Compound Dry–Hot Conditions in South and Southeast Asia Modulated by the Arctic Stratospheric Polar Vortex

Compound dry-hot conditions are intensifying across South and Southeast Asia, posing severe threats to the region's dense populations, agricultural systems, and rich biodiversity. To improve seasonal risk assessment, this study investigates the interannual drivers of these extremes, revealing a robust polar-low-latitude teleconnection. Utilizing observations and numerical simulations, including WACCM6-SC experiments, we demonstrate that the strength of the March Arctic stratospheric polar vortex (ASPV) significantly modulates compound dry-hot conditions across the region during the subsequent March-April period. The underlying mechanism is driven by stratosphere-troposphere coupling. When the March ASPV weakens, anomalous easterlies develop in the lower Arctic stratosphere and propagate downward. This induces corresponding tropospheric easterlies over the Arctic that persist from March through April, driving southward cold air advection that cools Siberia and strengthens the mid-latitude westerlies. Consequently, enhanced anticyclonic shear along the jet stream generates an anomalous anticyclonic circulation over northern South and Southeast Asia. This anomalous anticyclone suppresses regional convection, enhances subsidence, and reduces cloud cover. Aligning with the region's climatological dry-hot season, this dynamic decreases precipitation and increases surface solar radiation. These conditions are further intensified by a positive soil moisture-atmosphere feedback loop: drier soils weaken evaporative cooling and increase upward sensible heat fluxes, significantly warming the near-surface atmosphere. Crucially, this study reveals that the March ASPV outweighs the influence of preceding winter tropical sea surface temperatures (SSTs) in shaping compound dry-hot variability in the northern sector of South and Southeast Asia, whereas tropical SSTs remain the dominant driver in the south. These findings underscore the critical role of Arctic stratospheric anomalies in driving low-latitude climate variations, providing a new pathway to enhance seasonal risk forecasting and mitigation.