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The causes of the California drought during November to April winters of 2011/12 to 2014/15 are analyzed using observations and ensemble simulations with seven atmosphere models forced by observed SSTs. Dry winters in California most typically arise from internal atmosphere variability but La Nina conditions can tip the odds in favor of dry conditions. The first of the four dry winters was indeed a La Nina. Winters 2012/13 and 2013/14 were however different and ENSO-neutral. However, the SST-forced models suggest that SST anomalies in the Indian and Pacific Oceans drove a wave train that placed a ridge over the northeast Pacific and North American west coast that generated dry conditions. After an El Nino failed to develop in 2014, the tropical SSTs reverted to a state similar to that in winter 2013/14 and California's wet season came to an abrupt end creating a fourth winter of drought. However, SST …