Climatic Controls and Hydrologic Impacts of a Recent Extreme Seasonal Precipitation Reversal in Arizona

Abstract

The winter (December?February) of 2005/06 ranked as the driest in the instrumental record (since 1895) for nearly all regions of Arizona. The city of Phoenix, Arizona, recorded no precipitation during this time period, which was part of a record dry streak of 143 days without measurable precipitation. More important, the Salt and Verde watersheds, which supply the greater Phoenix area with approximately 50% of its water supply, received less than 3% of normal precipitation. Remarkably, this historically dry winter was preceded by the second wettest winter on record in 2004/05, a winter that filled reservoirs statewide and ameliorated a drought that has persisted since 1996 in some parts of the state. This study begins with a brief overview of the historical context of such reversals of extreme seasonal precipitation in Arizona followed by an analysis of the teleconnective impacts. The authors find that while an extreme reversal such as this has only happened once before in Arizona (1904/05 and 1905/06), there is a trend for increasing variability in winter precipitation from one year to the next in Arizona, especially since the 1960s. Large reversals of winter precipitation are followed by large reversals of the opposite sign in the summer monsoon more than 75% of the time. In general, large dry-to-wet reversals are associated with neutral ENSO?to?neutral ENSO conditions or a neutral ENSO?to?El Niño transition, whereas wet-to-dry reversals are associated with an El Niño?to?La Niña transition or, more commonly, with an El Niño?to?neutral ENSO transition. In addition, changes in the sign of the Atlantic multidecadal oscillation, eastern Pacific oscillation, and Pacific?North American (PNA) pattern are all significantly associated with precipitation reversals. During the seven winters when neutral ENSO and strongly positive PNA coexist, large wet-to-dry reversals occur in every case and nearly all rank among the largest such reversals. It is suggested that small reservoirs are more at risk for increasing climatic volatility than are large reservoirs.