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This paper focuses primarily on the effect of El Niiio on seasonal and annual precipitation at 26 locations in Arizona and western New Mexico for an 86-year period from 1900. Following Rasmusson (1984, each year is classified as either a non-El Niiio year (51) or an El Niiio year (33, with the El Niiios designated as being very weak (4), weak (8), moderate (12), or strong (1 1). Using the Mann-Whitney U-test for the 19 stations in Arizona, we find that, in the spring (March-May) and in the autumn (September-November), there is a positive relationship between precipitation in Arizona and both moderate and strong El Niiios.These results are verified by correlating seasonal and annual precipitation for the complete 26-station network with the pressure difference between Tahiti and Darwin. Plausible physical explanations for these results are given.

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The eigenvector or "empirical orthogonal function" approach is used t o determine the dominant precipitation anomaly patterns for the western United States for each month during the last 36 yr. I n all months there is enough intercorrelation among monthly precipitation amounts in different parts of the region that a t least 45 percent of the total variance can be explained by only three eigenvectors. Usually the most important pattern is one with a single large region of anomalous precipitation, centered in southern California, Arizona, or Nevada in winter and in Washington, Idaho, or hfontana in summer. Also important in all months is a pattern with anomalies of opposite sign in the Pacific Northwest and the Arizona-New Mexico-Texas area. 315-521 0-68-2 MONTHLY WEATHER REVIEW Vol. 96, No. 9 The author would like to thank Dr. D. L. Gilman of the Extended Forecast Section of ESSA for his comments on this paper and for helping to interpret the results. The assistance of Douglas R. Fancher and James A. Fogltance in the data reduction and analysis is also appreciated.

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