The increasing frequency of heatwaves over East Asia (EA) is impacting agriculture, water management, and people’s livelihood. However, the effect of humidity on high-temperature events has not yet been fully explored. Using observations and future climate change projections conducted with the latest generation of Earth System models, we examine the mechanisms of dry and moist heatwaves over EA. In the dry heatwave region, anticyclonic circulation has been amplified after the onset of heatwaves under the influence of the convergence of anomalous wave activity flux over northern EA, resulting in surface warming via adiabatic processes. In contrast, the moist heatwaves are triggered by the locally generated anticyclonic anomalies, with the surface warming amplified by cloud and water vapor feedback. Model simulations from phase six of the Coupled Model Intercomparison Project projected display intensification of dry heatwaves and increased moist heatwave days in response to projected increases in greenhouse gas concentrations.

In 2018, heatwaves (HWs), which are defined as period of abnormally hot weather with the daily maximum temperature (T_Max) exceeding its 95th percentile threshold for at least three consecutive days, were prevalent during June to August, and temperature records were broken in many countries over East Asia (EA) including China (CH), Japan (JP), and the Korean Peninsula (KP). In particular, extreme HWs through July to August lasted for the longest duration with 21.3 days with T_Max reaching 36.9°C. The highest T_Max, recorded since 1907, exceeded to 41°C in Hongcheon, located east of Seoul in the KP. Here, we examine the factors that influenced the 2018 HW, and how these relate to the 1994 HW, which was second longest HW recorded in the KP. The results show that abnormally strong and northwestward extended anticyclone features observed in July 2018 lasted as a persistent North Pacific anticyclone anomaly until August 2018 centered at the northern KP. These anticyclone features subsequently formed as a modon-like blocking with a cyclonic anomaly in the East China Sea. Meanwhile, in August 1994, the North Pacific High (NPH) extended to eastern EA, which broked the meridional dipole structure and HWs do not persist. The NPH, which persisted till August 2018, was accompanied by a sinking motion, suppression of precipitation, anomalous maximum temperature, weakening of the westerly jet stream, and increased insolation due to clear sky. We find that the prolonged and northwestward-shifted NPH including the KP drove the extraordinarily hot 2018 summer in Korea. In addition, low precipitation and massive evapotranspiration with the persistent insolation in July 2018 influenced the dry condition at the surface. We suggest that the predictions for the location and duration of the NPH associated with the HWs are required to reduce heat-related mortality and the impact on agriculture due to excessive evapotranspiration.

In this work, we projected future drought risks over East Asia from the perspective of evapotranspiration (ET) deficit using the new climate projections of the Coupled Model Intercomparison Project Phase 6 (CMIP6). For the regional drought projections, we employed a state‐of‐the‐art complementary relationship (CR) method to estimate ET and atmospheric evaporative demand (Ep) that quantify the combined impact of water deficiency and vapor pressure deficit stress. The performance of the CR method was validated using the reanalysis climate data and independent water‐balance ET estimates. The same CR method was again applied to hindcasts and projections of seven global climate models (GCM) in the CMIP6 archive. Results showed that the CR ET estimates and the GCM‐driven ET products were both able to approximately close the basin‐scale water balance and tightly correlated with each other. The application with the CMIP6 GCM hindcasts provided the same indication that the CR ET and the GCM‐driven ET are consistent. The drought assessments using the ET deficit (ET − Ep) from the CMIP6 projections implicates that future drought risks over East Asia would be much higher than indicated by precipitation or soil moisture projections. Rising trends of Ep are expected to outpace those of ET even under the medium‐level greenhouse gas emission scenario. Our new drought projections suggest that Ep amplified by soil water deficiency could raise agricultural and wildfire risks substantially by the end of the 21st century.

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Offline drought indices have been utilizable for monitoring drought conditions, but their reliability in projecting future drought risks is uncertain due to non-meteorological influences on atmospheric water demand (Ep). This study investigates the impact of surface resistance sensitivity (rs) to elevated CO2 (eCO2) on projections of future flash droughts (FD). We evaluate FD risks across an East Asian region during a historical period (1981-2020) and a future period (2021-2060) using two Evaporative Stress Index (ESI) series. One series employs the conventional Penman-Monteith (PM) equation for Ep, while the other incorporates a generic rs sensitivity to eCO2 derived from advanced Earth System Models (ESMs). We compare the FD risks identified by the two ESI series with assessments based on soil moisture data from atmospheric reanalysis and multiple ESM projections under two emission scenarios linked with the Shared Socioeconomic Pathways. Results showed that the response of rs to eCO2 has had minimal influences on temporal variations of ESI for the past decades, likely due to its low sensitivity and the masking effects of other environmental factors. However, for the future decades, the ESI projected by the conventional PM equation significantly diverged from soil moisture projections, overestimating future FD risks even under a low emission scenario. We found that incorporating the generic rs sensitivity into the PM equation did not simply resolve the disparity in FD frequencies between ESI and soil moisture projections. Many associated factors contributing to stomatal responses to eCO2 complicate the understanding of future flash drought risks. This study suggests that overreliance on the conventional Ep formula, which neglects non-meteorological effects, could decrease the ability of ESI to detect future FD events under eCO2.