Sources of Atmospheric Moisture for the La Plata River Basin*

Abstract

Abstract The La Plata River basin (LPRB) is the second largest basin of South America and extends over a highly populated and socioeconomically active region. In this study, the spatiotemporal variability of sources of moisture for the LPRB are quantified using an extended version of the Dynamic Recycling Model. Approximately 63% of mean annual precipitation over the LPRB comes from South America, including 23% from local LPRB sources and 20% from the southern Amazon. The remaining 37% comes mostly from the southern Pacific and tropical Atlantic Oceans. The LPRB depends largely on external sources during the dry winter season, when local evaporation reaches a minimum and moisture outflow increases. Variations in the transport of moisture from the Amazon to the LPRB depend more on variations of the atmospheric circulation than on evaporation, at both the monthly and daily time scale. In particular, weak atmospheric flow allows the accumulation of moisture over the Amazon basin, followed by an above-normal release of moisture downwind when the atmospheric flow strengthens again. Water vapor transport with these characteristics was observed for 20% of the days of the summer season during the 1980–2012 period, leading to higher-than-average convergence of moisture of terrestrial origin over the LPRB. During the positive (negative) phase of the El Niño–Southern Oscillation (ENSO), more (less) moisture from Amazonian evaporation reaches the LPRB. The Amazonian contribution to the LPRB is reduced (increased) during the positive (negative) phase of the Antarctic Oscillation (AAO), when surface pressure over southern South America is above (below) normal.