Three‐Dimensional Water Exchanges in the Shelf Circulation System of the Northern South China Sea Under Climatic Modulation From ENSO

Abstract

AbstractUsing the numerical calculations of exposure time (), this study investigated the three‐dimensional characteristics of water exchanges associated with the coastal‐shelf circulations of the Northern South China Sea (NSCS). The circulation connectivity and its interannual variability under the modulation of El Niño Southern Oscillation (ENSO) were investigated. The gradually increased from approximately 50 days over the outer shelf to more than 150 days in the shallower coastal seas with the Beibu Gulf displaying a significantly longer (approximately 300 days). The Beibu Gulf played an influential role in the water exchange that had a contribution of over 30% to in the most regions. The results from sensitivity experiments indicate that the wind forcing is the primary controller of and the boundary flux plays a secondary role. Specifically, the wind enhances water exchange in western NSCS, particularly in the Beibu Gulf and Yuexi region. Conversely, boundary fluxes facilitate exchanges in the eastern shelf, such as Yuedong region. Analysis revealed a clear modulation from ENSO on water exchange, wherein El Niño and La Niña years displayed symmetrical morphologies in distribution anomalies, albeit with varying intensities. During El Niño years, the weakened southwestward shelf current increased and the shelf water was less likely to reach the Beibu Gulf, and once arrived, these waters were trapped inside to elongate . In contrast, during La Niña years, the strengthened southwestward shelf current shortened and more shelf waters were transported away from the NSCS by an intensified southwestward shelf current.