An Atlantic influence on evaporation in the Orinoco and Amazon basins

Abstract. Tropical South America’s hydroclimate is influenced by ocean-atmospheric oscillations. The physical mechanisms that tele-connect the Atlantic modes of variability with the soil moisture and evaporation of the region remain unclear. This study uses composites of reanalysis and satellite data to identify the processes linking land-surface anomalies and ocean modes. It shows that the Atlantic Meridional Mode (AMM) generates cross-equatorial wind anomalies that affect moisture convergence, in turn modifying the cloud cover, precipitation, radiation availability and hence evaporation. The anomalies have important geographical differences depending on the analysed season; they migrate from the east in Austral autumn towards central Amazon and western Orinoco in Austral spring. The Atlantic El Niño (Atl3) affects the Guianas and eastern Orinoco by means of pressure and trade wind variability. Evaporation is water- or energy-driven depending on the position of the Intertropical Convergence Zone (ITCZ), but the anomalies are controlled by the phase of each mode which alter water and radiation availability. Both Atlantic modes mainly impact regions different from those impacted by El Niño Southern Oscillation (ENSO), although northeast Brazil and the Guianas might experience overlapping effects. Therefore, these ocean-atmospheric modes impact the water and energy cycles and might influence regional climate extremes (e.g. droughts and floods), and are critical for achiving sustainable development (SDG).