Classification and Causes of East Asian Marine Heatwaves during Boreal Summer

Abstract

Marine heatwaves (MHWs) are among the most severe climatic events under global warming conditions; they damage marine ecosystems and result in socio-economic losses. In East Asia, the sea surface temperature (SST) has increased significantly and is projected to increase further. Considering such increases in SST over East Asia, studies on MHWs are needed to minimize the damage caused. In this study, we classified the spatiotemporal characteristics of East Asian MHWs during boreal summer with a self-organizing map for 39 years (1982–2020) and determined their unexplored possible mechanisms. Four dominant modes of MHWs across East Asia were identified: the (1) Subtropical Gyre, (2) East China Sea, (3) East Sea/Japan Sea, and (4) Yellow Sea modes. The total annual number of MHW days has increased by about 1.23 days/year. The significant trend is mainly determined by the Subtropical Gyre mode, whereas other modes do not exhibit significant trends. As the East China Sea mode could be linked to a strong East Asian summer monsoon (EASM), the resultant low-salinity water from the Yangtze (Changjiang) River could stratify the ocean and increase the water temperature. The East Sea/Japan Sea and Yellow Sea modes could be linked to remote forcings over the subtropics, leading to persistent anticyclonic circulation over East Asia. The anomalous anticyclone contributed to the MHWs occurrences by enhancing downward shortwave radiation and reducing entrainment cooling caused by Ekman downwelling. These findings have important implications for distinguishing the characteristics of East Asian MHWs and reveal the possibility of predicting them by identifying their precursors.