Causes of the extreme dry conditions over California during recent winters

This study investigates the extreme dry conditions over California during early 2013 and the 2013-14 winter in the context of long-term climate change. Observations, MERRA Reanalysis and a series of modeling experiments using the NASA GEOS-5 AGCM are used. The results show that the extreme precipitation deficits over California during recent winters resulted from considerably less North Pacific storms reaching California, due to the blocking by persistent high anomalies over the northeast Pacific. The GEOS-5 AGCM results show that while the concurrent SST anomalies do force a predilection for dry events over California, it is the atmospheric internal variability that accounts for the extreme magnitude of these dry climate events. An assessment of the role of the long-term warming trend shows that it forces a high anomaly over the northeast Pacific resulting in less North Pacific storms reaching California. The warming trend, however, also leads to increased atmospheric humidity over the northeast Pacific, thus, facilitating wetter events over California. The above two effects appear to counteract each other, contributing to no appreciable long-term change in the risk for dry climate extremes over California since the late 19th century.->