Nature and causes of the 2002 to 2004 drought in the southwestern United States compared with the historic 1953 to 1957 drought

The 1950s drought (1953 to 1957) and the recent drought (2002 to 2004) are 2 of the longest and most severe droughts to affect the southwestern USA since 1895. The 1953 to 1957 drought was longer, more severe, and more spatially extensive than the 2002 to 2004 drought. Although the 1953 to 1957 drought was centered over New Mexico, it was quite severe over much of the southwestern USA, while the impact of the 2002 to 2004 drought was mostly felt in Arizona. Both of these droughts were associated with multi-year La Nina events, the cold phase of the Pacific Decadal Oscillation (PDO), the warm phase of the Atlantic Multidecadal Oscillation (AMO), and the positive phase of the Eastern Pacific Oscillation (EPO). These conditions resulted in anomalous atmospheric ridging over the southwestern USA, which prevented Pacific moisture from entering the study region. Therefore, both droughts were associated with increases in the number of dry tropical days and decreases in the number of dry moderate and moist tropical days. These statistically significant changes in air mass frequencies were relatively consistent across the southwestern USA and during both drought events. Generally, it appears that the occurrence of major droughts in the southwestern USA is associated with a cold PDO and warm AMO, conditions seen during both the 1950s and the recent drought.