PNNL

Abstract

Land-atmosphere interactions through moisture pathways are critical for precipitation over South America where terrestrial moisture constitutes a significant fraction of rainfall, specifically over the ecologically and socio-economically vital Amazon (AMZ) and La Plata (LPB) river basins. While previous work has quantified the seasonal importance of terrestrial moisture sources for precipitation (PPT), there is a dearth of knowledge on how the transport and recycling of evapotranspired moisture (ET) is modulated at intra-seasonal time scale. We employ numerical tracers embedded in the Weather Research and Forecasting (WRF) model and track the moisture originating from the AMZ and LPB basins to study its interaction with the intra-seasonal hydroclimate variability. First, we present high resolution simulated maps of annual and seasonal recycling ratio for South American PPT and isolate the role of AMZ and LPB ET in the regional atmospheric water budget. Our findings indicate that AMZ tracers contribute more than 40% of rainfall over the eastern foothills of Andes and LPB tracers account for greater than 30% over northern Argentina. We then analyze the patterns of terrestrial moisture transport and precipitation recycling during both phases of the well identified "see-saw" pattern over South America. Results show a clear intra-seasonal "sloshing" of terrestrial moisture between the South Atlantic convergence zone (SACZ) and the southeastern South America (SESA) regions. AMZ and LPB each account for approximately 6% of SACZ PPT highlighting the predominant role of oceanic moisture. Conversely, nearly on third of PPT over SESA is of terrestrial origin, of which LPB alone accounts for 23%.