The Growing Role of Evaporative Demand in Driving Extreme Droughts in the Amazon Basin

The Amazon basin is increasingly threatened by severe droughts, traditionally attributed to precipitation deficits. However, the amplifying role of rising evaporative demand, represented by potential evapotranspiration (PET), is not well quantified. Using a counterfactual decomposition framework based on Standardized Precipitation-Evapotranspiration Index (SPEI) from 1979 to 2024, this study quantifies the contributions of precipitation and PET to drought severity and coverage to better understand the evolving drought mechanisms in the region.

Our analysis of the record-breaking 2024 drought reveals that while precipitation deficit was the primary contributor, surging evaporative demand acted as a strong amplifier, nearly doubling the event's severity compared to a precipitation-only scenario. Consequently, 76% of the basin experienced exceptional drought conditions (or D4 drought, SPEI below 2nd percentile) during the peak of the 2024 event. We identify a fundamental regime shift in the 21st century where the contribution of PET to drought area has systematically increased. The basin is transitioning from a precipitation-dominated regime to a "hot drought" paradigm, where compound events, characterized by moderate rainfall deficits exacerbated by high atmospheric thirst, now drive the majority of exceptional drought coverage. Deconstructing the drivers of this rising evaporative demand shows that it can be attributed almost equally to both regional warming and increased surface shortwave radiation from reduced cloud cover. Overall, this study indicates that global warming and regional radiative feedbacks are making the Amazon basin more susceptible to rapid drying even without extreme rainfall deficits.