Marine Heatwaves in the East Asian Marginal Seas Facilitated by Boreal Summer Intraseasonal Oscillations

During the summer of 2016, the northern East China Sea and the southern Yellow Sea (NECS‐SYS) experienced one of the most severe and devastating marine heatwaves (MHWs) on record, with a temperature anomaly exceeding 4°C. This shallow semi‐enclosed continental shelf region is widely recognized as a significant hotspot for MHWs with associated incidences of harmful algae blooms. Previous studies have highlighted the importance of mixed layer shoaling as a crucial factor in the genesis of MHWs in the global ocean. The current study employed the Hybrid Coordinate Ocean Model reanalysis data set during 1994–2015 to delve into the mechanisms driving mixed layer shoaling during NECS‐SYS MHW genesis. Our findings reveal the significant role of the northward propagating boreal summer intraseasonal oscillation in promoting MHW genesis and intensification. Specifically, boreal summer intraseasonal oscillation phases 5, 6, and 7 contribute to the favorable conditions that facilitate MHW formation by inducing mixed layer shoaling and increasing solar influx, with mixed layer shoaling playing a more dominant role. The current study provides insights into the relative influences of wind, salinity, and temperature on mixed layer shoaling. We observe that wind plays the most significant role in mixed layer shoaling, followed by temperature and salinity. The boreal summer intraseasonal oscillation induced wind relaxation, increased shortwave radiation, and freshwater influx lead sea surface temperature by 7, 5, and 4 days, respectively. Importantly, mixed layer shoaling leads SST anomalies by 1–2 days. Therefore, the current study also suggests an intraseasonal predictability source for NECS‐SYS MHWs. The northern East China Sea and southern Yellow Sea (NECS‐SYS) experienced one of the most severe and devastating marine heatwaves (MHWs) during the summer of 2016. The region, situated in the northwest Pacific, is characterized by a shallow semi‐enclosed marginal sea, and receives a significant amount of freshwater influx from the Yangtze River. Previous studies have demonstrated that shoaling of surface mixed layer is a primary factor contributing to the genesis of MHWs in the global ocean. The present study emphasizes the role of the boreal summer intraseasonal oscillation in creating favorable conditions for MHWs genesis in these regions. Our research demonstrates that boreal summer intraseasonal oscillation phases 5, 6, and 7 contribute to the amplification of solar flux and the shoaling of mixed layer by reducing wind speed and introducing freshwater in this region. We provide a comprehensive understanding of the individual influences of wind, salinity, and temperature on mixed layer shoaling, highlighting the significance of mixed layer shoaling as a fundamental driver of MHWs. Moreover, our research also directs to an intraseasonal predictability source of the NECS‐SYS MHWs. Boreal Summer Intraseasonal Oscillation phases 5, 6, and 7 facilitate marine heatwaves (MHWs) in the northern East China Sea and southern Yellow SeaMixed layer shoaling is a more critical factor than the enhanced shortwave for MHW genesis and intensification in this regionOccurrences of MHWs in phase with boreal summer intraseasonal oscillation offer subseasonal predictability of MHWs Boreal Summer Intraseasonal Oscillation phases 5, 6, and 7 facilitate marine heatwaves (MHWs) in the northern East China Sea and southern Yellow Sea Mixed layer shoaling is a more critical factor than the enhanced shortwave for MHW genesis and intensification in this region Occurrences of MHWs in phase with boreal summer intraseasonal oscillation offer subseasonal predictability of MHWs