Warming Patterns Affect El Nino Diversity in CMIP5 and CMIP6 Models

Given the consequences and global significance of El Nino-Southern Oscillation (ENSO) events it is essential to understand the representation of El Nino diversity in climate models for the present day and the future. In recent decades, El Nino events have occurred more frequently in the central Pacific (CP). Eastern Pacific (EP) El Nino events have increased in intensity. However, the processes and future implications of these observed changes in El Nino are not well understood. Here, the frequency and intensity of El Nino events are assessed in models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), and results are compared to extended instrumental and multicentury paleoclimate records. Future changes of El Nino are stronger for CP events than for EP events and differ between models. Models with a projected La Nina-like mean-state warming pattern show a tendency toward more EP but fewer CP events compared to models with an El Nino-like warming pattern. Among the models with more El Nino-like warming, differences in future El Nino can be partially explained by Pacific decadal variability (PDV). During positive PDV phases, more El Nino events occur, so future frequency changes are mainly determined by projected changes during positive PDV phases. Similarly, the intensity of El Nino is strongest during positive PDV phases. Future changes to El Nino may thus depend on both mean-state warming and decadal-scale natural variability.