Contributions of Climate Change and ENSO Variability to Future Precipitation Extremes Over California

The El Niño/Southern Oscillation (ENSO) affects the occurrence frequency and intensity of extreme precipitation through modulations of regional heat and moisture fluxes. California experiences particularly strong ENSO influences and models project different to its extreme precipitation. It remains unclear how diverse projections of future precipitation extremes relate to inter‐model differences for those changing signals. Here, we use “large ensemble” simulations with multiple climate models along with the Coupled Model Intercomparison Project Phase 5 to investigate the range of precipitation extreme changes over California and the influences from ENSO‐related teleconnections. We found that precipitation amount increases are much larger during El Niño relative to La Niña years, mainly caused by the differences in frequency of extreme events during different phases. The ENSO‐driven effect is even larger than the overall climate change signal for the most extreme events, implying uncertainties from inter‐model differences in ENSO‐related SST variability for extreme precipitation changes. Plain Language Summary Regional projected precipitation changes over California are associated with large uncertainties due to both climate variabilities, external forcing, and model internal uncertainties. Extreme precipitation in California is projected to intensify, and how mean‐state climate change and El Niño/Southern Oscillation (ENSO)‐related SST variability contribute to different changes are focused on with a set of “large ensemble” simulations. Precipitation extremes, as a significant topic for climate impacts, are focused in this study for future changes and distributions within models' large ensemble. We found that the effect of ENSO variability is comparable to the magnitude of overall future changes and even larger than mean climate impacts for the most precipitation extreme events. Projections of future precipitation extremes are diagnosed for the influences from changes to ENSO variability with mean‐state forcings. Key Points The contributions from El Niño/Southern Oscillation (ENSO)‐driven SST variability to extreme precipitation are quantified using multiple model ensembles and Coupled Model Intercomparison Project Phase 5 data The effect of ENSO variability is comparable to or even larger than the overall impact from climate change for extreme precipitation events Better constraining ENSO‐related SSTA patterns and associated teleconnections is necessary fo