Tracing the Waters through the Trees: North American Monsoon Dynamics over the past Four Centuries

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The North American Monsoon (NAM) regulates summer precipitation over Central America, Mexico, and the American Southwest, reaching as far north as Las Vegas and the Great Basin. Based on instrumental records, both the onset and total amount of NAM rainfall have oscillated over time, with a tendency towards increased variability in recent years. Given the interplay between ocean, atmosphere, and land that characterizes the NAM region, this research project aims at placing modern instrumental observations (which are limited to a few decades) within a longer temporal perspective in order to fully represent the range of NAM variability and improve our understanding of underlying processes and driving forces at annual to decadal scales. The researchers will obtain information on North American Monsoon variability over the past four centuries using stable isotope analysis of tree-ring records from two geographic endpoints of the NAM, namely Nevado de Colima, Mexico, at about 19 degrees N, and Great Basin National Park, USA, at about 39 degrees N. In addition, the investigators will perform an intensive monitoring of stem growth and climate using, respectively, automated dendrometers and weather stations. These sensors are already in place at Nevado de Colima, and the required equipment will be installed at Great Basin National Park. This research will provide detailed information on growing season length and daily to weekly relationships between radial growth and climate, information that is critical for correctly calibrating stable isotopic ratios against climate variability in the North American Monsoon region.The North American tropics are a heavily populated area that is highly vulnerable to natural disasters. Water treaties between the US and Mexico concern the NAM region, are traditionally contentious, and are likely to be challenged in the coming years by rapid population growth and climatic changes. It is unclear if the eastern tropical Pacific, the western tropical Atlantic, or a combination of both supplies moisture for the summer monsoon. Sources of moisture may also change from one year to the next or on longer time scales. Project results will provide a long-term perspective to the North American Monsoon Experiment (NAME), a joint CLIVAR-GEWEX process study aimed at determining the sources and limits of predictability of warm season precipitation over North America. In addition, both study sites are located in mountain environments, where monitoring stations are scarce despite the vulnerability of these regions to natural and human disturbance. The proposed project will be coordinated with the Consortium for Integrated Climate Research in Western Mountains (CIRMOUNT), an interdisciplinary group of experts recently formed to promote climate-related science in mountainous regions. This project will strengthen collaboration between national (the Department of Geography at UNR, Great Basin National Park) and international (the Universidad de Colima in Mexico, the Institute of Chemistry and Dynamics of the Geosphere Sedimentary Systems in Julich, Germany) research and education institutions. Results will be broadly disseminated using a mix of presentations at professional meetings, multi-media talks during seminars and class lectures, articles in peer-reviewed scientific journals, materials posted on the PI's web site, and as one-on-one conversations during outreach programs aimed at secondary teachers and Latino/a students.