Causes of extreme dry conditions over the western United States during early 2013

The western U.S. experienced extreme dry conditions during early 2013, in particular, January and February were the driest months ever recorded in California. This study investigates the underlying causes for this extreme event, utilizing observations, MERRA reanalysis, and NASA GEOS-5 AGCM simulations consisting of AMIP simulations and idealized runs forced with the leading patterns of SST variability.

The results show that several factors contributed to the strong precipitation deficits over the western U.S., including the SST anomalies in the Pacific, atmospheric internal variability, and modulations from climate variations on decadal and longer time scales. The extreme precipitation deficits during early 2013 resulted from considerably less than normal north Pacific storms reaching the west coast of the U.S., under the influence of notably weakened upper-level zonal wind at and to the east of the North Pacific jet exit region. Such a weakened upper-level zonal wind was primarily maintained by anomalies in transient vorticity flux convergences that were associated with changes in both atmospheric internal variability and the Pacific SST. While the Pacific was characterized by weak ENSO and PDO events during early 2013, the general cold SST anomalies in the tropical Pacific and over the northeast Pacific off the west coast of North America, and the warm SST anomalies in the central north Pacific contributed substantially to the dry anomalies over southwestern U.S. via atmospheric teleconnections. Additionally, the occurrence of this extreme dry event in the context of the ongoing tendency for a cold PDO and globally warming trend will be discussed.