From the Summit County Citizens Voice (Bob Berwyn):
A detailed new study of tree rings in the Southwest shows that patterns of alternating wet and dry seasons observed since the 1950s is not the norm. Rather, the analysis suggests that, during historic decadal droughts, both winter and summer precipitation was below the long-term average. The new 470-year-long history (1539 to 2008) of summer precipitation in the Southwest covers most of Arizona, western New Mexico and parts of northern Mexico. It was established by looking at rings from Douglas firs and ponderosa pines, including tree-ring samples stored in the archives of the University of Arizona Laboratory of Tree-Ring Research.
Here’s the abstract:
The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June–August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October–April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century.