Assessment of North American Monsoon Variability in the Lower Santa Cruz River Basin Using Dynamically Downscaled CMIP5 Projections

The General Circulation Models (GCMs) within the Climate Model Inter-comparison Project 5 (CMIP5) have been dynamically downscaled using WRF in the Lower Santa Cruz River Basin (LSCRB), Arizona. The study area is also known as the Tucson Active Management Area. The downscaled data were then evaluated to select those models that best represent the North American Monsoon (NAM) for the historic period 1979-2005. The MPI, HadGem2 and GFDL models have the most pronounced monsoon signal so were used for this project. The Bureau of Reclamation expressed a desire to understand the evolution of the climate in the LSCRB, principally for current and future water resource management, specifically in two future time periods 2020-2049 and 2050-2079. Two specific climate metrics important for water resource management are the timing of the monsoon onset and the length of the preceding dry period. The definition for monsoon onset is 3 consecutive days with an average dew point temperature above 53F (National Weather Service). The beginning of the preceding dry period is given by >0.1 inches of precipitation total for two weeks averaged over the basin. These metrics are inter-compared using the dynamically downscaled GCMs using WRF and statistical downscaling of the GCMs, to highlight the difference between the two methodologies. Initial results show that the timing of monsoon onset is shifting earlier in the season, which counters results from Cook & Seager (2013). The domain considered here is a small portion in the northwest of the domain in Cook & Seager and fewer models/members are considered here. This emphasizes the value added when using dynamical downscaling to resolve local features that are perhaps missed or represented differently on larger scales.