Rainy weather patterns in the southwestern United States are becoming less frequent. It's a trend that could signify the region's transition to a drier climate state — one characterised by megadroughts and dramatic changes to the environment. Researchers from the National Center for Atmospheric Research (NCAR) analysed 35 years (1979-2014) of meteorological data to spot common weather patterns to determine if these patterns were becoming more or less frequent over time. Led by Andreas Prein, they were looking for common drivers of precipitation, including arrangements of high and low pressure systems. The team discovered that these critical weather systems are decreasing in frequency in the southwest, suggesting the region is drifting into a drier state. The results of their work now appear in Geophysical Research Letters.
This map shows the overall changes in precipitation across the US that can be attributed to changes in weather system frequency. The grey dots represent statistically significant results. Via Andreas Prein, NCAR
"A normal year in the Southwest is now drier than it once was," said Prein in AtmosNews, a publication of the NCAR. "If you have a drought nowadays, it will be more severe because our base state is drier."
The Southwest is already the most arid region in the US, but the prospect of prolonged droughts — even decades-long megadroughts (some lasting upwards of 35 years) — is a significant cause for concern. Last year, researchers from Cornell used climate models to predict that the chances of the southwestern US experiencing a megadrought in the coming decades is at least 50 per cent. Recent droughts in California and Oklahoma may actually be precursors. The new NCAR study adds credence to the models.
Lack of precipitation and prolonged droughts can cause any number of adverse effects, including bushfires, dust storms and desertification. It places pressure on afflicted regions to store and supply water, and could even spark population migrations.
The NCAR researchers isolated three patterns that deliver precipitation to the Southwest, all of which involve low pressure in the North Pacific off the coast of Washington. These low-pressure systems are not forming as often as they used to, replaced instead by persistent high pressure systems that have been linked to the severe drought in California. These observations corroborate climate models, which predict that a belt of higher average pressure near the equator will migrate north. The resulting sinking air causes drier conditions and inhibits the development of rain-producing systems.
At the same time, the researchers detected an opposite, though smaller, effect in the Northeast, where precipitation appears to be increasing.
Many of the world's deserts are located in regions of sinking air, typically around 30 degrees latitude on either side of the equator. The Sahara is one such example. Models show that these zones are poised to move towards the poles, producing a drier southwest. However, while climate change is a possible explanation, the researchers make no such connection. Looking forward, they're planning a follow-up study to examine that very possibility.
Top image: Earth to Sky Calculus