We've always been comforted by the common belief that wet summers follow dry winters and vice versa - but that has only been the case for the last half of the 20th century.
A new interpretation of tree rings that separates winter and summer rain markers found that dry summers often pair with dry winters.
A dry winter is no harbinger of a good monsoon.
When double-dry seasons lasted multiple years, they may have contributed to all manner of social disruption - from the great cattle die-off of the early 1900s to the Pueblo revolt of the 1670s.
The findings, by a large team of dendro-climatologists affiliated with the UA Laboratory of Tree-Ring Research, were published last week in Geophysical Research Letters, a journal of the American Geophysical Union.
The study notes some periods in which the summer monsoon and the winter rains fizzled for a decade or more.
"The major message for us is that these decadal droughts of the last 470 years were not just a winter phenomenon. The failure of the monsoon is now implicated in the record of the past," said Daniel Griffin, lead author of the report.
"It was a little surprising, given this 'folk' weather forecasting that people seem to have accepted," said Connie Woodhouse, associate head of the School of Geography and Development at the University of Arizona. "There is something new here."
In addition to Woodhouse, the research team included four other UA faculty researchers, Griffin and three other graduate students, and about a dozen undergraduates.
The resulting database has 948,077 rings representing about 2,500 trees, Griffin said.
About 1,500 tree samples were already in the tree-ring archive, and the researchers collected samples from an additional 1,000 trees between 2008 and 2011.
All were taken from ponderosa pine and Douglas fir in mountain forests from northern Mexico to above the Mogollon Rim in Arizona, the westernmost forests of New Mexico and a sliver of northern Chihuahua. The area includes watersheds for Arizona's most populous areas in Tucson and Phoenix.
The study found that, over the last 470 years, years in which both winter and summer rain fell beneath the norm were more common than previously thought.
The alternating wet signal was the exception rather than the rule.
Each tree ring, which marks the annual growth of a tree, contains a light band representing early or winter growth and a darker ring of "latewood" representing summer growth.
Previous studies had compiled annual climate data for the region, but had not differentiated between winter and summer rain.
Researchers knew, for example, that the decades preceding the Great Puebloan Revolt of 1680 were dry ones.
This study found that the failure of the summer monsoon started 20 years earlier and the period from 1666-76 was "the most extreme monsoon drought episode of the last 470 years."
It is further corroboration of earlier tree-ring studies of annual precipitation, the historical record and the oral history of the descendants of those Pueblo people, said Thomas Sheridan, research anthropologist and historian with the UA's Southwest Center, who was not part of the study.
"There are mentions of drought and famine, as well as quite a bit of social unrest in the Spanish documents, " said Sheridan. "This is a corroboration of that and also indicates how major a drought that was."
Crop failure combined with Spanish demands for tribute and a ban on ceremonies to fuel what Sheridan called "the most successful revolt in colonial history." The Spanish were driven from New Mexico in 1680 and did not return for 12 years.
"These people were pushed closer and closer to the margin of famine. It's hard not to believe that they saw that drought as punishment for not performing all their ceremonies, which were prohibited by the Franciscans."
Sheridan said there has "always been an intimate tie between tree-ring studies and anthropology. The first people to realize how the tree-ring record could be used were archaeologists like Emil Haury. It really helps us reconstruct the past and, unfortunately, lets us peer into the future and what Arizona is going to be like," he said.
"The major message for us is that these decadal droughts of the last 470 years were not just a winter phenomenon. The failure of the monsoon is now implicated in the record of the past."
Daniel Griffin, lead author of the report
Contact reporter Tom Beal at tbeal@azstarnet.com or 573-4158.
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