Effects of Excessive Equatorial Cold Tongue Bias on the Projections of Tropical Pacific Climate Change. Part II: The Extreme El Nino Frequency in CMIP5 Multi-Model Ensemble

Under increased greenhouse gas (GHG) forcing, climate models tend to project a warmer sea surface temperature in the eastern equatorial Pacific than in the western equatorial Pacific. This El Nino-like warming pattern may induce an increase in the projected occurrence frequency of extreme El Nino events. The current models, however, commonly suffer from an excessive westward extension of the equatorial Pacific cold tongue accompanied by insufficient equatorial western Pacific precipitation. By comparing the Representative Concentration Pathway (RCP) 8.5 experiments with the historical simulations based on the Coupled Model Intercomparison Project phase 5 (CMIP5), a "present-future" relationship among climate models was identified: models with insufficient equatorial western Pacific precipitation error would have a weaker mean El Nino-like warming pattern as well as a lower increase in the frequency of extreme El Nino events under increased GHG forcing. Using this "present-future" relationship and the observed precipitation in the equatorial western Pacific, this study calibrated the climate projections in the tropical Pacific. The corrected projections showed a stronger El Nino-like pattern of mean changes in the future, consistent with our previous study. In particular, the projected increased occurrence of extreme El Nino events under RCP 8.5 forcing are underestimated by 30-35% in the CMIP5 multi-model ensemble before the corrections. This implies an increased risk of the El Nino-related weather and climate disasters in the future