Multiscale land-atmospheric feedbacks intensify hot droughts over global river-basins

Somnath Mondal , Auroop R Ganguly

When drought and heatwave coincide as hot drought, its impacts exceed the sum of individual hazards. The understanding that hot drought intensifies through soil drying is incomplete.
We instead find amplification and suppression of hot droughts over both dry and wet soils, with wet (dry) soil-associated amplification (suppression) being the secondary pathway. Moreover, soil-moisture control on land–atmospheric coupling is scale-dependent. So, we hypothesize that hot drought severity reflects not only regional mean land-atmospheric feedbacks but also the spatial coherence of drying within and (propagation) across a region. This three-nested hypothesis explains up to 32% of hot drought severity across 30 major river basins over 1980–2023. At the basin scale, hot droughts over subtropics have come to intensify more through soil moisture-associated warming than precipitation deficits, except in eastern Asia. Within basins, the spatial coherence of soil drying explains up to 12% of hot-drought severity, and this contribution strengthens with drying itself in one-third of basin segments. Contrasting moisture deficit and heat propagation amplify hot droughts over basins in southwest USA and monsoon-influenced Asia, respectively. These show mean feedback, intra-basin coherence of soil moisture, and inter-basin propagation as first-order and nonstationary controls of hot drought severity, with implications for regional risk estimation under warming.