Why CMIP5 models projected precipitation changes over California are uncertain

Abstract:

Future planning for adaptation to and mitigation of the impacts of anthropogenic climatic changes depends mainly on the robustness of climate-model projections. California is one of the global regions where CMIP5 models projections for end-of-century precipitation changes are most uncertain. However, over other mediterranean-climate regions the same models robustly project drier futures. Here we investigate wintertime precipitation projections by the end of the century from 30 CMIP5 models over the five global mediterranean regions (California, South America, the Mediterranean basin, South Africa, and Australia) to disentangle the causes of uncertainty in projections over California.

Our findings reveal that the projections of increases in wintertime precipitation over California by two thirds of the models are mainly driven by a comparably weaker increase in the frequency of dry days and a stronger increase in the frequency of heavy precipitation events (99^th percentile) along with an increase in the intensity of moderate precipitating events (50-75^th percentile) compared to the other mediterranean regions. California is projected to have 5-7 dry days and one more once in year heavy precipitation events in ensemble mean per year. The significant increases in the frequency of heavy events and weak increase in dry days’ frequency are mostly attributed to the intensification of the north Pacific low-pressure system along with the strengthening of the sub tropical jet stream. The uniqueness of California’s circulation change compared to other mediterranean regions is partially due to the modulation of the large-scale circulation by the low frequency Pacific climate variability.