Influences of different intensities of El Nino-Southern Oscillation on South American precipitation

El Nino-Southern Oscillation (ENSO) phenomena occur in the equatorial Pacific and differentially affect the global climate. Recently, studies have begun to analyse the influence of ENSO intensity on the global climate, but how this intensity affects South American precipitation remains unclear. This work defined the ENSO intensities each season (versus annually, as is more commonly done) because understanding this seasonal intensity is essential for seasonal forecasting. The atmospheric patterns identified in this study showed that the stronger the sea surface temperature (SST) anomalies (SSTA) in the equatorial eastern Pacific were, the stronger the atmospheric responses. The known South American precipitation pattern occurred at all ENSO intensity categories but had different intensities. However, the most important patterns in this study differed from the main pattern. Central-eastern South America did not have a notable precipitation pattern during all El Nino (EN) years, but there was a negative pattern of strong ENs. In addition, the intensity of the precipitation anomaly during weak EN and La Nina (LN), mainly in northeastern South America, was stronger in austral autumn than in austral summer because the intensity in a previous season can influence the current season's precipitation pattern and the total SST in autumn is stronger than that in summer. Asymmetries between EN and LN events are important to consider; EN tends to be more intense than LN, and deep convection anomalies during EN are displaced eastward relative to those during LN. In this study, categories were defined based on ENSO intensities, but asymmetries appeared in atmospheric patterns, mainly in omega patterns during strong events. The intensities of SSTAs over the equatorial eastern Pacific affected the intensities of the Walker circulation and Rossby waves and, consequently, the intensity of precipitation patterns in different South American regions.

Automated summary

This study analyzed the impact of ENSO intensity on South American precipitation and found that stronger sea surface temperature anomalies in the equatorial eastern Pacific resulted in stronger atmospheric responses. South American precipitation patterns varied in intensity, and the anomaly of weak El Nino and La Nina events was stronger in autumn than in summer.