Large-Scale Weather Regimes and Local Climate Over the Western United States

Weather regimes are used to determine changes in the statistical distribution of winter precipitation and temperature at eight locations within the western United States. Six regimes are identified from daily 700-mb heights during 46 winters (1949–95) over the North Pacific sector applying cluster analysis; these include the Pacific-North American (PNA) pattern, reverse-PNA, and a Pacific Ω-block. Most of the regimes are shown to have a statistically significant effect on the local median temperature, as well as daily temperature extremes, with differences between locations being secondary. Local precipitation frequency is also conditioned significantly by certain weather regimes, but differences between groups of locations are larger. Precipitation extremes are dispersed and hard to classify. The extent to which the El Niño-Southern Oscillation (ENSO) modulates the probability of occurrence of each of the six weather regimes is then investigated. Warm-event (El Niño) winters are found to be associated with a significant increase in prevalence of a PNA-like regime, in which negative height anomalies exhibit a northwest-southeast tilt over the North Pacific. During La Niña winters, this PNA-like regime occurs significantly less frequently, while a regime characterized by a ridge over southwestern North America becomes more prevalent. These two regimes are associated with opposing regional precipitation-frequency anomalies that contribute to a north-south contrast in precipitation anomalies over the western United States during El Niño and La Niña winters.