Strengthening ENSO amplitude asymmetry: historical trends and future projections in a warming climate

Abstract The amplitude asymmetry between warm (El Niño) and cold (La Niña) phases of the El Niño-Southern Oscillation (ENSO) has been observed to undergo changes in response to greenhouse warming. However, there is still no consensus on how this asymmetry may evolve in the future, especially in terms of complex types of ENSO events. In this study, we analyze observational sea surface temperature (SST) datasets spanning over a century to investigate the long-term trends of amplitude asymmetry in the eastern Pacific (EP) and central Pacific (CP) ENSO events. Our findings reveal a significant strengthening trend in the El Niño-La Niña amplitude asymmetry over the past century for both CP and EP events. This strengthening trend is attributed to the mean state change induced by greenhouse warming over the equatorial Pacific, and its impact on the modulation of ENSO feedback processes. Furthermore, using a large ensemble model projection, we suggest that the ongoing enhanced ENSO amplitude asymmetry could continue to strengthen until 2100 under the RCP8.5 greenhouse scenario.