Asymmetric Impacts of El Nino and La Nina on Equatorial Atlantic Warming

Despite an insignificant linear relation between the boreal summer equatorial Atlantic sea surface temperature anomaly (EA SSTA) and preceding winter El Nino-Southern Oscillation (ENSO), an EA warming event was found to be mostly preceded by either phase of ENSO. Physical mechanisms of this asymmetric impact of El Nino and La Nina on EA warming were investigated through observational and modeling analyses. For the El Nino-induced EA warming group, the El Nino-related SSTA induces a warm SSTA in the South Atlantic during developing fall and winter via the Pacific-South American (PSA) pattern. The induced extratropical SSTA extends to the equator through local air-sea interaction processes and favors EA warming development. In contrast, for the La Nina-induced EA warming group, there is no significant SSTA in the South Atlantic from developing fall to winter. It is not until La Nina decaying spring that a significant westerly anomaly is generated and thus an EA warming through the remote Gill-type response. The asymmetric South Atlantic response is attributed to the stronger (weaker) heating strength over the central Pacific and Maritime Continent for El Nino (La Nina). The distinctive evolutions of El Nino (fast transition) and La Nina (long persistence) also contribute to the asymmetric EA response. The persistent La Nina makes a stronger equatorial Atlantic response during ENSO decaying spring. Our results suggest that the South Atlantic SSTA and ENSO temporal evolution are important in explaining the asymmetric impacts of the El Nino and La Nina on EA warming. Furthermore, the Indian Ocean SSTA is also suggested to contribute to the asymmetric impacts.