Understanding Dominant Tracks of Moisture for the North American Monsoon Region

Abstract:

Summer monsoon rains contribute more than half of the total annual rainfall in the semi-arid region of Southwest United States, also providing important input to river systems like the Colorado River. The North American Monsoon region or Southwest United States experiences great climatic variability on a range of spatial and temporal scales. This region has also been experiencing significant climate and hydroclimate changes over the last few years. Understanding the interannual variability of moisture delivery in this region will help in natural resources management such as water resources, ecology, etc.. In this study, we investigate the major sources of moisture and their interannual variability during the monsoon season. To this end we selected eight locations in the region from the states of Arizona, New Mexico, Colorado and Utah to cover the monsoon region of U.S and generated backward moisture trajectories for each wet day during the monsoon season (Jun–Sep) over the historical period 1964-2013, using the HYSPLIT model developed by NOAA. The tracks show clear source preferences. Gulf of Mexico is the dominant source for south eastern part of the domain, Gulf of California is dominant for the south western domain, a combination of these for regions in between and the Pacific provides the source for northern part of the domain. Decreasing trends in the frequency of the dominant moisture source events corresponds well with the decreasing trends in the rainfall over the domain. The frequencies when correlated with large scale climate variables indicate coherent patterns in the tropical Pacific and Atlantic. Furthermore, the population means for each source during El Nino and La Nina years were found to be significantly different. Since the moisture from the dominant sources is also responsible for causing extreme rainfall in this region, these trajectories will provide potential predictability of monsoon rainfall and extremes.