Rapidly Changing East Asian Marine Heatwaves Under a Warming Climate.

The East Asian marginal seas (EAMS) are one of the fastest‐warming ocean regions globally. This study presents the long‐term trends (1982–2020) of extreme ocean warming events called "marine heatwaves" over the EAMS and examines the relationships between marine heatwave trends and mean SST warming trends. We focus on five subregions with different influences from atmospheric perturbation and ocean currents: the northern East Sea (N‐ES), southern East Sea, Yellow Sea, Korea Strait (KS), and East China Sea (ECS). During the past four decades, marine heatwave duration and intensity in the EAMS have increased to approximately +4 days and +0.3°C per decade on average, respectively. In summer, the positive trend of marine heatwaves is the highest in the ECS, primarily due to the rapidly increasing mean sea surface temperature (SST). In winter, the N‐ES reveals remarkably rapid increases in marine heatwave properties in the last two decades, with increasing rates of approximately 6.2 (4.9) times longer total duration (stronger intensity) than the global average changes. Beyond the impact of the rapid increase in mean SST, the N‐ES marine heatwaves can be further extended due to the northward shift of the East Korea Warm Current. In general, mean SST changes are critical to the increasing trend in marine heatwave duration and intensity. This study further emphasizes that the changes in ocean circulation may expedite more rapid changes in extreme ocean events, which can produce more vulnerability in some places, such as the N‐ES, to marine heatwaves under continued global warming. Plain Language Summary: Extreme warming events of the ocean occurring from several days to weeks significantly impact marine ecosystems. This study investigates the increasing trends of extremely high ocean warming, namely "marine heatwave," over East Asian marginal seas. During the past four decades, marine heatwave duration and intensity have increased to approximately +4 days and +0.3°C per decade on average, respectively. Comparing the earlier period (1982–2000) and the recent period (2001–2020), the most dramatic changes in marine heatwave properties, including frequency, duration, and intensity, occur in the northern East Sea in winter, significantly increasing 763.33% in duration. During summer, the East China Sea reveals the most significant increasing ratio of marine heatwave properties, with change rates of 227.07% in duration. Marine heatwave characteristics have a statistically increasing trend in both seasons in most regions due to global warming. Beyond the impact of the change in mean SST, we find the change in SST variance via shifting ocean current leads to the excessive trend of marine heatwaves. Key Points: East Asian Marine heatwave duration and intensity have enhanced to approximately +4 days and +0.3°C per decadeDuring winter, the marine heatwave duration over northern East Sea regions experiences a dramatic increase of 763.33%, much higher than the global average increase of 122.63%Northward ocean current variance is a significant factor in the excessive increase of marine heatwave duration, in addition to the changing mean sea surface temperature [ABSTRACT FROM AUTHOR]