The weather patterns that typically bring moisture to the Southwest are becoming more rare, an indication that the region is sliding into the drier climate state predicted by global models, according to a new study.
"A normal year in the Southwest is now drier than it once was," said Andreas Prein, a researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, who led the study. "If you have a drought nowadays, it will be more severe because our base state is drier."
Climate models generally agree that human-caused climate change will push the southwestern United States to become drier. And in recent years, the region has been stricken by drought. But linking model predictions to changes on the ground is challenging.
For the study, the researchers analyzed 35 years' worth of data to identify common weather patterns -- arrangements of high and low pressure systems that determine where it's likely to be sunny and clear or cloudy and wet.
They identified a dozen patterns that are usual for the weather activity in the contiguous U.S., then looked to see whether those patterns were becoming more or less frequent.
"The weather types that are becoming more rare are the ones that bring a lot of rain to the southwestern United States," Prein said. "Because only a few weather patterns bring precipitation to the Southwest, those changes have a dramatic impact."
The Southwest is especially vulnerable to any additional drying. The region, already the most arid in the country, is home to a quickly growing population that is putting tremendous stress on its limited water resources.
"Prolonged drought has many adverse effects, so understanding regional precipitation trends is vital for the well-being of society," says Anjuli Bamzai, program director in the National Science Foundation (NSF) Division of Atmospheric and Geospace Sciences, which funded the research. "These researchers demonstrate that subtle shifts in large-scale weather patterns over the past three decades or so have been the dominant factor in precipitation trends in the southwestern United States."
The study also found an opposite, though smaller, effect in the Northeast, where some of the weather patterns that typically bring moisture to the region are increasing.