No Long-Term Trends in Southwestern United States Drought
McCabe, G.J., Legates, D.R. and Lins, H.F. 2010. Variability and trends in dry day frequency and dry event length in the southwestern United States. Journal of Geophysical Research 115: D07108, doi:10.1029/2009JD012866.
McCabe et al. (2010) found for the period of study "El Niño events have been more frequent, and this has resulted in increased precipitation in the southwestern United States, particularly during the cool season. The increased precipitation is associated with a decrease in the number of dry days and a decrease in dry event length."
The plot below (Figure 1) nicely reveals what has happened in the region. The number of dry days dropped over much of the study period but increased since 2000
Figure 1. Five year moving time series of the mean fraction of days with daily precipitation below 2.54 mm for water years (October through September), cool seasons (October through March), and warm seasons (April through September) (from McCabe et al., 2010).
Most of the rainfall in the southwest occurs with the winter cyclones (most common in El Niño years and also with late summer monsoon showers. Winter rains and mountain snows are more effective for water supplies than summer thunderstorms which runoff and whose water evaporates more quickly in the summer temperatures. Winter rains may fall over several days as storms pass. The pattern above clearly shows the El Niño and the effect of the Pacific Decadal Oscillation which effects El Niño frequency. The PDO was in an 'El Niño favored stage' from 1977 to 1998 and the frequency of dry days was lowest. The La Niñas are wetter and the PDO was in the "La Niña favored stage' from 1947 to 1977 and after 1999.
Prior to this study, McCabe et al. (2044) did a study of the effect of the PDO and the Atlantic Multidecadal Oscillation (AMO) on drought frequency in the United States and found more than half (52%) of the spatial and temporal variance in multidecadal drought frequency over the conterminous United States is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).
The lack of a signal for increased drought in this data agrees with NOAA's NCDC Climate at a Glance analysis for the southwest regional annual precipitation (Arizona, New Mexico, Utah and Colorado) which shows a slight, statistically insignificant upward trend.
"Our results are consistent with analyses of trends in discharge for sites in the southwestern United States, an increased frequency in El Niño events, and positive trends in precipitation in the southwestern United States are negative trends for water years and cool seasons."
McCabe, G.J., Palecki, M.A. and Betancourt, J.L. 2004. Pacific and Atlantic ocean influences on multidecadal drought frequency in the United States. Proceedings of the National Academy of Sciences, U.S.A. 101: 4136-4141.