Contrasting seasonal rainfall responses to deforestation in the Southwestern Amazon Basin

Abstract The Amazon Basin continues to be impacted by deforestation, causing a decrease in evapotranspiration and the expansion of hotter, drier land surfaces, two factors that impact spatial precipitation distribution differently 1,2 . Recent modeling studies show a wide range of predicted impacts on Amazonian precipitation, and this stems from uncertainty around surface characteristics and moisture affect how and where convection initiates and develops 3–7 . To explore how changes in evapotranspiration and increased surface heating have impacted convection and rainfall distribution, we analyzed 20 years of satellite observations of precipitation and skin temperature from a large contiguous deforested region and surrounding forest in southwestern Brazil. We find that wet season precipitation is enhanced over the downwind forested area, while dry season precipitation is preferred over the downwind side of the deforested area. By examining the cause of these differing responses using reanalysis data, we have found evidence of a key mechanism causing the seasonal nuances in the location of downwind enhancement: in both seasons, a mesoscale circulation enhances convection over the hotter deforested surface in the late afternoon. During the dry season, weak winds keep the precipitation in the deforested area, whereas stronger monsoonal winds and higher moisture levels in the wet season shift this enhancement further downwind over the forest. Over the 20 years of the study, total annual, wet, and dry season precipitation decrease markedly over the study region, likely driven by a reduced moisture supply. During the dry season, we observe a slight increase over recently deforested areas, indicating a redistribution of moisture based on the changing land surface. Our results show that Amazonian deforestation alters precipitation by modulating convective initiation and evolution through complex interactions of large-scale dynamics, evapotranspiration, and land-surface change.