Peak flow
The cover shows a lightning strike as a storm in the monsoon moves through the southeastern corner of Arizona. A band of intense rainfall extends for more than 1,000 kilometres along the west coast of Mexico into the southwestern United States during the summer months, constituting the North American monsoon. Monsoons are generally thought to result because land and the ocean are warmed differently by the Sun. This causes changes in air pressure, ultimately creating winds that draw cooler, moist air from the ocean, resulting in rainfall on the land. In this week’s issue, William Boos and Salvatore Pascale show that the North American monsoon is different. Here the monsoon is a result of winds being diverted by mountain ranges. Mexico’s Sierra Madre mountains deflect the jet stream, which in turn lifts up warm moist air and generates convective rainfall. The researchers note that although land heating does occur, its effects are insufficient to explain the monsoon, meaning that the North American monsoon should be thought of as mechanically forced.
https://www.nature.com/nature/volumes/599/issues/7886
