Winter circulation anomalies in the western United States associated with antecedent season and interdecadal ENSO variability, 1948-1998

Many users of climate information across the western United States look to fall season conditions in the El Niño-Southern Oscillation (ENSO) system as an indicator of potential winter season precipitation anomalies. In this study, the reliability of fall season ENSO conditions as a predictor of Western U.S. winter circulation anomalies associated with canonical precipitation impacts is shown to vary over multi-decadal time periods consistent with phasing of the Pacific Decadal Oscillation (PDO) pattern. During the PDO cold phase of 1948-1976, fall season El Niño events tended not to precede the canonical winter troughing pattern over the West typically associated with above-normal precipitation in many areas, particularly the southwestern states. During the PDO warm phase of 1977-1998, fall season El Niño conditions were more reliably associated with winter troughing, but typically only during moderate-to-strong El Niño events. Fall season La Niña conditions during both the cold and warm phases of the PDO generally correlated well with the occurrence of wintertime high-pressure ridging centered off the Pacific coast. The results presented here highlight uncertainty over multi-decadal time scales surrounding the use of fall ENSO conditions, particularly during El Niño events, as a seasonal climate forecast tool for winter precipitation in the western United States.