| Summary: | <p> In southwestern United States, the North American monsoon (NAM) is the main driver of severe weather in the Southwest. How the monsoon has behaved in the past and how it will change in the future is a question of importance for natural resource management and infrastructural planning. In this dissertation, I present the results of three studies that have investigated NAM variability and change from the perspective of paleoclimate records, future climate change projections, and simulation of the low-frequency variability with the longest retrospective atmospheric reanalysis. </p><p> In the first study, a monsoon-sensitive network of tree-ring chronologies is evaluated within its ability to reproduce NAM variability during the past four centuries. The tree-ring chronologies can reasonable characterizes the dominant modes of NAM climate variability and reveal low-frequency climate variability at decadal and longer timescales that is beyond the ability of the instrumental record to temporally well resolve. This low-frequency climate variability seems to coincide with the occurrence of multiyear persistent droughts. </p><p> In the second study, we consider the modes of climate variability to assess the degree of physical uncertainty in climate change projections models used in the North American Regional Climate Change Assessment Program (NARCCAP). NARCCAP models are evaluated mainly on their ability to represent warm season driven by quasi-stationary Rossby wave trains and El Niño Southern Oscillation – Pacific Decadal Variability (ENSO-PDV). Only one out of eight NARCCAP models has a reasonable representation of the seasonal cycle of monsoon precipitation and ENSO-driven variability in both the 20<sup> th</sup> and 21<sup>st</sup> centuries. No decadal variability was observed in any of the NARCCAP models. </p><p> In the third study, the low-frequency drought signal found with tree-ring chronologies is further explored within the framework of a regional climate modeling. The Twentieth-Century Reanalysis is dynamically downscaled (DD-20CR) and its statistic analysis suggests that low-frequency drought signal in the Southwest is driven by atmospheric circulation changes on global to continental scales that affect precipitation in Central American as well. Low-frequency climate variability is therefore likely responsible for the multiyear persistent droughts in the last four centuries, as independently evaluated from the tree-ring monsoon-sensitive network.</p>
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