Frequency of extreme El Niño and La Niña events under global warming

Abstract

The El Nino-Southern Oscillation (ENSO) is a quasi-periodical natural phenomenon occurring in the tropical Pacific whose characteristics can be influenced by global warming. Using outputs of 14 general circulation models (GCMs) participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), the numbers of extreme El Nino and La Nina events are analyzed for the 50-year period in the future (2050–2099) under the RCP4.5 and RCP8.5 scenarios relative to those during the historical period 1950–1999. Analyses are based on the Oceanic Nino Index (ONI) and the modified Cai index that is defined in this study. Our approach deviates from that of Cai et al. (Nat Clim Change 4:111–116, 2014, Nat Clim Change 5:132–137, 2015b), who used rainfall thresholds to identify extreme El Nino events but sea surface temperature (SST) thresholds to identify extreme La Nina events. Analysis of SST and rainfall in the tropical Pacific indicated that under global warming the eastern equatorial Pacific warms faster than the surrounding ocean waters, which is accompanied by an increase of rainfall in the eastern equatorial Pacific. Thus, changes in the mean state of the tropical Pacific climate will be like an El Nino pattern under global warming. By applying the ONI, it is found that the number of very strong El Nino events slightly increases under global warming, while the rate of increase in the number of very strong La Nina events is greater than that of very strong El Nino events. Analysis based on the modified Cai index indicates a slight decrease in the number of extreme El Nino events and a slight increase in the number of extreme La Nina events under global warming. Thus, results of several previous studies which concluded that the number of extreme El Nino events nearly doubles under global warming are not supported by results of this study.