Snow in the Rockies, dry summers in the Southwest?

New simulations of summer rains in the arid American Southwest show that they are influenced by the previous winter’s snowpack in the Rocky Mountains.

Summer rains, called "monsoons," are the predominant source of rain in eastern Arizona and western New Mexico, says Michael Notaro , a climate scientist who is associate director at the Nelson Institute Center for Climatic Research at the University of Wisconsin-Madison.

Anything that can help predict those rains could help protect people and an environment that are severely stressed by water shortages.

Monsoons are summer rains that occur when moist air above an ocean, in this case the Pacific, flows over land and releases its water as rain. Monsoons are driven by a summertime temperature differential between hot air above the land and the cooler air over the ocean.

Repeated studies have shown that the East Asian monsoon, which delivers heavy rain that supports intense agriculture in China and India, is influenced by snowpack in the Himalayas and Tibetan Plateau. Deep, extensive snowpack in the early spring translates into less rainfall in that summer, Notaro says, but nobody had tried to check for the same phenomenon in the North American monsoon.

Heavy snowpack cools the air through two mechanisms, Notaro says. "Snow reflects more sunlight than plants or bare ground. Also, after all this extra snow melts in spring, the soil remains extra wet in early summer," he says.

And because a temperature differential is what drives the monsoon — cool air from the ocean pushes in to replace hot air above the land — it makes sense that heavy snowpack would reduce the intensity of the ensuing monsoon.

Some studies have found this relationship in weather data for the American Southwest, but the new study is the first experimental evidence that the correlation is real, Notaro told the annual meeting of the American Geophysical Union in San Francisco today.

Using a computer model that could predict climate conditions from past data reasonably well, Notaro and co-author Azar Zarrin, a recent post-doctoral fellow at the Center for Climatic Research, then doubled the snowpack, or removed it entirely, in early spring.

Removing all snow did not have a significant effect on the monsoon. But doubling the average snowpack had a strong influence on rainfall during August, the peak of the North American monsoon.

"We found about a 25 percent reduction in rainfall amounts," Notaro says, "and that would really matter, as the monsoon in the Southwest is the primary source of rainfall for the year."

It’s ironic, he says, that more frozen water in the Rockies would reduce the amount of liquid water falling in subsequent months, but the physics makes sense, and the finding is in line with measurements. "Snow in the mountains means less monsoon rain in the region to the south, even though the rivers fed by Rocky Mountain snowmelt may carry more water."

Climatologists have found many examples of such "teleconnections," in climate, he adds. The cycle of El Niño and La Niña — driven by variations in water temperatures in the tropical Pacific Ocean — affects U.S. weather. During La Niña conditions, the eastern Pacific is cooler, pushing the subtropical jet stream northward, contributing to dry weather in Texas, the Southwest and Florida.

The new study also raised a suggestion of a second teleconnection, says Notaro — a significant reduction in rainfall in Kansas, Oklahoma, Missouri and Arkansas. The model shows that heavy snowpack over the Rockies contributes to higher atmospheric pressure, which increases northerly winds in the Central plains during late spring and summer. "And so with less moist air flowing from the Gulf of Mexico, there is more likely to be hot, dry conditions in the Central plains," Notaro says. "I am not sure anyone had explored that before."

One study, no matter how careful, does not prove the snow-monsoon connection, Notaro says, but the model did succeed in recreating real weather in the Southwest. And since only the amount of snowpack was changed, the results do finger an important factor at monsoon strength.

A better understanding of monsoons could contribute to a sorely needed improvement in long-term forecasts, Notaro says.

"Drought is a regular visitor to the Southwest, and with a growing population facing greater water restrictions, knowing how much rain is likely to fall should help land and water managers anticipate droughts before they happen," Notaro says.