The contribution of eastern North Pacific tropical cyclones to the warn season rainfall climatology of the southwestern United States

Unlike the much larger and stronger Asian Monsoon, no single rainfall mechanism has been found to dominate the North American monsoon system. Surges of low-level moisture from the Gulf of California into the southwestern United States account for a significant part of the intraseasonal variation of monsoon rainfall. These surges have been found to be associated with the passage of tropical easterly waves near the southern end of the Gulf of California. As the eastern North Pacific is climatologically the most active basin for tropical cyclone development, the question is raised whether eastern North Pacific tropical cyclones also play a significant role in the summer rainfall climatology of the southwestern United States. The present study seeks to address this question using high resolution, daily rainfall and best track tropical cyclone data for the summer months of the years 1958 to 2003.

Forty tropical cyclones and their remnants were found to bring rainfall to the southwest United States in the 46 years studied, representing just 7% of the total number of tropical cyclones that formed within the eastern Pacific. The first two weeks of September were the most common time for tropical cyclone rainfall in the monsoon region. Averaged over all tropical cyclone rainfall events, ~15% of the monsoon seasonal rainfall is contributed by tropical cyclones, with individual storms accounting for as much as 95% of the summer rainfall experienced in the region. The distribution of rainfall in these events reveals two dominant tracks of storms from the eastern Pacific into the monsoon region, one directly north into southern California and the other cutting northeastward from southwestern Arizona through extreme northwestern New Mexico and into southwestern Colorado.

Preliminary results indicate that tropical cyclones are twice as likely to influence the southwest in years that are climatologically dry, underscoring their importance to the summer rainfall in the region. Tropical cyclone rainfall is also twice as likely to occur during La Niña years. The large-scale flow patterns over the western United States and eastern Pacific associated with these signals are currently being studied and will be explored in detail in the talk.