EAPS

The California Drought

California is in its fourth year of an epic drought.    The prime cause of the drought is a drop in winter precipitation summing to more than a year of precipitation missing.  Atmosphere modeling strongly suggests that the persistent west coast ridge that blocked Pacific storms and caused the precipitation drop was forced by a combination of SST anomalies in the Indian and tropical Pacific Oceans, of presumed natural origin.  In addition to the precipitation drop, California was warm during much of the drought and, from the surface moisture perspective, this was responsible for about a quarter of the drought with the long term warming trend accounting for at least some of that.  The drought is not representative of expected human-driven climate change in California.   Models project the California wet season to become shorter and sharper.  The midwinter projected wetting is partly a simple thermodynamic case of 'wet getting wetter' but is also importantly influenced by a shift to more southwesterly prevailing winds at the west coast.  This wind shift occurs as part of a change in the intermediate scale stationary wave field that is itself a response to accelerated subtropical upper troposphere westerlies.   Modeling and analysis suggests, however, that the midwinter wetting of California is likely overestimated by the CMIP5 models.  Finally the implications of the current El Nino for California precipitation in the upcoming 2015/16 winter will be discussed and whether it has the potential to end the drought.   


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