Intraseasonal Variability of the Winter Western Boundary Current in the South China Sea Using Satellite Data and Mooring Observations

The intraseasonal variability of the winter western boundary current (WBC) in the South China Sea (SCS) is investigated using satellite data and mooring observations. The intraseasonal intensification of the northeasterly wind enhances the cross-shelf sea-level gradient, and, thus, causes the basin-scale cyclonic circulation to spin up within around one week. The Southern Cyclonic Gyre in the southwestern SCS further strengthens due to the intensified inertial recirculation of the WBC. Complex empirical orthogonal function analysis reveals that the large sea-level fluctuation around the downstream WBC can mainly be explained by two dominant modes. The first mode is related to intraseasonal variations of the wind field and its spatial field represents the response of the WBC to the westward Rossby waves originated from the interior basin, while the second mode reflects the southward propagation of mesoscale signals along the WBC. Mooring observations recorded the accelerations of the southward WBC three times in winter 2004/2005. While the first two could be linked to the wind intensification, the third one was associated with a cyclonic eddy closely next to the WBC. Initially, this eddy propagated from the Luzon Strait, where active mesoscale processes associated with the Kuroshio intrusion act as important disturbance sources for the SCS.