Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2010 Synoptic circulation of the North American Monsoon System and precipitation within the Lower Colorado River Basin Jessie Frances McCann Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Social and Behavioral Sciences Commons Recommended Citation McCann, Jessie Frances, "Synoptic circulation of the North American Monsoon System and precipitation within the Lower Colorado River Basin" (2010). LSU Master's Theses. 1296. https://digitalcommons.lsu.edu/gradschool_theses/1296 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. SYNOPTIC CIRCULATION OF THE NORTH AMERICAN MONSOON SYSTEM AND PRECIPITATION WITHIN THE LOWER COLORADO RIVER BASIN A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural Mechanical College In partial fulfillment of the Requirements for the degree of Master of Science in The Department of Geography and Anthropology by Jessie McCann B.S., Mississippi State University May 2010 ACKNOWLEDGEMENTS First, I would like to thank my committee members Dr. Barry D. Keim and Dr. Jeremy E. Diem, and specifically my advisor Dr. David P. Brown, for all their assistance during my thesis research. Without all the helpful advice, willing assistance, and instruction I would not have been able to complete this study. I would also like to show my gratitude to my officemates for their tremendous assistance to me throughout my classes, projects, and thesis research. Specifically, I acknowledge Anna Trevino for all of her Excel assistance, Matlab help, paper revisions, coffee breaks, etc. I could not have made it through these last two years without her help. Also, I would like to thank Katie Lavigne for all her technical assistance and support throughout my research. The Southern Regional Climate Center and Department of Geography and Anthropology at LSU provided me with the support and funding to complete my thesis. Lastly, I want to sincerely thank my parents, grandfather, and Marc who have supported me throughout my education, and always assisted me in any way they possibly could. ii Table of Contents ACKNOWLEDGEMENTS ....................................................................................................................ii ABSTRACT ......................................................................................................................................... v CHAPTER 1 – INTRODUCTION ......................................................................................................... 1 1.1 Introduction and overview of studies of the North American Monsoon System ................. 1 1.2 NAMS Seasonal Development and Processes ....................................................................... 4 1.3 Teleconnections .................................................................................................................... 6 1.4 Geography of the Lower Colorado River Basin ..................................................................... 7 1.5 NAMS impacts and vulnerability in the LCRB ........................................................................ 9 CHAPTER 2‐ RESEARCH QUESTIONS AND THESIS STRUCTURE ..................................................... 11 2.1 Research Questions and Objectives .................................................................................... 11 2.2 Significance of Research ...................................................................................................... 12 2.3 Structure of Thesis .............................................................................................................. 13 CHAPTER 3‐ SYNOPTIC TYPING OF CIRCULATION ANOMALIES ASSOCIATED WITH THE NORTH AMERICAN MONSOON, 1948‐2008 .............................................................................................. 14 3.1 Introduction ......................................................................................................................... 14 3.2 Data and Methods ............................................................................................................... 16 3.3 Results and Discussion ........................................................................................................ 19 3.3.1 Description of Synoptic Types ...................................................................................... 19 3.3.2 Trends in Type Frequencies .......................................................................................... 26 3.3.3 Intra‐seasonal (daily) type frequencies ........................................................................ 34 3.4 Conclusion ........................................................................................................................... 34 CHAPTER 4‐ PRECIPITATION TRENDS IN THE LOWER COLORADO RIVER BASIN AND LINKAGE TO SYNOPTIC‐SCALE CIRCULATION VARIABILITY ............................................................................... 38 4.1 Introduction ......................................................................................................................... 38 iii 4.2 Data and Methods ............................................................................................................... 39 4.3 Results and Discussion ........................................................................................................ 44 4.4 Conclusion ........................................................................................................................... 55 CHAPTER 5‐ CONCLUSIONS ........................................................................................................... 57 REFERENCES .................................................................................................................................. 62 VITA ............................................................................................................................................... 64 iv ABSTRACT 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. v CHAPTER 1 – INTRODUCTION 1.1 Introduction and overview of studies of the North American Monsoon System Every year, the southwestern United States and northwestern Mexico are affected by a summer phenomenon known as the North American Monsoon System (NAMS). The NAMS is also referred to as the southwestern United States monsoon, the Arizona monsoon, or the Mexican monsoon (Douglas et al. 1993, Adams & Comrie 1997). A monsoon is defined as a seasonal reversal of winds, and the shift of winds from the west (winter) to south (summer) denotes the beginning of the NAMS circulation (Douglas et al. 1993, Grantz et al. 2007). The NAMS begins in Mexico and moves northward into Arizona and New Mexico affecting these areas most strongly in July and August (Diem & Brown 2006, Ray et al. 2007) with some locations receiving between 50%‐70% of their annual rainfall totals during the monsoon season (Grantz et al. 2007). The largest overall precipitation amounts are received in July and August for the southwestern United States (Diem & Brown 2006). Studies of the NAMS date back around 100 years (Adams & Comrie 1997), but early analyses were limited to lack of widespread data across the region. Interest regarding the NAMS began with the observation of summer thunderstorms in the Arizona and New Mexico region, and the simultaneous lack of precipitation in southern California (Bryson & Lowrey 1955). The onset of this summertime precipitation was explained by Bryson and Lowrey (1955) as the reversal of wind direction in the middle troposphere. A shift in winds from west to east along with horizontal advection explained