Every year in late April or May, when the sun is moving north in its seasonal progression, large, high-elevation land surfaces in Mexico's Sierra Madre Occidental Mountains begin to warm up. As the warm air over the land rises, it draws moist air from the Sea of Cortez, or Gulf of California, further west.
"Moisture comes in and as the land continues to heat up moving northward, that moist air also moves northward," Monson explained. As it advances, daily warming at the land surface gives the air a lot of upward lift, which creates the late-afternoon thunderstorms in May and June in Northern Mexico.
By July the storms begin to reach Tucson.
This process continues to proceed northward as summer progresses. By late summer and early fall, the sun begins to recede southward. The major weather fronts and regional pressure systems are changing, the land-sea temperature difference begins to diminish and the monsoons slowly dissipate."
Monson said much research in the UA's atmospheric sciences has focused on modeling how the monsoon system evolves, what determines its intensity, and how it responds to El Nio and La Nia years, caused by unusually warm or cold surface temperatures in the equatorial Pacific.
"How far north the monsoon travels from its origin in the Sierra Madre Occidental, its east-west expanse and the intensity of the monsoon are all driven by land-sea temperature differences, which are in turn affected by climate change," Monson said. "What we want to do now is bring the ecological perspective into play with our strength in weather modeling. How does the monsoon and its variability influence tree growth, spread of invasive grasses in the western U.S. and the propagation of fire cycles?"
Monson also said the monsoon has direct effects on tree growth in the region, where water is sparse and limits plant growth. "Using tree-ring analyses, we can go back hundreds of years and assess the relative intensit
|Contact: Daniel Stolte|
University of Arizona