Loss of Summer Rains Stoked Long Droughts in Southwest

Joint Release

WASHINGTON— Long-term droughts in southwestern North America often mean failure of both winter and summer rains, according to new tree-ring research. The finding contradicts a commonly held belief regarding the region – that a dry winter rainy season is generally followed by a wet summer season, and vice versa. In fact, when severe, decades-long droughts have struck the area in centuries past, both winter and summer rains generally were sparse year after year, the new study shows.

“One of the big questions in drought studies is what prompts droughts to go on and on,” said lead author Daniel Griffin, a doctoral candidate in the School of Geography and Development of the University of Arizona in Tucson. “This gives us some indication that the monsoon and its failure is involved in drought persistence in the Southwest.”

A monsoon is a season of heavy rains caused by air rising over warm land, which draws in cooler, more humid air from the ocean. In most of Arizona, western New Mexico, and parts of northern Mexico–where the monsoon lasts from late spring to early fall–moisture-laden winds blow in from the Gulf of California and the eastern tropical Pacific Ocean.

The new study’s results surprised Griffin because rain gauge records for the Southwest from 1950-2000 show that dry seasons alternated with wet ones. However, the team’s new 470-year-long record, going from 2008 all the way back to 1539, shows that the wet/dry pattern of the latter part of the 20th century is not the norm – either prior to the 20th century or now, he said.

The research report by Griffin and his colleagues was published today, March 11, in Geophysical Research Letters, a journal of the American Geophysical Union.

“This is the first time researchers have used tree rings to take a closer look at the monsoon in a large and important area of the American Southwest,” Griffin said. “Monsoon droughts of the past were more severe and persistent than any of the last 100 years,” he added. “These major monsoon droughts coincided with decadal winter droughts.”

Those droughts had major environmental and social effects, Griffin said, pointing out that the late-16th-century megadrought caused landscape-scale vegetation changes, a 17th-century drought has been implicated in the Pueblo Revolt of 1680 and the 1882-1905 drought killed more than 50 percent of Arizona’s cattle.

“The thing that’s interesting about these droughts is that we’ve reconstructed the winter precipitation, but we’ve never known what the summers were like,” said co-author Connie A. Woodhouse, also of the University of Arizona, Tuscon.

Because winter precipitation has the strongest influence on annual tree growth, previous large-scale, long-term tree-ring reconstructions of the region’s precipitation history had focused only on the winter rainy season. “Now we see – wow – the summers were dry, too,” Woodhouse said. “That has a big impact.”

“In the Southwest, the winter precipitation is really important for water supply. This is the water that replenishes reservoirs and soil moisture,” she said. “But the monsoon mediates the demand for water in the summer.”

Until recently, most tree-ring researchers, known as dendrochronologists, have looked at the total width of trees’ annual rings to reconstruct past climate. Few teased out the seasonal climate signal recorded in the narrow part of the growth ring laid down in late summer known as latewood.

To figure out the region’s past history of monsoon precipitation, the scientists needed to measure latewood from tree-ring samples stored in the archives of the University of Arizona Laboratory of Tree-Ring Research and go into the field to take additional samples of tree rings.

The team looked at annual growth rings from two different species, Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) throughout the weather forecast region called North American Monsoon Region 2, or NAM2.

In all, the researchers used samples from 50 to 100 trees at each of 53 different sites throughout southwestern North America. The team’s climate analyses focused on NAM2, which covers most of Arizona, western New Mexico and northern parts of the Mexican states of Sonora and Chihuahua.

Griffin said, “It was a massive undertaking — we employed about 15 undergraduates over a four-year period to measure almost 1 million tree rings.”

One possible next step, Woodhouse said, is to expand the current project to other areas of the Southwest and into Mexico, where the monsoon has a bigger influence on annual precipitation.

Another would be using tree-ring reconstructions of the Southwest’s fire histories to see how wildfires are related to summer precipitation.

“Before I moved to the Southwest, I didn’t realize how critically important the summer rains are to the ecosystems here,” Griffin said. “The summer monsoon rains have allowed humans to survive in the Southwest for at least 4,000 years.”

The National Science Foundation, the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency supported the research.