On the Fragile Relationship Between El Nino and California Rainfall

Abstract

The failed influence of the 2015-2016 El Nino on California rainfall has renewed interest in the relationship between El Nino and U.S. rainfall variability. Here we perform statistical data analyses and simple model experiments to show that sufficiently warm and persistent sea surface temperature anomalies (SSTAs) in the far eastern equatorial Pacific are required to excite an anomalous cyclone in the North Pacific that extends to the east across the U.S. West Coast and thus increases rainfall over California. Among the four most frequently recurring El Nino patterns considered in this study, only the persistent El Nino, which is often characterized by the warm SSTAs in the far eastern equatorial Pacific persisting throughout the winter and spring, is linked to such extratropical teleconnection patterns and significantly increased rainfall over the entire state of California. During the last 69 years, only three of the 25 El Nino events (i.e., 1957-1958, 1982-1983, and 1997-1998) are clearly identified as the persistent El Nino. In addition, the monthly rainfall variance explained by El Nino is less than half that caused by internal variability during the 25 El Nino. Therefore, the rarity of persistent El Nino events combined with the large influence of internal variability effectively explains the fragile relationship between El Nino and California rainfall.