Attribution of low precipitation in California during the winter of 2013-2014

The record-setting drought in the state of California was further aggravated by extreme low precipitation in the winter of 2013-2014 and the associated low snow cover over the Sierra Nevada. Attribution work on the decline in Northern Hemisphere spring snow cover (Rupp et al. 2013) has shown that the decrease was likely the result of combined natural and anthropogenic forcing but not by natural forcing alone. Regional model superensemble simulations of snow water equivalent (SWE) with the Hadley Regional Climate Model (HadRM3P) shows the decline as a statistically-significant, linear trend for the Western US from 1961 to 2010. The present work focuses on attribution of these events by employing a superensemble of regional climate model simulations from the climateprediction.net (CPDN) experiment, which allows for robust statistical analysis of extreme events. Specifically, we compare the decade of the 2000s and the 1960s, which had different levels of heat-trapping gases and forcing from natural variability, among other factors. A linear regression of wet days and number of days with precipitation above 40 mm shows a strong drying pattern for the winter months of December, January, February, March (DJFM), especially for northern California and the Sierra Nevada. A strong warming pattern is also present during the winter months, with the minimum temperatures outpacing maximum temperatures for the Pacific Northwest. We will also investigate how simulations for DJFM 2013-2014, using only natural forcing provided CMIP5 HistoricalNat boundary conditions, compare against the model simulations using observations as boundary conditions. Results from this experiment also highlight the influence of increasing number of simulations on confidence intervals, which significantly reduces the uncertainty of both the change in magnitude of a given event and its corresponding return period.Rupp, David E., Philip W. Mote, Nathaniel L. Bindoff, Peter A. Stott, David A. Robinson, 2013: Detection and Attribution of Observed Changes in Northern Hemisphere Spring Snow Cover. J. Climate, 26, 6904-6914.doi: http://dx.doi.org/10.1175/JCLI-D-12-00563.1