Winter Circulation Anomalies in the Western United States Associated with Antecedent and Decadal ENSO Variability

Previous research has shown that the impacts of ENSO variability on the winter climate of the western United States vary on decadal time scales. In this study, the relationship between fall season ENSO conditions and winter circulation anomalies over the western United States is shown to vary with phasing of the Pacific decadal oscillation (PDO). During the PDO cold phase of 1948-76, the majority of fall season El Nino events did not precede winter troughing anomalies over the southwestern states that are typically associated with above-normal winter precipitation in this region. In contrast, during the PDO warm phase of 1977-98, fall season El Nino conditions did precede southwestern winter troughing anomalies in all but one instance. Fall season La Nina conditions during both the cold and warm phases of the PDO reliably preceded winter season high pressure ridging centered off the Pacific coast. These results highlight uncertainty on decadal time scales surrounding the use of fall ENSO conditions, particularly El Nino events, as a seasonal climate forecast tool.