ANOMALOUS NORTH PACIFIC ATMOSPHERIC CIRCULATION AND LARGE WINTER FLOODS IN THE SOUTHWESTERN UNITED-STATES

Specific anomalous atmospheric circulation conditions over the North P acific are conducive to the occurrence of the largest winter floods (g reater-than-or-equal-to 10-yr return period) on rivers in six hydrocli matic subregions of Arizona and southern Utah, Nevada, and California. Composite maps of anomalies in daily 700-mb heights indicate that flo ods in all of the subregions are associated with a low pressure anomal y off the California coast and a high-pressure anomaly in the vicinity of either Alaska or the Aleutian Islands. Of these two major circulat ion features, the presence of the low is the controlling factor in det ermining whether large floods will occur. Shifts in the locations of t he low and high pressure anomalies over the North Pacific appear to co ntrol which subregions experience floods, with high-elevation topograp hic features and proximity to air masses forming a major influence ove r the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region. Concerning the interannual variab ility of flooding in the Southwest, there is an increased frequency of large winter floods during multiple-year periods dominated by negativ e SOI and a virtual absence of large floods during the intervening per iods. This suggests that global-scale climatic anomalies exert a stron g influence on the occurrence of severe regional winter floods.