Projected change in wintertime precipitation in California using projected changes in extratropical cyclone activity

Abstract

Wintertime extratropical cyclones in the east Pacific region are the source of much of the precipitation over California. There is a lot of uncertainty in future projections of Californian precipitation associated with predicted changes in the jet stream and the midlatitude storm tracks. The question this work seeks to answer is how the changes in the frequency and the intensity of extratropical cyclones in the Pacific storm track influence future changes in Californian precipitation. We used an objective cyclone identification method applied to 25 CMIP5 models for the historical and RCP8.5 simulations and investigated the changing relationships between storm frequency, intensity and precipitation. Cyclone data from the historical simulations and differences between the historical and RCP8.5 simulations were used to “predict” the modeled precipitation in the RCP8.5 simulations. In all models, the precipitation predicted using historical relationships gives a lower future precipitation change than the direct model output. In the future, the relationship between track density and precipitation indicates that for the same number of tracks, more precipitation is received. The relationship between track intensity and precipitation (which is quite weak in the historical simulations) does not change in the future. This suggests that other sources, likely enhanced moisture availability, are more important than changes in the intensity of cyclones for the rainfall associated with the storm tracks.