Comparison of future rainfall projections over California: CMIP6 vs. CMIP5

Future projections of rainfall changes remain one of the most prominent challenges for climate models due to the high degree of uncertainties associated with them. This is especially true for regions featuring Mediterranean climate, where the uncertainties in projections of future rainfall changes in combination with increasing temperatures, pose a significant problem. One particular uncertainty hotspot is California, where models tend to disagree about the sign of future precipitation changes.

We analyse the simulations from the Coupled Model Intercomparison Projects Phase 5 (CMIP5) and 6 (CMIP6) and study their respective intermodel differences in projections of future rainfall changes over the state of California. We employ 30 models from CMIP5 and 30 models from CMIP6 initiatives and focus on the business-as-usual scenarios (SSP585 - RCP8.5). We consider the precipitation changes over the two time horizons  - mid-century (2040-2059) and the end of century (2080-2099).

Our results point at higher uncertainty in rainfall projections for the wet winter season December-January-February (DJF) than for the rest of the seasons. Comparing CMIP5 and CMIP6, we see that CMIP5 simulations show larger intermodel disagreement for the end-of-century period while CMIP6 show larger intermodel disagreement for the mid-century time horizon. For the new CMIP6 simulations, more than 70% of the models project that northern California is going to experience an increase in precipitation in the future, while the projections for southern California are more uncertain, with models being almost equally spread between those suggesting drier and those suggesting wetter conditions. This is an improvement compared to the CMIP5 simulations where 60% models suggested wetter conditions (and 40 % drier conditions) over northern California.

Finally, we analyse how the uncertainties in future model simulations of geopotential height, outgoing longwave ration (OLR) and ENSO changes relate to the described uncertainties in California’s rainfall in CMIP6 and CMIP5.