TWENTIETH-CENTURY CLIMATE ANOMALY PATTERNS OVER THE SOUTHWESTERN UNITED STATES

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Abstract
Winter precipitation anomaly patterns between 1911 and 1977 over the southwestern United States are investigated by empirical orthogonal function (EOF) analysis. The first four EOFs explain 74% of the total variance in the original data assemblages. The first three were statistically significant at the 95% confidence level. The spatial distribution of the coefficients and the time plots of the amplitudes of the first two EOFs indicate that the first three decades of the study period, identified as encompassing the most anomalously warm decades in the western Untied States this century, was a period when negative precipitation anomalies predominated in the Great Basin and in the upper Colorado and Green River basins, but positive anomalies predominated in the lower Colorado and the Colorado Plateau region of Arizona. In the early 1950s not only was there a hiatus in winter precipitation distribution, but the earlier anomaly patterns reversed to a predominantly wet northern section and a predominantly drier south. This hiatus in anomaly patterns coincided with the change to increased meridionality in upper tropospheric circulation patterns. These results, when viewed in the light of those from Euler et al. (1979), Wigley, Jones and Kelly (1980), Williams (1980) and Jäger and Kellogg (1983), support a strong case for an anomalously warm Southwest during strong hemispheric and hence Arctic warming and therefore a predominance of the anomaly patterns exemplified by the first two EOFs. The third EOF depicts the influence of physiography and the nature and trajectory of precipitation-inducing systems in winter on the variance pattern. The spatial distributions of the EOFs raise questions about the inferential use of surrogate data for environmental reconstruction in the presence of anomaly patterns that lead to nonsynchroneity in the response of samples in time and space. Power spectra of the amplitudes of the first three EOFs show the biennial oscillation and oscillations with periodicities greater than 30 years to be significant at the 95% confidence level on a white noise continuum.