Tongue-shaped frontal structure and warm water intrusion in the southern Yellow Sea in winter

Abstract

In winter, a thermohaline front forms at the Yellow Sea (YS) entrance where the warm and saline Cheju Warm Current (CWC) water meets cold coastal water. The frontal structure, as well as the northwestward intrusion of the warm water, was investigated by analyzing conductivity-temperature-depth (CTD) data, tracks of drifting floats, moored current data, and satellite images. The CWC water advances westward as a tongue with its tip heading westward across the YS entrance, not intruding northward along the deepest part of the YS trough, whereas a secondary warm water band is developed northwestward along the western flank of the trough. Floats in the warm tongue moved northwestward across the western frontal zone where the frontal structure was weak, reflecting the northwestward intrusion along the 50-70 m isobaths through the frontal zone. Currents in the trough do not always meet the upwind flow theory that, in an elongated basin, the wind-driven flow is upwind in the deep channel, but rather show the predominance of the southward downwind flow. Model experiments show that on the western flank of the trough, both the southeastward tide-induced residual flow and the southeastward flow due to the nonlinear effect between tide and wind are generated at the same time. Thus the intermittent intrusion of the CWC across the western frontal zone may be closely associated with the predominance of the upwind flow over the southeastward nonlinear interacted flow.