Moisture transport regimes associated with large-scale organized convection over subtropical South America

Precipitation variability in the La Plata river basin of subtropical South America has wide-ranging social and economic consequences. Much of the region's warm season precipitation is generated by large-scale, long-lasting, intense thunderstorm complexes known as mesoscale convective systems (MCSs). Nearly all previous research on La Plata basin MCS activity has assumed that atmospheric moisture flux from the Amazon river basin is a required precondition for MCS development. However, the presence of the warm Brazil current in the Atlantic Ocean along the southeast coast of Brazil, along with the frequent passage of extratropical cyclones originating in the Pacific Ocean through the South American subtropics, suggests that Pacific and Atlantic moisture sources may make significant contributions to MCS activity in the region. This study investigates the relative contribution of Amazon basin, South Atlantic Ocean, and Pacific Ocean moisture sources to La Plata basin MCS activity using geostationary satellite imagery, atmospheric reanalysis data, and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Results indicate that the South Atlantic moisture source is most influential in the lowest levels of the atmosphere, with flow from the Amazon basin more prevalent above the tropospheric boundary layer. South Atlantic and Pacific Ocean moisture sources were found to be particularly important for MCSs occurring in the higher latitudes of central Argentina. Future work based on in situ observations is needed to confirm these initial results, which hold important implications given the potential sensitivity of regional precipitation to Amazon deforestation and Atlantic and Pacific Ocean variability.