Dynamics of the cross-shelf circulation in the Yellow and East China Seas in winter

Abstract

A general ocean circulation model is used to study the dynamics of the cross-shelf circulation in the Yellow and East China Seas in winter. The model results show that the shelf circulation in winter is induced primarily by the northerly monsoonal winds. Compared with the wind-driven circulation, the Kuroshio-forced circulation over the shelf is very weak. The westward shift of the Yellow Sea Warm Current is produced by the northerly winds through the generation of a sea-level trough off the Subei coasts and a sea-level ridge along the center of the Yellow Sea topographic trough. The sea-level trough off the Subei coasts is produced because the sea level at the coast is tied to the depression in the northern Yellow Sea and is moderated by the surface Ekman transport of the northerly monsoon. The sea-level ridge along the center of the Yellow Sea topographic trough is produced because the sea level along both the east China and the west Korea coasts decreases. The structure of the sea level gives rise to a geostrophic intrusion against the wind along the western side of the Yellow Sea trough and southward movement of the Korean coastal currents along the eastern side of the trough. Under the northerly wind forcing, the currents over the East China Sea shelf flow southward in an opposite direction to the Yellow Sea Warm Current. The resulting divergence, therefore, forces the Yellow Sea Coastal Current to move offshore to form the East China Sea Current. At the head of the submarine canyon off the Changjiang mouth, a northward intrusion is produced by downwind pile-up of sea level and by the wind-curl forcing. The Yellow Sea Warm Current and the canyon intrusion then extend southward through the propagation of topographic Rossby waves to reverse the wind-driven currents over the East China Sea shelf, generating the offshore and inshore branches of the Taiwan Warm Current, both of which serve to raise the sea level over the East China Sea shelf. The final adjustment to the northerly wind forcing is a sea-level tilt to the south, which forces the East China Sea Current to flow cross the continental shelf and slope into the Okinawa Trough. The numerical experiments thus suggest that both the inshore and offshore branches of the Taiwan Warm Current are forced primarily by the northerly winds in winter.