Long-term droughts in the Southwestern North America often mean failure of both summer and winter rains, according to new tree-ring research. For the severe, multi-decadal droughts that occurred from 1539 to 2008, both winter and summer rains were sparse year after year. The finding contradicts the commonly held belief that a dry winter rainy season is generally followed by a wet monsoon season, and vice versa.
http://www.sciencedaily.com/releases/2013/03/130311151120.htm