Comparison of CMIP5 and CMIP6 mid- and end-century precipitation changes in California

Climate models have projected an overall drying in Mediterranean climate zones, with the exception of California, where the models have been split between predicting wetter or drier future conditions. This uncertainty is problematic for a major economy, where water scarcity, especially in southern California, has been an issue of concern during the last few decades. Here we compare the future projections of California’s winter (December, January, February) precipitation changes from the latest Coupled Model Intercomparison Project Phases 6 (CMIP6) and 5 (CMIP5). Over northern California the models from both ensembles agree on wetter future conditions. However, over southern California the models are almost equally divided between wetter or drier conditions, with projections ranging from -30% to +70% in CMIP5 and -20% to +80% in CMIP6 for the end of the century. The CMIP6 ensemble indicates wetter overall conditions, and features a larger model disagreement compared to CMIP5.

Interannual precipitation indicates more extremely wet or dry years over southern California in CMIP6 than in CMIP5. Some models even suggest that the five wettest years will account for as much as ~55% of the total 20-year rainfall considered, and the five driest for as little as ~5%. Dynamically, both ensembles project weakened subsidence over Baja California. This effect is stronger in CMIP6 than in CMIP5, consistent with the wetter mean conditions in CMIP6. In the western tropical Pacific our results point to strengthening of the Hadley circulation in CMIP6 that is not seen in CMIP5, and El Niño conditions prevailing over La Niña. CMIP6 models also project a stronger overall impact of ENSO on California’s precipitation than CMIP5 models.