Now researchers can begin to unravel the tale of the monsoon's influence on earlier cultures -- and help us understand monsoonal cycles that influence our own culture -- thanks to a breakthrough at the University of Arizona's Laboratory of Tree-Ring Research.

A pilot study by LTRR research specialists David Meko and Christopher Baisan on Arizona's San Pedro watershed indicates tree ring analysis can successfully pinpoint years when the monsoon failed to bring its summer rains to the Southwest.

"It's a pretty striking pattern," explained Meko, who specializes in dendrochronology and hydrology. Dendrochronologists use the annual growth rings of trees to create timelines that can span thousands of years in order to date ancient ruins, answer ecological questions, and reveal climate details otherwise lost to the modern world.

"In this study, we're looking at the ability of a tree-ring network to identify a poor monsoon. And it does that pretty well, from what the results show," Meko said.

In fact, all but one of the 14 "dry summers" pinpointed using their technique reflected the instrumental record (1868-1992). They considered a summer dry if average rainfall for July and August fell below 124 mm (about 5 inches), based upon instrumental records drawn from 39 stations in their region of study, the San Pedro River Basin in southeastern Arizona.

Their extrapolation back into 1791 using their tree-ring record picks up another 14 dry summers, including a three-year run of poor summer rains starting in 1822.

The success of this pilot study has encouraged Meko and Baisan to expand their findings. They are constructing a timeline for the Southwest that will show previous episodes of particularly wet summers as well as dry ones. In addition, they are working with other researchers to develop monsoonal tree-ring chronologies for sites in New Mexico and Mexico.

Although their five sites on the San Pedro watershed seem to provide a good representation of the monsoon's overall variability in the southwestern United States, the monsoonal rains that reach Tucson actually represent only the northern tip of a system that stretches deep into Mexico.

By knowing more about the patterns of rainfall throughout the monsoon region, scientists hope to better understand this mysterious weather pattern that defines the ecology of the Southwest.

Few rainfall gauge records in the southwestern United States go back more than 110 years, Meko noted, while researchers feel lucky when they find even a few decades of such instrumental records near their sites in Mexico.

"This will give us an idea of mechanism behind the monsoon," Meko said. "And anything that helps us understand the mechanism of the monsoon can help us predict future occurrences."

Previous attempts to link tree-ring growth to monsoonal rains have been relatively unsuccessful. But Meko and Baisan adopted an approach that differs from earlier efforts in several ways.

First of all, the researchers selected and cored trees fitting a very specific profile: low-elevation pines and Douglas firs known from previous research to have an abundance of "false rings." Second, they honed in on the rings produced during the first century or two of each tree's life, and measured two separate sections of each of these rings. Finally, they used statistical maneuvers to reduce the influence of the previous growth, making summer growth -- or lack of it -- more distinctive.

Many dendrochronologists find false rings a nuisance because their darker color mimics the latewood marking the end of a year's growth. But Baisan had established in a previous project that they could pinpoint the exact year of each ring's formation, whether it contained a false ring or not.

As a result, the researchers could view the false rings as a record of the growth slowdown that occurs during the bone-dry late springs typical of the Southwest. Then they could interpret the flush of renewed growth after the false rings as a likely response to summer rainfall.

"These trees are most sensitive to the early monsoon," Meko said. "It's been so dry in the spring before the rains start up that they are primed to respond to whatever happens."

Because the dendrochronologists were able to visually divide each tree-ring measurement into two separate parts, Meko was able to set up an equation to remove the impact of early ring growth on late ring width. This reduced confusion that could result, for instance, when vigorous spring growth carried the trees through the rest of the year, making summer growth look normal despite a poor monsoon.

The LTRR researchers weeded out another potential source of confusion by focusing their analysis on what Meko called the "relatively young" portion of the tree: the first 100 to 200 years of their lives. Baisan had previously found the late ring size differences in the Doug firs tended to fade away as the trees reached 200 years or more in age.

"The age of the tree is important because the older trees lose their response," Baisan said. "When they're young, they have false rings typically every year."

The International Journal of Climatology (Volume 21, pages 697-708) has a paper describing this pilot study that was published online on April 19, 2001, at this URL

[Contact: David Meko, Christopher Baisan, Lori Stiles]

04-Oct-2001