A hydroclimatic index for examining patterns of drought in the Colorado River Basin

Abstract

To better understand drought occurrence in the Colorado River Basin (CRB) of the southwestern United States we used a hydroclimatic index to create a historical record of drought coverage and analysed the linear trend and relationships with key climate teleconnections. The past century was characterized by an increase in drought coverage during the warm portion of the year almost exclusively as a result of climatic warming. In recent decades, a significant increase in the drought coverage occurred earlier in the year, during the spring season, primarily as a function of warming, but in combination with a decline in precipitation for a significant portion of the basin. The El Niño (La Niña) phase of the El Niño-Southern Oscillation (ENSO) phenomenon is associated with a smaller (larger) area of drought during fall and winter, and the ENSO phase during the preceding six months is a significant predictor. The area of drought within the CRB is larger (smaller) during the warm (cold) phases of the Atlantic Multi-decadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), although the relationship with the PDO is weak. Monthly AMO values for the two years preceding drought provide minor predictability. Decadal averages of drought coverage closely follow those of both the AMO and PDO index. However, the nature of the PDO-drought relationship is reversed over the two halves of the historical record, possibly indicating a dominance of the AMO over the PDO in influencing drought in the region. Teleconnection-drought relationships are stronger for the southern portion of the basin. Trends in drought coverage, the current phases of the AMO and PDO, climate change projections of regional warming, and the likelihood of continued rapid population growth could result in significant water resource problems within the CRB. Copyright © 2009 Royal Meteorological Society