ENSO Influence on Regional Atmospheric Circulation and Thermodynamics Over Northern South America

Abstract The interannual variability of hydroclimatic conditions in Northern South America (NOSA), specially precipitation, is mainly influenced by the El Niño-Southern Oscillation (ENSO). We explored potential mechanisms that affect precipitation occurrence in NOSA during El Niño and La Niña events over the period 1980-2019, using data from the ERA5 reanalysis. We looked at the atmospheric moisture contribution from different sources using the Dynamic Recycling Model to track water vapour trajectories. Interestingly, conditions with reduced precipitation during El Niño events can take place along with increased precipitable water. To understand this, we analyzed thermodynamic conditions in the atmosphere that are necessary for precipitation to occur over the region, such as convective available potential energy, convective inhibition, lifting condensation level and low--level relative humidity. With this approach, we found more favorable atmospheric conditions for the occurrence of precipitation during La Niña events, even if the content of water vapor was equal or even less than during El Niño events. We also looked at the structure of the regional Hadley circulation in both types of events and found a weakening of the rising motion of the cell during El Niño, which further reduces convective processes over this region. This study provides an integral picture of how precipitation anomalies over NOSA during ENSO events are related both to thermodynamic conditions and sources of atmospheric moisture.