Tree rings and monsoon season rainfall variability in the Southwestern US: Strategies for sensitive chronology development

The climate of southwestern North America is characterized by two seasonally discrete precipitation regimes. Frontal storms deliver cool season precipitation whereas warm season rainfall is associated with the North American Monsoon. For the cool season, tree-ring reconstructions have revealed a detailed history of moisture variability across the region for the last 500-1000 years. These reconstructions have been developed primarily from annual ring-width chronologies, which are not typically useful for reconstructing the warm season rainfall variability. However, using chronologies developed from the intra-annual, anatomically distinguishable components of the growth ring, commonly referred to as earlywood (EW) and latewood (LW), great potential exists for reconstructing monsoon-season moisture variability in the region. Towards this end, we are currently developing a network of EW and corresponding LW chronologies from moisture-sensitive tree-ring sites around the southwestern United States. We have updated over forty tree-ring collections to 2009 and are measuring EW and LW width of the modern and archived specimens. Here we report results from an experiment designed to refine and improve the published methods for developing LW chronologies that are sensitive to monsoon-season precipitation. Challenges we face include 1) the evaluation of competing techniques for the treatment of intra-annual latewood density variations (i.e. “false” rings), 2) the assessment of sample signal strength in LW-width time series, particularly since LW-width variability often diminishes substantially with increasing biological age, 3) the improvement of statistical techniques for removing the covariance between EW and LW, a step proven to be necessary for developing chronologies sensitive to warm season precipitation in the region.