Modulations of Subsurface Circulation in a Shallow Marginal Sea by Extreme Typhoon Forcing, Revealing Hourly-Scale Transient Dynamics

Typhoons can substantially perturb the thermal and circulation structure of marginal seas, yet in the Yellow Sea most studies have focused on surface cooling, vertical mixing, and coastal upwelling/downwelling, while the hourly-scale response of subsurface circulation below the thermocline remains poorly understood. Using the Finite Volume Community Ocean Model (FVCOM) combined with ERA5 reanalysis and satellite remote sensing, this study investigates the subsurface response of the South Yellow Sea during Typhoon Lekima (2019). Results reveal a pronounced transient sub-basin-scale anticyclonic circulation below the thermocline (30–70 m depth, maximum at 40–60 m) lasting approximately 15 h. This circulation exhibits significantly negative relative vorticity (O(10−5) s−1) and coherent vertical structure, driven by barotropic pressure forcing. The barotropic sea surface height anomaly was traced to coastal wind forcing along the Shandong Peninsula and northern Jiangsu margins, which subsequently extended into the central basin. These findings reveal an overlooked pathway by which typhoons rapidly reorganize subsurface circulation in shelf seas, with implications for understanding the hourly-scale response of shelf seas to intensifying typhoon activity.