The North American monsoon in the U.S. Southwest: Potential for investigation with tree-ring carbon isotopes

The North American Monsoon (NAM) contributes critical summer moisture to the U.S. Southwest from July through September, but instrumental records of monsoon precipitation are limited to 100 years or less. Tree-ring investigation offers a means of improving our understanding of its long-term spatial and temporal variability. Available evidence indicates the stable-carbon isotopic composition (δ¹³C) of tree rings in this region is strongly linked to moisture. In addition to latewood width as a precipitation proxy, the δ¹³C of latewood also appears to be a strong proxy, largely manifesting water stress effects on stomatal conductance and their consequence to isotopic discrimination against ¹³CO₂. In one promising study, the δ¹³C of 11 years of latewood from 8 sites regressed against their corresponding precipitation exhibited a coefficient of -0.061‰ per cm of July + August + September precipitation (r² = 0.41). Long latewood δ¹³C chronologies are currently being developed from tree rings of ponderosa pine and Douglas-fir at several sites in NAM core regions in the Southwest to evaluate its usefulness in supplementing precipitation reconstructions derived from latewood widths. Among planned outcomes, the improved monsoon precipitation records can be used to better evaluate natural variability of NAM precipitation and its linkage to winter precipitation, document the character of monsoon season droughts, and test accuracy of regional climate models.