The Amazon and La Plata River Basins as Moisture Sources of South America: Climatology and Intraseasonal Variability.

Land‐atmosphere interactions are critical for precipitation (PPT) over South America where terrestrial evapotranspiration (ET) constitutes a significant fraction of moisture for rainfall over the ecologically and socio‐economically vital Amazon (AMZ) and La Plata (LPB) river basins. We quantify the contribution of ET from AMZ and LPB to PPT over the continent with a focus on the intraseasonal time scale. Using numerical water tracers embedded in the Weather Research and Forecasting model we track the moisture originating from the two basins. Our findings indicate that approximately 40% of annual rainfall over the eastern foothills of the Andes originates as AMZ ET, and nearly 30% of rainfall over northern Argentina originates as LPB ET. Analysis of moisture transport during both phases of the dominant intraseasonal oscillation pattern over South America reveals an intraseasonal "sloshing" of LPB moisture between the South Atlantic convergence zone (SACZ) and southeastern South America (SESA) regions. AMZ and LPB each supply approximately 6% of moisture for SACZ PPT during periods of intraseasonal enhancement (positive anomalies), highlighting the importance of moisture from the Atlantic Ocean. For the SESA region, LPB supplies 26% of the moisture for PPT during periods of intraseasonal enhancement while AMZ supplies 5%. Plain Language Summary: Terrestrial moisture ‐ moisture that previously evapotranspired from the land surface ‐ plays a crucial role in the regional hydroclimate over South America through "recycling", a process by which terrestrial moisture contributes to precipitation (PPT) within the same region. Previous work has shown that the Amazon (AMZ) and the La Plata (LPB) basins, the largest river basins of South America, receive a large amount of PPT through recycling. This study investigates the changes in transport and recycling of terrestrial moisture from these basins at the 30–75 days timescale. We want to understand how variability in the continental‐scale circulation can affect the transport of terrestrial moisture. By using a numerical weather prediction model with the added capability of tracking moisture that originates as evapotranspiration within a specified source region (in this case, AMZ and LPB), we are able to track the motion, phase change and PPT of AMZ and LPB moisture throughout the domain. First, we present a detailed climatological analysis of recycling ratios for South American PPT and then we focus on the 30–75 days time‐scale to understand how the intraseasonal circulation modifies the geographical location of continental recycling patterns. Key Points: Evapotranspiration from Amazon and La Plata basins plays a crucial role in the regional water balance through recycling and moisture exportThe dominant intraseasonal hydroclimate oscillation over South America drives the transport of terrestrial moistureThere is a clear "sloshing" of terrestrial moisture between central‐east and southeastern South America at intraseasonal time scale [ABSTRACT FROM AUTHOR]