Do CMIP5 Models Show El Nino Diversity?

Whether the state-of-the-art CMIP5 models have different El Nino types and how the degree of modeled El Nino diversity would be impacted by the future global warming are still heavily debated. In this study, cluster analysis is used to investigate El Nino diversity in 30 CMIP5 models. As the method does not rely on any prior knowledge of the patterns of El Nino seen in observations, it provides a practical way to identify the degree of El Nino diversity in models. Under the historical scenario, most models show a poor degree of El Nino diversity in their own model world, primarily due to the lopsided numbers of events belonging to the two modeled El Nino types and the weak compactness of events in each cluster. Four models are found showing significant El Nino diversity, yet none of them captures the longitudinal distributions of the warming centers of the two El Nino types seen in the observations. Heat budget analysis of the sea surface temperature (SST) anomaly suggests that the degree of modeled El Nino diversity is highly related to the climatological zonal SST gradient over the western-central equatorial Pacific in models. As the gradient is weakened in most models under the future high-emission scenario, the degree of modeled El Nino diversity is further reduced in the future. The results indicate that a better simulation of the SST gradient over the western-central equatorial Pacific might allow a more reliable simulation/projection of El Nino diversity in most CMIP5 models.