‘A normal year in the Southwest is now drier than it once was’
A subtle long-term shift in atmospheric patterns driven by global warming could lead to longer and more intense droughts in the southwestern U.S. and other semi-arid regions. Most climate models suggest that that a belt of higher average pressure that now sits closer to the equator will move north. This high-pressure belt is created as air that rises over the equator moves poleward and then descends back toward the surface.
That shift may already be affecting the climate of the Southwest, as moisture-bearing weather patterns have become more rare in the region, according to a new study. Previous research has suggested that the region’s forests and fish and birds are in big trouble. In Australia, researchers are nearly certain that global warming was a factor in a record-breaking 2013 heatwave. A federal climate assessment released in 2013 also identified similar concerns for the Southwest.
“A normal year in the Southwest is now drier than it once was,” said Andreas Prein, a postdoctoral researcher at the National Center for Atmospheric Research who led the study. “If you have a drought nowadays, it will be more severe because our base state is drier.”
Linking model predictions to changes on the ground is challenging. But the new study, published in the journal Geophysical Research Letters, a publication of the American Geophysical Union, grappled with the root cause of current drying in the Southwest to better understand how it might be connected to a warming climate.
The researchers looked at 35 years of data to identify common weather patterns that determine where it’s likely to be sunny and clear or cloudy and wet, among other things.
“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.
“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,” said Anjuli Bamzai, program director in the National Science Foundation’s Division of Atmospheric and Geospace Sciences, which funded the research.
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.
“Understanding how changing weather pattern frequencies may impact total precipitation across the U.S. is particularly relevant to water resource managers as they contend with issues such as droughts and floods, and plan future infrastructure to store and disperse water,” said NCAR scientist Mari Tye, a co-author of the study.
The three patterns that tend to bring the most wet weather to the Southwest all involve low pressure centered in the North Pacific just off the coast of Washington, typically during the winter. Between 1979 and 2014, such low-pressure systems formed less and less often. The associated persistent high pressure in that area over recent years is a main driver of the devastating California drought.
This shift toward higher pressure in the North Pacific is consistent with the climate models projecting a northward migration of the subtropical high pressure belt — a zone created when air rising over the equator moves poleward and then descends back toward the surface. The sinking air causes generally drier conditions over the region and inhibits the development of rain-producing systems.
Many of the world’s deserts, including the Sahara, are found in such regions of sinking air, which typically lie around 30 degrees latitude on either side of the equator. Climate models project that these zones will move further poleward. The result is a generally drier Southwest.
While climate change is a plausible explanation for the change in frequency, the authors caution that the study does not prove a connection. To examine this potential connection further, they are studying climate model data for evidence of similar changes in future weather pattern frequencies.
“As temperatures increase, the ground becomes drier and the transition into drought happens more rapidly,” said NCAR scientist Greg Holland, a co-author of the study. “In the Southwest the decreased frequency of rainfall events has further extended the period and intensity of these droughts.”