New research, based on two decades of data, shows that in the ten years since its onset in 2000, the Southwestern North America (SWNA) megadrought caused a 30 % of gravity wave activity in Earth’s upper atmosphere.
More than 30 years ago, Chester Gardner of the Department of Electrical and Computer Engineering at UIUC and Chiao-Yao She of the Department of Physics at Colorado State University teamed up to study the middle atmosphere of the Earth. Using sodium resonance laser radar (lidar), Gardner and She developed and demonstrated an important new technique for measuring temperature profiles in Earth’s upper atmosphere.
Subsequently, they were able to observe changes in upper atmosphere gravity waves at two locations (Albuquerque, New Mexico and Ft. Collins, Colorado) over a 20-year period. Their results, describing how the waves changed after the drought began, have now been published in Geophysical Research Letters.
In 1994, a team led by Gardner installed a lidar system on a large telescope at the Starfire Optical Range outside Albuquerque, NM at Kirtland Air Force Base. The lidar system measured temperature and winds in the upper atmosphere, using atomic sodium as the target species until the end of 2000.
The cosmic dust particles that are vaporized in the Earth’s atmosphere are the source of this atomic sodium. Using a laser beam, atomic sodium can be excited, causing it to glow. A ground-based telescope collects backscattered light from sodium fluorescence and computers process this information to derive profiles of sodium density, temperature and radial wind speed. His team at CSU made similar observations in Ft. Collins, CO, ultimately compiling a large dataset spanning 20 years from 1990 to 2010.
Finding changes in wave activity during the mega-drought was a happy by-product of other research. The researchers were studying how temperature and winds fluctuated in the upper atmosphere due to waves generated in the lower atmosphere and were surprised to find dramatic changes in gravity wave activity after the drought began.
“We never expected to make observations that would provide insight into how a drought might affect Earth’s upper atmosphere,” Gardner said.
After looking at Gardner’s data from Albuquerque and She’s data from Ft. Collins, what they found was a significant (~30%) reduction in wave activity after the onset of the drought. Changes in gravity wave activity may be related to reduced wave generation by tropospheric storms during the mega-drought and an altered geographic distribution of precipitation events in the western and midwestern states. -United. Simply put, less precipitation means fewer storms, so fewer waves are generated by storms.
What is a mega-drought?
A mega-drought is a prolonged period of drought that lasts at least two decades. The southwestern North America mega-drought began in 2000 and is still persisting 22 years later with no end in sight.
Gardner and She say the SWNA mega-drought is significant not only because of its duration, but also because of the size of the geographic region affected, which stretches from northern Mexico to the northern borders of Oregon and the Wyoming, and from the Pacific Coast to the eastern borders of Wyoming, Colorado, and New Mexico.
This particular mega-drought is believed to be the region’s driest 22-year period since 800 CE. It has been argued that human-induced warming of the Earth’s atmosphere may have contributed more than 40% of the severity of the mega-drought.
Gravity waves (not gravitational waves)
gravity waves– not to be confused with gravitational waves associated with cosmological phenomena such as black holes – occur at the interface between two media when gravity or buoyancy attempts to restore equilibrium. Gardner explains that this effect is like dropping a rock into a pond of water. The stone moves the surface of the pond, pushing the water down; the buoyancy returns the water, which then oscillates, generating a ring of waves that propagates outward. These are gravity waves.
One of the ways waves are generated in the lower atmosphere is through thunderstorm convection, which triggers gravity waves by causing vertical motion that results in oscillation. Waves can also be generated by air flowing over topographic features like mountains, which push air upwards.
While waves in a pond propagate only along the air-water interface, gravity waves in the atmosphere propagate in all directions. These waves drive the global circulation of the upper atmosphere and can affect space weather and satellite orbits.
Gardner and She say the work is important because it demonstrates that regional changes in the lower atmosphere can also impact the upper atmosphere. They believe this is the first time a regional climate effect has been observed in the upper atmosphere.
The results can be used to test and validate the next generation of high-resolution regional atmospheric computer models that can resolve small-scale waves observed by lidar.
Gardner explains: “Current atmospheric models cannot see waves because the resolution, even on the fastest computer models, is not sufficient to see the scale of these waves. Now, scientists are developing very high-resolution regional models so models can see larger-scale waves. Our observations can be used to test the accuracy of these models and to validate them.
Chester S. Gardner et al, Contemporary Southwestern North America Megadrought Signature in Mesopausal Region Wave Activity, Geophysical Research Letters (2022). DOI: 10.1029/2022GL100569
Provided by the University of Illinois Grainger College of Engineering
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