El Nino's impact on California precipitation: seasonality, regionality, and El Nino intensity

California has experienced severe drought in recent years posing great challenges to agricultural production, water resources, and land management. El Nino, as the prime source of seasonal to interannual climate predictability, offers the potential of amelioration of drought in California. Here El Nino's impacts on California winter precipitation are examined, focusing on variations by season, region, and the strength of El Nino using observational data for the period 1901-2010. The El Nino influence on California precipitation strengthens from early to late winter and is stronger in the south than the north. Eight of ten moderate-to-strong El Ninos in the late winter put southern California in the wettest tercile and none of these ten events put northern California in the driest tercile. The early to late winter strengthening of the El Nino impact on precipitation occurs even as El Nino weakens and is associated with a strengthening and eastward extending tropical deep convection anomaly allowed by the late winter warming of the climatological mean sea surface temperature over the tropical eastern Pacific.