Arizona and New Mexico receive up to half of their annual rainfall during the summer monsoon (Figure 1). The monsoon season suppresses much of the hot summer temperatures, replenishes water resources, and nourishes the vegetation. The monsoon arrives with much flash and fanfare, with its trademark thunderstorms and flooded streets in southwestern cities. Monsoon rainfall events tend to be short and spotty, with intense, local storms drenching some neighborhoods but not others. The water the storms bring quickly flows off the landscape into streets and rivers, with much of the remnant moisture evaporating in the summer sun.
The monsoon is driven by the sun heating up the land and the Pacific Ocean at different rates, with land surfaces warming more quickly than the ocean. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, the winds shift to fill in the vacuum. With this shift in the winds, the monsoon begins in northern Mexico in May. The moisture-laden monsoon air travels north to Arizona and New Mexico, encouraged by the pressure difference between the hot, parched southwestern air and the cooler Mexican air. In 2008, National Weather Service officials decided to consider June 15–September 30 as the U.S. Southwest monsoon season in Arizona, although the thunderstorms that bring the rain may form in different times and places across the region.
The monsoon’s driving rains are most dramatic in southeast Arizona and in western New Mexico, tapering off in Phoenix and Yuma, AZ (Figure 2). In addition to being variable in space, some years produce weak monsoons, while others provide ample rain. There are no evident trends in annual monsoon strength, making it difficult to predict. This variable tendency has been consistent over the past 100 years when record keeping generally began.
A number of factors affect the monsoon. El Niño–Southern Oscillation (ENSO) can have a strong impact, with El Niño events often bringing more moisture up from the Gulf of Mexico. Changes in tropical Pacific convection and temperature patterns in the Midwest are also factors that can influence the summer monsoon.
Along with the relief that the summer monsoon provides to the region, it also brings flash floods, dust storms, strong winds, lightening, and dangerous fires that can harm people and property. In the Southwest, lightening has ignited more than 2,300 fires annually since 2001, burning approximately 277,000 acres per year. For much of the Southwest, the monsoon season sparks the fire season but also can bring the fire season to a close after the first few weeks of rainfall.
The monsoon brings rains that can help reverse the downward draw from reservoirs; it is also important to farmers who depend on natural rainfall. The monsoon fuels the growth of summer grasses, which can make or break southwestern ranchers struggling to eke out a living in harsh desert lands. Southwest summers can be especially harsh in years when the monsoon falters, leaving shriveled plants and baked soil in its wake.
Figure 1. Percent of average annual precipitation that falls in the West between July and August. Figure courtesy of Western Regional Climate Center, based on PRISM data produced at Oregon State University.