This study will address variability in the North American monsoon associated with easterly waves and tropical disturbances. More than 25 years of observations and short-term forecasts from a regional model will be used. Previous work suggests that easterly waves and tropical storms have a significant influence on the location and amount of precipitation within the North American monsoon region.
Several key processes of the North American monsoon are not well represented in global or regional models, making it difficult to establish the physical mechanisms linking monsoon synoptic variability with more slowly varying processes associated with the tropical ocean-atmosphere system. The investigators will use short-term Weather Research and Forecasting (WRF) model simulations to determine how easterly waves and tropical disturbances affect monsoon precipitation. The model will be run at high enough resolution to avoid the use of convective parameterizations in the core monsoon region, thus eliminating one of the primary contributors to errors in regional model simulations of monsoon precipitation.
This work has the potential for improving monsoon forecasts and it addresses the scientific objectives of the North American Monsoon Experiment (NAME), a field campaign that took place in the summer of 2004 with the goals of determining the sources and limits of predictability of monsoon precipitation. This study builds on the work of the NAME community, including that of two of the principal investigators (PIs), as well as recent work on easterly waves including that of the third PI. The modeling aspect of the proposed study takes advantage of the University of Arizona WRF model, the model providing the highest resolution forecasts of the Southwest, and it will contribute to the establishment of a regional atmospheric model for emerging research applications and societal needs in the Southwest region.
Broader impacts of this project include potential improvements in monsoon forecasts that can help local governments in the Southwest to respond to flooding and weather-related hazards and to manage water resources for their growing populations. The project will contribute to multidisciplinary interactions on the University of Arizona campus, including the development of a new hydrometeorology program; it will support undergraduate research in the sciences and provide students with examples of the achievements of Hispanic and women scientists in a physical science and mentoring from such scientists. A graduate student and one or more undergraduate research projects will be supported.
The North American Monsoon (NAM) accounts for approximately 50-70% of the annual precipitation in northwest Mexico and the Southwest U.S. The NAM is important for both Mexico and the United States, as it impacts water supply and demand, severe weather, extreme heat, drought, and wildfire during the warm season. Development of improved capabilities to climatologically monitor the NAM and forecast severe weather events is a pressing need on both sides of the border, particularly as anthropogenic climate change is generally increasing the intensity of precipitation and increasing aridity in subtropical regions. This project aimed to better understand the key regional aspects of the NAM through a better understanding of the synoptic (~1-week, ~1000 km) variability within the NAM region. The key outcomes of this study are 1) an assessment of the primary modes of NAM sub-seasonal rainfall variability, 2) an improved understanding of the distinct roles of tropical and mid-latitude troughs in NAM rainfall events, and 3) an improved understanding of the origins and intraseasonal (~30-60 day) modulation of tropical troughs in the East Pacific. These results are based on the June-September 2002-2009 monsoon seasons. Our analysis suggests that the primary mode of NAM sub-seasonal (< 120 day) rainfall variability is associated with the passage of tropical storms near the mouth of the Gulf of California, which result in severe weather events throughout the NAM region. If these tropical storm events are eliminated from the data set, the first two primary modes of NAM rainfall variability reflect a lower tropospheric low-pressure disturbance of tropical origin and an upper tropospheric low-pressure disturbance of mid-latitude origin, respectively. Our results further suggest that tropical troughs in the East Pacific (first mode) are primarily associated with rainfall events in the southern most part of the NAM region in the Mexican states of Sinaloa and southern Sonora, while upper-level inverted troughs (second mode) are primarily associated with rainfall events in the northern portion of the domain including the Southwest U.S. and northern Sonora. Tropical troughs impact NAM rainfall events through enhancing moisture transport up the Gulf of California. This low-level moisture transport also plays a role in the second mode of NAM rainfall variability, supporting the idea that the phasing of lower and upper tropospheric low-pressure centers is important to their overall impact. The results of this analysis have significance for forecasting severe weather during the monsoon within both Mexico and the Southwest U.S. This study also explored the modulation of tropical easterly waves by the Madden-Julian oscillation (MJO), the primary mode of intraseasonal variability in the tropical East Pacific. Results of this aspect of our study suggest that the MJO convectively active phase enhances easterly wave activity and that this activity is directed along the west coast of Mexico and past the Gulf of California, bringing it closer to the NAM region. We hypothesize that this shift in the easterly wave tracks increases the opportunity for NAM activity associated with tropical troughs, which will be explored in an upcoming publication resulting from this work. Another interesting outcome of this aspect of the study is that the modulation of cyclogenesis drawn from an eight-year subset within 1990-2009 (the most reliable period for hurricane observations in the East Pacific) is not fully consistent with the full period. The absence of consistency across the epoch in the pattern of modulation supports our conclusion that the MJO only provides modest gains in medium range (2-4 week) predictability of cyclogenesis. This project supported two graduate students in atmospheric science and produced two master’s theses and three peer-reviewed journal articles. One student was awarded a Student Career Experience Program (SCEP) job at a local air force base as a result of her experience working on this project and has also entered the PhD program in Arid Lands Resource Sciences at the University of Arizona. The results have been discussed in graduate courses within the Department of Atmospheric Sciences as well as have informed the department’s annual monsoon discussions, which occur each summer during the monsoon season and are posted to the general public on our department website. Through these discussions and other regular communication between the UA Department of Atmospheric Sciences and the NWS Tucson office and Davis-Monthan Air Force Base 25th Weather Squadron, the results of this study also contribute to the improvement of civilian and military monsoon severe weather forecasts for southeast Arizona. We have additionally disseminated our results to graduate students at the Universidad Nacional Autónoma de México (UNAM) helping to train future forecasters in both countries. The UA continues to work with the Mexican Meteorological Service (SMN) regarding the forecasting of mesoscale convective organization and severe weather during the NAM under new projects.
