Discrepant effects of atmospheric adjustments in shaping the spatial pattern of SST anomalies between extreme and moderate El Niños

Abstract

AbstractThe surface heat flux anomalies during El Niño events have always been treated as an atmospheric response to sea surface temperature anomalies (SSTAs). However, whether they play roles in the formation of SSTAs remain unclear. In this study, we find that the surface net heat flux anomalies in different El Niño types have different effects on the development of the spatial pattern of SSTAs. By applying the fuzzy clustering method, El Niño events during 1982–2018 are classified into two types: extreme (moderate) El Niños with strong (moderate) positive SSTAs, with the largest SSTAs in the eastern (central) equatorial Pacific. The surface net heat flux anomalies in extreme El Niños generally display a “larger warming gets more damping” zonal paradigm, and essentially do not impact the formation of the spatial pattern of SSTAs. Those in moderate El Niños, however, can impact the formation of the spatial pattern of SSTA, by producing more damping effects in the eastern than in the central equatorial Pacific, thus favoring the largest SSTAs being confined to the central equatorial Pacific. The more damping effects of net heat flux anomalies in the eastern equatorial Pacific in moderate El Niños are contributed by the surface latent heat flux anomalies, which are mainly regulated by the negative relative humidity–SST feedback and the positive wind–evaporation–SST feedback. Therefore, we highlightthat these two atmospheric adjustments should be considered during the development of moderate El Niños in order to obtain a comprehensive understanding of the formation of El Niño diversity.