Large-scale weather regimes and local climate over the western United States

Weather regimes are used to determine changes in the statistical distributi on of winter precipitation and temperature at eight locations within the we stern United States. Six regimes are identified from daily 700-mb heights d uring 46 winters (1949-95) over the North Pacific sector applying cluster a nalysis; these include the Pacific-North American (PNA) pattern, reverse-PN A, a tropical-Northern Hemisphere (TNH) regime, and a Pacific Omega block. Most of the regimes have a statistically significant effect on the local me dian temperature, as well as daily temperature extremes; differences betwee n locations are secondary to the large-scale effects. Local precipitation f requency 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 dependence of local temperature sta tistics on the warm- or cold-air advection associated with particular weath er regimes is discussed, as is the dependence of precipitation anomalies on the regimes' displaced storm tracks. The extent to which the El Nino-Southern Oscillation modulates the probabil ity of occurrence of each of the six weather regimes is then investigated. Warm event (El Nino) winters are found to be associated with a significant increase in prevalence of a TNH regime, in which negative height anomalies exhibit a northwest-southeast tilt over the North Pacific. During La Nina w inters, this TNH regime occurs significantly less frequently, while a regim e characterized by a ridge over southwestern North America becomes more pre valent. These two regimes are associated with regional precipitation-freque ncy anomalies of opposite sign, that contribute to a north-south contrast i n precipitation anomalies over the western United States during El Nine and La Nina winters. On interdecadal timescales, the frequency-of-occurrence o f the PNA pattern is found to be notably higher during the 1970s and early 1980s.