Upper ocean responses to three sequential tropical cyclones in the stratified yellow sea during summer 2020

Abstract

Three sequential tropical cyclones (TCs), Bavi, Maysak, and Haishen, passed over the Yellow Sea (YS) during summer 2020, separated by intervals of seven and five days. Focusing on this unique synoptic event, this study investigated the corresponding sea surface temperatures (SSTs) variations and the underlying hydrodynamic mechanisms using numerical simulations and satellite and in-situ observations. Notable SST decreases occurred in the eastern YS following each TC due to intense vertical mixing, with Bavi causing the strongest cooling due to its intensity and the undisturbed ocean stratification. Subsequently, despite similar weather conditions, Maysak induced almost twice as much surface cooling as Haishen. Upper ocean salinity generally increased in the eastern YS under strong mixing which override the freshening effect of precipitation. By conducting TC vortex removal experiments, we demonstrated that the low SSTs following Maysak and Haishen were primarily due to cold water entrained into the upper layer left by Bavi. Surface cooling caused by Maysak was intensified by Bavi's prior influence, whereas that caused by Haishen was weakened by both Bavi and Maysak's influence. This was because rapid SST rewarming after Bavi created a high pre-TC SST for Maysak; however, rewarming only occurred in the thin surface layer, leaving the water below still cold. For Haishen, the pre-TC SST remained low and the mixed layer was deeper, resulting in much weaker surface cooling despite its intensity being comparable to Maysak. This study enhances our understanding of the responses of stratified shelf waters to sequential TCs.