Synoptic circulation of the North American Monsoon System and precipitation within the Lower Colorado River Basin

The North American Monsoon System (NAMS) is an important climate phenomenon that affects the southwestern United States during the warm season, most significantly in July and August. Climatic variability associated with the NAMS impacts a wide range of both physical and socio-economic systems in this region, with a broad range of stakeholders concentrated in the Lower Colorado River Basin (LCRB) that includes portions of five southwestern states. This study examines variations in synoptic circulation patterns associated with the NAMS with respect to the LCRB. Using a manual classification, daily 500-mb geopotential height patterns were classified for monsoon seasons (July 1-September 15) from 1948 -2008. Ten synoptic types were identified and their frequency and variability assessed. Rainfall patterns at four key sites within the LCRB were also examined for trends in both total seasonal rainfall and the frequency of extreme events, and average daily rainfall at each site was linked to each of the ten synoptic types. Results of this study show that the occurrence of synoptic-scale circulation patterns during the NAMS season has not been constant over time. In addition, the frequency of heavy precipitation events is increasing within the LCRB, and the occurrence of NAMS-related rainfall is not consistent among each of the synoptic types. The findings presented here may be used to reduce vulnerability within the LCRB, through an improved understanding of inter-annual and intra-annual variability of the NAMS that can inform forecasting applications and vulnerability assessments