5. Causes and predictability of the 2011 to 2014 California drought - PDF

The causes and predictability of the California drought during the November to April winters of 2011/12 to 2013/14 are analyzed using observations and ensemble simulations with seven atmosphere models forced by observed sea surface temperatures (SSTs). Historically, dry California winters have most commonly been associated with a ridge off the west coast but no obvious SST forcing. Wet winters have most commonly been associated with a trough off the west coast and an El Niño event. These attributes of dry and wet winters are captured by many of the seven models. According to the models, SST forcing can explain up to a third of California winter precipitation variance. SST-forcing was key to sustaining a high pressure ridge over the west coast and suppressing precipitation during the three year drought. In 2011/12 the forced component was a response to a La Niña event whereas in 2012/13 and 2013/14 it was related to a warm tropical west Pacific SST anomaly. All models contain a mode of climate variability that links west Pacific SST anomalies to a northeastward propagating wave train with a ridge off the North American west coast. This mode explains less variance than ENSO and Pacific decadal variability and its importance in 2012/13 and 2013/14 was unusual. The CMIP5 models project that rising greenhouse gases should increase California winter precipitation but that changes to date are small compared to the recent drought anomalies. As such, the recent drought was dominated by natural variability, a conclusion framed by discussion of differences between observed and modeled< tropical SST trends over past decades.

NOAA DROUGHT TASK FORCE ASSESSMENT REPORT: Causes and Predictability of the 2011 to 2014 California Drought

• Seager, R., M. Hoerling, S. Schubert, H. Wang, B. Lyon, A. Kumar, J. Nakamura and N. Henderson, 2015: Causes of the 2011-14 California Drought.J. Climate28 , 6997-7024, DOI:10.1175/JCLI-D-14-00860.1. PDF