The role of upper ocean stratification in resurgent marine heatwaves in the East/Japan Sea

The East/Japan Sea (EJS) has experienced increasingly frequent and intense marine heatwaves (MHWs) in recent years, characterized by prolonged periods of record-breaking sea surface temperatures (SSTs). This study investigates the two most severe MHW events in the EJS during recent summers—2021 and 2023—highlighting the resurgent nature of the 2023 event in contrast to the 2021 MHW, and exploring key differences in the regional coupled ocean–atmosphere system, including the mixed-layer heat budget and various oceanic and atmospheric processes influencing MHW evolution. In both years, the MHWs weakened in mid-August following typhoon passage; however, their subsequent evolution diverged markedly. The 2023 event developed into a resurgent MHW, recovering rapidly after temporary cooling and persisting for nearly three months. This resurgence was facilitated by favorable oceanic preconditioning—rapid shoaling of the mixed layer due to weak wind stress and enhanced stratification—which effectively trapped anomalous heat within the upper ocean, producing a sustained ocean heat-trapping effect. On the atmospheric side, the westward expansion of the Western North Pacific Subtropical High maintained persistent southerly winds that advected warm, moist air into the EJS. The resulting increase in extreme atmospheric humidity from the surface to the upper troposphere further reinforced SST anomalies by suppressing latent heat release from the ocean surface. The coupled feedback between the ocean and atmosphere generated a synergistic double heat-trapping effect that played a key role in sustaining the 2023 MHW. In contrast, although a high-pressure system was also present in 2021, stronger vertical ocean mixing during and after typhoon passage induced upper-ocean cooling, preventing MHW recovery. These findings underscore the crucial role of upper-ocean stratification in controlling MHW duration and recovery, offering new insights into the mechanisms governing extreme ocean temperature events in the EJS.