Inter-El Nino variability in CMIP5 models: Model deficiencies and future changes

Inter-El Nino variability, which represents the diversity in spatiotemporal evolution among El Nino events, has been identified using the first two leading modes of sea surface temperature anomalies along the equator. The first mode represents the transition from El Nino into La Nina and the second mode reveals El Nino's persistence through the following spring. Here we examine the ability of models to capture inter-El Nino variability and predict future changes due to global warming using historical and Representative Concentration Pathway 8.5 simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5). Most CMIP5 models realistically reproduce the first mode, but three fifths of the models fail to capture the second mode, with considerable intermodel diversity. The 10 best models are therefore selected according to a measurement of pattern correlation coefficients and normalized root-mean-square errors with respect to reproduction of the secondmode. The results aggregated from the best models indicate that the first "transition" mode will remain unchanged fromthe present climate to the future climate; in contrast, the second "persistence" mode changes stochastically across the CMIP5 models. Consequently, we conclude that El Nino's transition into La Nina is the most dominant characteristic of simulated inter-El Nino variability and will remain unswayed under global warming conditions. Model deficiency in simulating El Nino's persistence is a key source of uncertainty in modeling inter-El Nino variability, resulting in difficulty predicting how certain characteristics of El Nino events may change with global warming.This study was supported by GRL grant of the National Research Foundation (NRF) funded by the Korean Government (MEST 2011-0021927). K.S. Yun was supported by NRF-2015R1C1A1A01054992. We acknowledge the data providers for producing HadISST1.1 SST data (http://www.metoffice.gov.uk/hadobs/hadisst/) and CMIP5 model output (https://pcmdi.llnl.gov/search/cmip5/)