Increasing heat stress across Southeast Asia driven by compound warming and moistening

Abstract. Heat stress is intensifying across Southeast Asia under global warming, yet the relative influences of atmospheric warming and moistening across timescales remain insufficiently quantified. This study investigates the thermodynamic drivers of warm-season (April–October) heat-stress intensity, measured by the daily maximum wet-bulb globe temperature (WBGTmax), and frequency, defined as the annual number of extreme heat-stress days (Nxday), across Southeast Asia and its 20 climatic sub-regions. Using observations together with dynamically downscaled CORDEX–SEA simulations, we apply a unified attribution framework to separate the effects of air temperature, specific humidity, and residual nonlinear processes on historical (1985–2014) trends, interannual variability, and projected late-century changes (2071–2100 relative to 1985–2014) under SSP5–8.5. Historical increases in WBGTmax and Nxday are dominated by temperature across much of the region. However, humidity already provides important amplification in monsoon-influenced lowlands, including Indochina, the Philippines, and parts of the Malay Peninsula. Future projections indicate a coherent basin-wide shift toward compound warm–humid conditions. In many monsoon regions, rising moisture contributes roughly 30–50 % of the increase, consistent with enhanced atmospheric water-holding capacity in a warmer climate. In contrast, interannual variability, particularly over the Maritime Continent, is strongly governed by nonlinear temperature–humidity interactions, which generate substantial unexplained components. Because Nxday depends on threshold exceedance, it shows a stronger amplification of future change than WBGTmax, even though its long-term evolution remains primarily temperature controlled. Overall, the results demonstrate that heat-stress escalation in Southeast Asia increasingly reflects rising atmospheric moist enthalpy rather than dry-bulb warming alone, underscoring the need for adaptation strategies that explicitly consider both temperature and humidity.