Extratropical control of monsoonal surges into the northern Great Basin

The northwestern fringe of the North American Monsoon (NAM) covering the Great Basin of the United States is characterized by highly variable intraseasonal monsoonal activity. Although typically dry during the summer months, sporadic moisture pulses associated with transient monsoonal surges interacting with mid-latitude disturbances yield low-risk high-impact events including lightning-ignited wildfires and flash floods. Mountainous terrain in the Northern Great Basin is extremely vulnerable to the dangers of monsoonal convection as the initial development typically occurs over high-altitude terrain due to differential heating. We have incorporated expert knowledge from four National Weather Service Offices in the Northern Great Basin along with 30 years (1980-2009) of data from the North American Regional Reanalysis (NARR) to answer the question: What mechanisms fuel monsoonal moisture surges into the Northern Great Basin?

In order to better understand the nature of these surges, an objective means of identifying surge events was done using precipitation and vertically-integrated moisture flux during the months of July-September. Preliminary results indicate that moisture surges are associated with either a progressive mid-latitude trough that infiltrates the west coast, or a strong four-corners ridge. Trough event signals reveal statistically significant moisture flux extending from the northern Gulf of California northward into southeast Idaho with positive flux anomalies on the order of 70-100 kg/m2/s. With ridge events, we see a similar trend but with a greater northward extension of moisture flux into northern Idaho with similar anomaly values as that of trough events. Additionally, 330K isentropic potential vorticity (IPV) was plotted for all 46 case studies in order to better understand if there exists a Rossby Wave Break signal (e.g., Postel and Hitchman 1999; Abatzoglou and Magnusdottir 2006) prior to the monsoonal outbreaks in the Northern Great Basin. While a definite answer cannot be concluded, there is a strong possibility that these extratropical wave breaks play a significant role in adjusting the subtropical dynamics which lead to enhanced moisture surges northward. Our results also suggest large interannual variability with a large number of these events occurring during 82/83 and 97/98 coincident with strong El Nino events.

In addition, a component of our analyses also shows proof of the interactions of a mid-latitude trough and the subtropical jetstream. These adjustments provided a convectively unstable environment that lead to widespread convection over the Gulf of Mexico during the North American Monsoon Experiment case study of 6 – 9 July, 2004. Deep convection over the Gulf of Mexico was observed during 10–13 July which provided mid-level moisture that ultimately was transported into the four-corners region of the U.S. during a monsoonal surge.

Ultimately, we would like the results of our work to be readily translatable to forecasters in order to improve the predictability of these events. Further research is needed but preliminary results are promising. Future work includes (1) understanding how easterly waves play a role in the enhancement of monsoonal surges into the Northern Great Basin, (2) establishing an algorithm to locate cut-offs in the eastern Pacific which significantly influence monsoonal surges during trough events, (3) understanding the role of intraseasonal to interannual climate variability and moisture surges and (4) applying what we have learned from observational analysis into a modeling environment using the OMEGA (Operational Multi-scale Environment model with Grid Adaptivity) (e.g., Bacon et al. 2000).