The North American Monsoon (NAM) is a prominent summertime feature over northwestern Mexico and the southwestern United States that significantly impacts humans and their property. The NAM is characterized by a distinct shift in winds to a more southerly direction below approximately 700 hPa as well as a sharp, marked increase in rainfall. This maximum in rainfall accounts for 60-80% of the annual precipitation in northwestern Mexico and nearly 40% of the yearly rainfall over the southwestern United States. Gulf surges, or coastally trapped disturbances that occur over the Gulf of California, are important mechanisms in supplying the necessary moisture for the monsoon. It has been hypothesized in previous studies that there is a relationship between gulf surges and the passage of a tropical easterly wave (TEW). The actual number of TEWs varies from year to year suggesting that they are responsible for producing some of the interannual variability in the moisture flux seen in the NAM.

The Fifth-Generation National Center for Atmospheric Research /Pennsylvania State University Mesoscale Model (MM5) is used to simulate the monsoon over several one month periods during July and August. Through harmonic analysis, waves with periods of approximately 3.5 to 7.5 days located south of 30ºN are removed from the boundary condition file in order to eliminate the presence of tropical easterly waves in the model simulations. The simulation without waves and a control run containing TEWs are explored to determine the impact of the waves on the NAM, and fields such as meridional moisture flux, rainfall totals, and surge occurrences are examined to resolve any similarities and differences between the model runs. Results suggest that the removal of TEWs produces substantial environmental changes in the NAM region. Influences of TEWs on gulf surges and their structure also is examined.