Date of Award:

5-2017

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Jiming Jin

Committee

Jiming Jin

Committee

Lawrence Hipps

Committee

Patrick Belmont

Committee

Scott Jones

Committee

Bruce Bugbee

Abstract

The North American Monsoon (NAM) is a dominant climate feature across the southwestern United States and Mexico during the summertime. Rainfall from the NAM elicits a significant response from the regional ecology as well as supports the socioeconomic well-being of both rural and urban populations. Due to the large variability in the yearly intensity and location of where the NAM rainfall occurs, the region is highly susceptible to prolonged and exceptional periods of drought; compounded by significant population growth projections through the 21st century. Reliable prediction for the NAM is highly beneficial on timescales ranging from shortterm weather forecasting to longer climate timescales spanning seasonal, decadal, or even century intervals. This project aims to better understand the NAM processes and mechanisms in order to improve the reliability of NAM predictability. To improve seasonal predictability, we examined biases across a network of western United States snowpack observations used to infer the NAM onset timing as well as the seasonal intensity. Furthermore, by using a high-resolution regional climate model (RCM), we improved upon current future projections of the NAM, which are based on global climate models (GCM). Whereas these coarse-resolution GCMs have proven to be inappropriate at resolving many of the processes controlling the NAM, the increased model resolution of the RCM (leading to a better representation of surface features and the land-atmosphere interactions represented in the model) yielded an improved historical evaluation period. Our RCM simulations suggest the overall future drying trend predicted by GCMs likely doesn’t factor in important processes responsible for thunderstorm activity. By including these effects, our RCM future projections indicate the average state of the future NAM will (1) onset earlier in the year, (2) bring greater total seasonal rainfall and, (3) contain more extreme daily rainfall amounts. However, despite the suggestion that average rainfall from the NAM will increase, analysis of year-to-year variability indicates a greater propensity for severe and frequent drought conditions during below-average years, suggesting the continued need to develop a more sustainable approach to long-term water resources across the region.

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