Greenhouse gas-induced heating inhibits early monsoon precipitation
By Bob Berwyn
SUMMIT COUNTY —Global warming result in a significant shift of the North American monsoon, with less rain during the early part of the season, in June and July, and more rain later in the summer and early autumn. The trend toward a later start to summer precipitation has already started, but will become more pronounced — and easier to distinguish from the background “noise” of natural variability — during the next few decades, according to researchers with NASA and Columbia University’s Lamont-Doherty Earth Observatory.
“We expect that increased greenhouse gases will make the atmosphere more stable and more difficult for precipitation to occur … When the warming is strong enough, it effectively delays the start of the monsoon,” said NASA researcher Benjamin Cook. “One way to overcome that is when the air near the surface is really moist. That’s what happens at the end of the monsoon season. At that point, it leads to an increase in rainfall,” Cook said, explaining that the study points to big change in the total amount of monsoon precipitation, but that the change in timing is still likely to have significant ecological societal impacts.
A second factor driving the change in timing is less surface moisture at the local level resulting from reduced evapotranspiration.
The North American monsoon may not be as pronounced as its famed Asian cousin, but it still dominates the seasonal cycle of precipitation over northwestern Mexico, southern Arizona and large parts of New Mexico and Texas, accounting for up to 70 percent of the annual precipitation in the core of the region.
The strength and timing of the monsoon is crucial to agriculture and rangeland management, for soil and groundwater recharge, and also triggers the growing season for native vegetation. Any changes in timing can also potentially affect wildfire behavior. Last summer’s destructive High Park Fire, west of Fort Collins, was extinguished when the monsoon rains started.
“Because there’s such a pronounced long dry season, the start of the monsoon is so important … at the end of the monsoon doesn’t matter how much wetter it gets,” Cook said.
The study focused on the core area of the monsoon, which doesn’t include Colorado. Cook said it’s more difficult to pinpoint the seasonal shifts farther north because there are other factors in play, including non-monsoonal precipitation, as well as local mountain topography, which makes it more challenging to apply global-level climate models.
Other studies have found similar impacts to monsoon timing in different parts of the world. The biggest changes in the North American monsoon are expected in the southern end of the core region and along the Pacific Coast of Mexico, where large precipitation reductions start in June and intensify in July. During the August peak of the monsoon, the models show drying in the western part of the region, but increases in the central area.
The research lends support to the idea of worsening drought conditions across the southwestern and southern plains, with a trend toward drying soils and less available moisture for evapotranspiration beginning early in the spring, in March and into April. During the heart of the monsoon, in June and July, those drying trends move north and east into Arizona, New Mexico and Texas.