strait could be used as actual transport during synoptic weather events.