Mechanisms of Tropical Intraseasonal Oscillations Modulating Humid Heatwaves in the Asian Monsoon Region

Abstract Humid heatwaves—extended periods of high temperatures and humidity—pose significant risks to human health and ecosystems. While long-term changes have been examined, their subseasonal variations and mechanisms in the Asian monsoon region remain less well understood. Spectral analysis of daily wet-bulb temperature (Tw) data from June to August (1979–2021) reveals significant 10–30 day variability across South, Southeast, and East Asia, suggesting modulation by the quasi-biweekly oscillation (QBWO). The occurrence and intensity of humid heatwaves, defined as Tw exceeding the 90th percentile for at least 3 days, fluctuate with the QBWO life cycle, with regionally distinct mechanisms. For central India, the humid heatwave probability nearly doubles during phase 2 of the real-time BSISO2 index, driven by a 10–30-day anticyclonic anomaly over the northern Bay of Bengal and India, promoting moisture advection from the Bay of Bengal towards India and enhanced shortwave radiation due to locally reduced cloud cover. In northern Indochina, phase 3 leads to an 81% increase in humid heatwave occurrence through suppressed convection-induced diabatic (evaporative) and adiabatic (warming) processes at the boundary layer. In southern China, phase 5 enhances the heatwave probability by 54%, with a southwesterly anomaly producing moisture convergence and warm advection. Operational subseasonal-to-seasonal prediction models from the ECMWF and NCEP skillfully predict humid heatwaves up to 3–6 pentads in advance, with ECMWF outperforming NCEP, although both face challenges in accurately capturing moisture anomalies. These findings on the physical processes driving humid heatwave occurrence provide insights for improving model predictions.