Relationships between extremes of the Southern oscillation and the winter climate of the Anglo‐American Pacific Coast

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Abstract
Warm and cold events in the central and eastern equatorial Pacific represent the two extreme climatic states of the Southern Oscillation and are related to pronounced winter climate anomalies along the Pacific coast of Anglo‐America. Previous research has demonstrated that the Pacific/North American (PNA) pattern may be the typical response of the northern hemisphere winter atmosphere to tropical forcing during warm events. In general, the anomalous flow associated with the PNA pattern raises winter temperatures along the entire length of the Anglo‐American west coast. Precipitation is enhanced in the vicinity of the Alaskan panhandle, but is suppressed to the south. During cold event winters, the dominant 700 mb anomaly strongly resembles the PNA pattern, but with a reversed polarity. Most cold event winters are associated with below normal temperatures along the west coast, while precipitation significantly decreases in the Alaskan panhandle and increases to the south. Thus, the climate anomalies experienced along the Anglo‐American west coast during cold events are in the opposite sense to those of warm events.Precipitation, and to a lesser extent temperature anomalies, are sensitive to minor fluctuations in the general placement and orientation of the PNA pattern, which in turn are related to shifts in the position of the polar front jet stream and associated storm track. Thus, a number of disparate climate anomalies are possible in the study area during warm event winters, depending upon the positioning of the long‐wave pattern over the north Pacific and North America.Although the PNA and ‘reverse PNA’ patterns can occur in any winter, they are significantly more frequent during warm and cold events, respectively, than in other winters. The west coast temperature and precipitation anomalies associated with the PNA (‘reverse PNA’) during warm event (cold event) winters can occur during any winter displaying the PNA (‘reverse PNA’) pattern, regardless of the presence of a warm (cold) event in the equatorial Pacific. A dynamical model is proposed to explain the observed teleconnection between the Anglo‐American west coast and cold Southern Oscillation events.