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Uncovering the Effects of Mountain Winds


By Tara Roberts

Over the past 60 years, scientists have recorded decreasing mountain snowpack and stream flow in the Northwest. Increased air temperature due to climate change is a major cause, but University of Idaho’s John Abatzoglou and researchers from the U.S. Forest Service have found another, less expected ingredient: weaker westerly winds.

Abatzoglou, an assistant professor of geography in the UI College of Science, co-authored a paper published Nov. 29 in the premier journal Science that explains how these decreased wind speeds are affecting the Northwest’s vital water resources.

In the Cascades and the Rockies in northern Idaho and western Montana, mountains play a major role in precipitation levels.

“If you imagine pushing a big air mass toward a mountain, it can’t go down, it can’t go through, so it has to go up,” Abatzoglou says. “As air is forced to rise, you squeeze additional moisture out of the clouds, and you get what’s called orographic precipitation.”

However, much of the data collected about precipitation in the Northwest doesn’t reflect these mountainous areas. Precipitation records go back to the 1880s, but only for lower elevations where people were likely to set up weather stations.

“What really matters is those higher elevations, where we haven’t had long-term records,” Abatzoglou says.

In the last 30 years or so, researchers have established stations across a broader range of elevations, providing Abatzoglou and his colleagues with the information they needed to study the factors affecting water resources.

“We saw a really strong relationship between the strength of the westerly wind at 10,000 feet and the amount of precipitation at higher elevations, where much of our precipitation is collected in mountain snowpack,” Abatzoglou says.

They then used data on westerly wind speeds over the past 60 years, and saw a 5-10 percent decline in speed during the winter months. Though they are not sure precisely what has caused the decrease, Abatzoglou and the Forest Service researchers were able to conclude that as wind speed decreased, the ratio of precipitation falling in the mountains compared to the amount falling in the lowlands decreased along with it.

This change is small, but its effects are noticeable when it comes to snowpack and stream flow.

“Together, decreasing wind speed and increasing air temperature create a double-whammy of impact for our water resources,” Abatzoglou says.

The research has implications for the climate models used to predict how the Northwest will change in the future and how those changes may affect the region’s economy and ecosystems.

Most climate models today predict a small increase in precipitation for the Northwest by the middle 21st century. But decreasing winds may make that increase even more subtle, Abatzoglou says.

“It’s another factor to consider when trying to project what our water resources will be in a changing climate.”