Land‐Atmosphere Interactions During Onset and Demise of the Rainy Season Over West‐Central Brazil

This study investigates the contrasting land–atmosphere coupling mechanisms that modulate the onset and demise of the South American Monsoon (SAM) rainy season over West‐Central Brazil (WCB) based on the European Centre for Medium‐Range Weather Forecasts Reanalysis 5th (ERA5) data for 1991–2021 to examine the role of soil moisture anomalies, evaporative regimes and atmospheric circulation in shaping monsoon transition timing. Results show that the monsoon onset is associated with concurrent land‐surface and atmospheric anomalies: During years with early onset, positive surface and subsurface soil moisture anomalies emerge from late austral winter and persist into spring, co‐occurring with enhanced evapotranspiration, strengthened vertically integrated moisture convergence and the establishment of deep convection over the WCB. Conversely, dry soil anomalies in late‐onset years are accompanied by suppressed surface fluxes, weaker low‐level moisture convergence and delayed convective development. In contrast, the demise of the rainy season is most clearly associated with atmospheric moisture transport anomalies. Early (late) termination years are characterised by widespread anomalous moisture divergence (convergence) during March–May, reflecting weakened (enhanced) large‐scale convective support. While soil moisture remains correlated with rainfall anomalies during the demise phase, the evapotranspiration–rainfall association is weak, indicating a less prominent land‐surface signal during monsoon retreat.