The North American Monsoon (NAM) is a critical source of moisture for northwestern Mexico and the U.S. Southwest. Monsoon research over the past several decades has investigated moisture source regions, spatial and temporal variability, timing and intensity of monsoon precipitation, and relationships to large-scale circulation. Although the NAM is now much more clearly understood through this body of research, questions still remain, particularly with regard to the long-term, low frequency behavior of the monsoon, and how it is related to both winter precipitation and natural variability of ocean-atmosphere circulation at decadal and longer time scales. Key questions about how the NAM may respond to global climate change also remain unanswered.

Instrumental records for the monsoon region are limited to the past century, at best, and are inadequate for investigating the long-term natural variability of the NAM, which will likely underlie future changes due to anthropogenic climate change. Proxy data can be used to reconstruct records of past climate variability, and great promise exists for using paleoclimatic data from sources such as tree rings and speleothems to document monsoon variability. Several key studies have indicated the potential for tree- ring based reconstructions of monsoon precipitation in Mexico and the Southwest, while other studies have pioneered new sampling and laboratory techniques to accentuate the summer moisture signal in trees.

The goal of this National Science Foundation-funded project is to design and apply these new techniques to the first tree-ring network, utilizing both existing and new collections, to specifically target long-term NAM variability in the US Southwest. We will use partial-width indices (earlywood and latewood) from this network, in combination with stable-carbon isotope measurements from tree rings, to reconstruct and examine the long-term variability of monsoon season precipitation, its relation with winter precipitation and widespread droughts in western North America, and to ocean-atmosphere circulation in the Pacific Ocean. The consistency of downscaled general circulation model (GCM) simulations with paleoclimatic evidence of NAM variability will also be examined.