UA tree ring research offers hope for wet monsoon season

It has happened before, although it is not the prediction for this year, nor the normal pattern.

Wet summers usually follow dry winters.

Atmospheric scientists trace that to the cooling and warming of the Pacific Ocean - the phenomena known as La Niña and El Niño.

In a La Niña year, when Pacific temperatures cool, winter rains are sparse and a better-than-average monsoon often follows. That's the situation this year, and the prediction and hope is for a decent monsoon soaking.

That's also the historical weather data, said Connie Woodhouse, an associate professor of geography and development at the University of Arizona.

Woodhouse is overseeing a study that is collecting data from tree rings to extend that data and discover, among other things, how often a dry monsoon has followed a dry winter.

Preliminary data show that below-average winter rains are followed by below-average summer rains about 28 percent of the time, she said.

In tree rings, the width of the light wood reflects winter precipitation, she said. The dark "latewood" rings record the growth spurt of the summer monsoon.

Using Douglas fir and ponderosa pine rings from Southern Arizona's "sky island" forests produces a chronology that goes back to 1650, with a few specimens that record events into the 1500s.

Woodhouse says researchers already know of a period from 1899 to 1903 when below-average summers were followed by below-average winters.

"We want to know, is that unusual, or does that happen from time to time?" she asked.

There is some preliminary evidence that the turn-of-the-last-century dry periods may extend back into the 1880s, she said.

That could correlate nicely with historical accounts of big fires in the region, said fire historian Tom Swetnam.

Swetnam said summer and winter rains failed to produce significant precipitation for an extended period of years in the 1880s and again in the 1950s. Large fires burned in those years, he said.