Unprecedented Marine Heatwave Significantly Exacerbated the Record‐Breaking 2023 East Asian Summer Heatwave

Abstract The 2023 East Asian summer experienced a record‐breaking compound hot‐humid extreme and an unprecedented concurrent marine heatwave (MHW) in surrounding oceans. Understanding and quantifying the impacts of such MHWs on land heatwaves can enhance seasonal prediction skills for these extremes. Here we evaluate the impact of the 2023 MHW on the record‐breaking atmospheric heatwave in East Asia during the summer. Through a set of regional atmospheric model experiments, we demonstrate that the MHW significantly exacerbated the 2023 East Asian summer atmospheric heatwave, particularly as a compound hot‐humid extreme. The extremely warm ocean amplified both surface air temperature and humidity anomalies, on top of the contributions of background atmospheric circulation anomalies. The MHW influence on air temperature manifests through radiative effects of cloud and water vapor changes. Our results also indicate that the atmospheric heatwave amplifies despite the MHW acts to dampen the western North Pacific subtropical high against large‐scale background atmospheric conditions. By examining the effects of both temperature and humidity anomalies through wet‐bulb globe temperature, we find that the MHW explains ∼20%–50% of the aggravation and prolonged duration of hot‐humid conditions in East Asia, with notable impacts in Japan. Additionally, the recent trend of sea surface warming is shown to substantially amplify the MHW's impact on the heatwave. Our findings underscore the key role of ocean variability and air–sea interactions in surrounding oceans on atmospheric heatwaves that occur in the summer climate in East Asia, highlighting a unique process in the humid and low cloud‐abundant maritime East Asia region.