Austral Autumn Precipitation Anomalies Across the Amazon Basin Under the Combined Influences of the Equatorial Pacific and Tropical Atlantic

Abstract

Under normal conditions, the austral autumn (March–April–May [MAM]) season represents the onset, peak or end of the rainy season, depending on the Amazon sector. In this study, the relative influences of the equatorial Pacific (EqP) and tropical Atlantic (TA) on precipitation anomalies over northern South America were revisited, focusing on the Amazon Basin and the MAM season. We systematically analysed the combined and isolated influences of the EqP and TA to identify the regions of the Amazon that are most sensitive to the atmospheric and oceanic patterns during MAM and described the main related mechanisms responsible for anomalous precipitation. The main features of the El Niño-Southern Oscillation (ENSO) and the TA meridional sea surface temperature gradient (tropical Atlantic gradient—TAG) were described for each scenario. Our results indicate that a positive (negative) TAG alone increases (decreases) precipitation over the northern and central-eastern regions. On the other hand, the isolated influence of ENSO depends on its persistence during the season, affecting mainly the northern and north-eastern regions, with a stronger impact on La Niña occurrence. Under the occurrence of ENSO events and TAGs of the same sign, the whole equatorial Amazon is affected by the sum of circulation effects. In this context, the driest scenario was observed under the combined influence of an El Niño and a positive TAG, which strongly impacted the northern and central-eastern sectors of Amazon. Finally, TAGs and ENSO events of opposite signs resulted in anomalous precipitation mainly in the northern, north-eastern and western Amazon, where the TAG-related circulation prevailed against the influence of ENSO events. This study improves the current understanding of austral autumn spatial rainfall variability in the Amazon under the influence of ENSO and tropical Atlantic patterns.