Regional Climate Dynamics

Global climate models (GCMs) that participated in the Coupled Model Intercomparison Project 5 (CMIP5) are analyzed here to examine their projections of wintertime (December through February, DJF) precipitation change over California. Most models agree that California winters are expected to become significantly wetter by the end of the 21st century (2070-2099 average minus 1961-1990 average) under the Representative Concentration Pathway 8.5 (RCP8.5) warming scenario (Figure 1). It is interesting to note that this model agreement is much stronger than equivalent projections in the older generation of GCMs (i.e. those included in CMIP3).

Figure 1: DJF precipitation change in CMIP5 GCMs for the end of the century (2070-2099 averaged minus 1961-1990) under the RCP8.5 forcing scenario. (a) Multimodel ensemble mean (15 models) precipitation change (mm per day). Hatched regions do not pass a significance test for the multimodel ensemble mean relative to internal variability at the 95% level. (b) Agreement on the sign of precipitation change among the model ensemble. Blue and green colors indicate a higher number of models (out of 15) agreeing on positive precipitation change; red colors indicate agreement on negative precipitation change. The box indicates the region used for averaging jet stream changes off the California coast.

Significant California precipitation increases in the CMIP5 GCMs are associated with an eastward extension of the region of strong Pacific jet stream by the end of the 21st century (r = 0.76, Figure 2). The extended jet steers more storms towards the California coast and provides for overall wetter conditions. Projected jet extensions over the Pacific (and teleconnections to California precipitation) in a warmer future climate are similar to those found in natural variability patterns, such as the El Niño-Southern Oscillation phenomenon.

Figure 2: End of century changes in DJF precipitation vs. DJF 200-mbar zonal wind speed averaged over the box in Fig. 1 (i.e. the storm-track extension region in the eastern Pacific and California coast) for the 15 CMIP5 GCMs.

Download the publication (Neelin et al. 2013) describing these results in more detail.

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