Classification and Causes of East Asian Marine Heatwaves during Boreal Summer

Marine heatwaves (MHWs) are among the most severe climatic events under global warming conditions; they damage marine ecosystems and result in socioeconomic losses. In East Asia, the sea surface temperature (SST) has in-creased 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 yr -1. The significant trend is mainly determined by the Subtropical Gyre mode, whereas other modes do not exhibit significant trends. We found that the East China Sea mode could be attributed to a strong East Asian summer monsoon via ocean stratification by the low-salinity water from the Yangtze (Changjiang) River. 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 MHW 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.

Automated summary

Marine heatwaves (MHWs) are severe climatic events under global warming, causing damage to marine ecosystems and socioeconomic losses. The sea surface temperature (SST) in East Asia has increased significantly, requiring studies to minimize the impact of MHWs.