The changing nature of the North American monsoon in the Southwest

The North American monsoon is a unique climatological feature of the Southwestern United States, accounting for approximately 30 – 50% of the annual precipitation in this region from late June through the end of September. Monsoon precipitation is mostly from thunderstorms, that typically initiate over the highest terrain during the day and propagate into lower elevations in the late afternoon and evening hours. With favorable thermodynamic and dynamic conditions thunderstorms may grow into organized mesoscale convective systems, and these trigger much of the monsoon severe weather episodes in urban areas like Tucson and Phoenix. Some monsoon severe weather dangers unique to the Southwest include flash flooding, microbursts, dust storms or haboobs, and lightning. Wildfire initiation and spread is an additional concern, most acute just as the monsoon starts.

The nature of the North American monsoon is changing, in association with global climate change. Consistent with other areas of the United States and the world, atmospheric conditions for monsoon thunderstorms are becoming more thermodynamically favorable, with increased atmospheric instability and moisture. However, the dynamic triggering mechanisms that facilitate monsoon thunderstorms are becoming less frequent. As a result, monsoon precipitation is becoming less frequent but more extreme. The most profound increases in extreme precipitation are occurring in conjunction with increased intensity of mesoscale convective systems, and severity of their associated severe weather. The monsoon may also be becoming more extreme and variable from year to year, going from very dry to very wet over consecutive years. A less reliable and more extreme and variable monsoon in the future will likely have adverse impacts in the Southwest, possibly exceeding the current resiliency of natural and human systems.