image: A monsoon thunderstorm forms over the pinon-juniper woodlands of northwest New Mexico in July 2009. view more
Credit: Copyright: Daniel Griffin, UA LTRR
Sweeping across the desert Southwest in the summer months, the monsoon not only brings rains and relief from the scorching heat to the people who live here, it also acts as a major driver of ecological processes.
To better understand the highly complex processes by which this unique weather phenomenon influences ecosystems and natural cycles, the National Science Foundation, or NSF, has awarded almost $3 million to fund a groundbreaking research endeavor aimed at unraveling the profound interactions between weather, soil and plants linked to the monsoon phenomenon.
The grant is one of only two research proposals considered "outstanding" across all categories evaluated by the NSF from a pool of 50 competing applications.
"We are constructing a highly coupled computer modeling system between summer weather and ecology that has never been constructed before for this part of the world," said Russell Monson, principal investigator of the grant. Monson recently joined the University of Arizona's School of Natural Resources and the Environment and the Laboratory of Tree-Ring Research.
"Traditionally, ecologists have tended to focus on individual ecosystems, small study sites and limited populations of organisms," he added. "What we're doing under this NSF grant is broad, regional-scale ecology. It's a new way of practicing, and thinking about, ecology. The monsoon seasonal weather phenomenon organizes and synchronizes virtually all the ecological systems in the southwestern United States, from low-elevation deserts to high-elevation spruce-fir forests."
The North American Monsoon System was selected "because it is not only a principal ecological driver at a large regional scale, but also susceptible to perturbation due to climate change," Monson explained. "Fluctuations in sea surface temperatures due to climate cycles, especially those that influence near-coastal waters, such as in the Sea of Cortez, exert a strong control on the magnitude and extent of summer monsoon storms and thus indirectly influence regional ecological cycles on land in the southwestern U.S."
Funding for the grant, called Processes and Patterns in the North American Monsoon Macrosystem, is for five years and comes with $2.95 million to the UA, $402,000 to New Mexico State University and $281,000 to Utah State University.
Co-investigators from the UA include Chris Castro, Francina Dominguez, Xubin Zeng, Guo Yue Niu, Tom Swetnam, Steve Leavitt, Connie Woodhouse and Julio Betancourt.
The investigation will consist of advanced computer modeling approaches combining weather forecasting and biological processes associated with the vegetation on the surface to determine how the highly variable rains associated with the monsoon system determine the timing and spatial spread of plants in the monsoon region.
Studies will include explicit investigations on how these rains control the spread of invasive grasses in the southwestern U.S., including cheat grass and buffelgrass, and how the spread of these invasive grasses is likely to influence weather patterns associated with seasonal propagation of the monsoon system through changes in the reflection of solar energy at the surface and evaporation of water into the atmosphere.
In a second line of research, extensive tree-ring analyses will be conducted across the southwestern U.S. to discern historical patterns in the magnitude and distribution of the monsoon system and to validate the predictions of the models using historical data.
The studies also will explore the connectivity between lower elevation arid grasslands and higher elevation forests with regard to the regional fire cycle.
Bringing an annually recurring, second rainy season in addition to winter rains, the monsoon makes the Sonoran Desert stand out from other deserts across the world by supporting an unparalleled diversity of fauna and flora.
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 Niño and La Niña 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 intensities of winter and summer rains," he said.
Monson and his collaborators are planning to take this a step further, studying oxygen and carbon isotope composition of the wood in tree rings.
"By looking at the isotope ratios in the wood, we can potentially resolve wet and dry years along very sensitive quantitative scales, providing even more accurate reconstructions of past monsoon intensities," he said.
The researchers will use tree-ring data to test and validate computer models that will be developed to model the monsoon and how it is expected to change over time.
A central focus of the grant is on the interconnections among invasive plants, monsoon weather patterns and fire. Monson and his co-workers decided to focus on pinion pine/juniper forest at intermediate elevations as a critical connector between low-elevation desert and high-elevation forests with respect to the upward spread of wildfires.
"Many wildfires start in the grassland at lower elevations, where dry grasses provide ready tinder for ignition by lightning strikes," Monson explained. He said grass doesn't carry a lot of fuel, but ignites quickly, causing fire to spread rapidly across the surface.
"With invasive grasses like cheat grasses and buffelgrass advancing into higher elevations, those fires can jump to pinion pine and juniper forests at intermediate elevations where there is a lot of wood and flammable needles to burn, and then to the higher-elevation ponderosa pine ecosystem where there is even more fuel," Monson said.
"We are trying to study that dynamic both through tree-ring analyses of past fire frequencies and modeling of current potential for fire frequencies in different years with different amounts of precipitation," he also said.
"An important part of what we want to study is how those invasive grasses change the fire frequency and how the monsoon rains affect that transmission of the fires from lower to higher elevations," Monson added. "There is a cascade of ecological effects that are linked to the variability in the monsoon system. We want to work all that out."
In addition to training students and post-doctoral researchers, the grant will fund public outreach programs, including a public display at the UA's Biosphere 2, describing the relevance of summer monsoon climate to past Native American peoples and contemporary societies. In addition, K-12 teachers and college professors from the Navajo Nation will be recruited to participate in workshops on paleoclimate reconstruction, mostly tree-ring research.
Xubin Zeng, a co-principal investigator and professor in the UA's department of atmospheric sciences and director of the UA's Climate Dynamics and Hydrometeorology Center, added: "A unique aspect of this grant is the equal focus on atmospheric science and ecology on the ground. This is the first time we integrate invasive plant species with weather and climate modeling. Nobody has done this before."
Zeng said the team has to develop new climate models accounting for interactions between vegetation and the atmosphere: Plants feed back to the monsoon through evapotransporation, a process by which plants move water from the soil through roots, trunk and leaves back into the atmosphere, and by changing the way solar energy reflects from the ground.
Monson, who holds the Louise Foucar Marshall Professorship, was recruited to the UA under the Provost's Environmental Hiring Initiative coordinated by the Institute of the Environment, a program to recruit high-potential and leading scholars in environmental science and policy.
"Dr. Monson was just the sort of outstanding scholar we hoped to attract when we partnered with the Provost's Environmental Hiring initiative," said Diana Liverman, co-director of the Institute of the Environment.
Travis Huxman, director of the UA's Biosphere 2, added: "There was interest across campus in his research and he is a wonderful addition to the UA interdisciplinary environmental expertise fostered by the Institute of the Environment and Biosphere 2."