Combined large-scale tropical and subtropical forcing on severe soil desiccation in South America

Abstract

Abstract Changes in the frequency and magnitude of dry spells have been recorded over the past few decades due to an intensification of the global water cycle. A long-term soil drying trend resulting from a joint contribution of natural decadal variability in precipitation levels and increasing temperatures, predisposed southeast South America (SESA) to experience during the 2019–2021 period outstanding soil desiccation. Flash droughts led to large areas (~100.000 ) in SESA covered by record-breaking soil dryness. April 2020 witnessed the most severe conditions when over 30% of the region experienced negative soil moisture anomalies larger than two standard deviations. Internal variability, particularly El Niño–Southern Oscillation, explained the dynamical forcing of this extreme drought. Large precipitation deficits and enhanced evaporation in SESA resulted from a coupled tropical and subtropical forcing associated to pronounced changes in the normal Walker and Hadley Cells' circulation and to the establishment of a Rossby wave extending from west south Pacific towards South America. These findings highlight the intricate interplay between different processes in the occurrence of climate extremes, stressing the need for an adequate representation of the dynamical processes associated with daily-to-multiyear timescales in climate models.