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Lenticular clouds over the Andes’s Torres del Paine are a sign of gravity waves.
Anton Petrus/GETTY IMAGES
By Eric HandMay. 12, 2020 , 5:00 PM
The Southern Ocean is famously stormy, home to waves taller than telephone poles. Yet 50 kilometers overhead, the weather is just as tempestuous, if less obviously so. Powerful waves in the air break and crash, dumping energy into the stratosphere and disrupting winds that help control the climate.
For about 2 decades, researchers have known that a region near 60° South, along the Drake Passage between the tip of South America and Antarctica, is the planet’s hot spot for these so-called gravity waves. They have long suspected that the waves (not to be confused with the gravitational waves rippling through space) are launched by the mountains of the southern Andes and the Antarctic Peninsula, which jut thousands of meters into westerly winds. But puzzlingly, the hot spot lies hundreds of kilometers away from the mountains. Now, a high-altitude aircraft has traced newborn gravity waves rising from the mountains and bending, or refracting, toward that hot spot.
The phenomenon helps explain why climate models predict unrealistically cold temperatures over the South Pole. “It’s really exciting,” says Tracy Moffat-Griffin, an atmospheric physicist with the British Antarctic Survey. “If we can get [refraction] represented in the models that will go a long way towards correcting the cold pole problem.” That, in turn, could improve forecasts of seasonal weather and of the ozone hole that develops over Antarctica in the southern spring.
In six flights in 2019, researchers gathered evidence that waves generated by the mountains are drawn to the powerful Antarctic polar vortex at 60° South. This collar of high-altitude winds swirls around the bottom of the world, penning in frigid air. Other measurements had already hinted at the refraction, which weakens the vortex and warms the Antarctic. But, “It’s fair to say this is the first experimental proof,” says Markus Rapp, director of Germany’s Institute for Atmospheric Physics and a leader of the €5 million SouthTRAC campaign, which presented results last week at the virtual meeting of the European Geosciences Union.
Around the world, gravity waves often arise when winds shove air over a mountain range, although storms and jet streams can also touch them off. In each case, a parcel of air gets pushed up, and gravity pulls it down. It overshoots and bobs back up. When confined by overhead winds, the train of undulating air parcels plows ahead horizontally, leading to so-called lee waves that can shake up commercial flights.
Gravity waves also reach upward: The pistoning air parcel can push on the parcel above it, and so on and so forth. The waves grow as they rise through thinner air; ultimately, says Yale University atmospheric scientist Ronald Smith, “They get so large that they kill themselves off and crash.” The turbulence unleashed in the breakups can be so strong that it temporarily reverses the direction of prevailing winds.
A vortex of winds draws gravity waves from mountains into a hot spot at 60° South. The map shows energy in winter at an altitude of 30 kilometers, tracked from space from 2006 to 2012.
N.P. Hindley et al., Atmos. Chem. Phys., 15, 7797, (2015) Adapted by N. Cary/Science
Some gravity waves crash in the upper stratosphere, about 50 kilometers up, but most keep rising into the mesosphere, where they can be seen causing ripples in the fluorescent glow of air molecules 90 kilometers up. They are even thought to create giant bubbles of plasma in the ionosphere more than 200 kilometers up, halfway to the International Space Station, causing trouble for radio communications. As one of the few vertical modes of energy transport, “they’re the glue that holds the atmosphere together,” says Dave Fritts, an atmospheric physicist at GATS, a satellite instrumentation company. “They play critical roles in accounting for our weather and climate.”
Yet climate modelers struggle to take the waves into account. That’s not only because their sources are so variable, but also because their wavelengths of tens of kilometers can be smaller than the size of the grids that modelers break up Earth’s atmosphere into. Instead, modelers rely on “parameterizations”—formulas that approximate the behavior of the gravity waves. Weather modelers know the approximations work pretty well, says Annelize van Niekerk, an atmospheric modeler at the U.K. Met Office, because when they turn them off, forecasts go haywire. Without them, the quality of the Met Office 5-day forecast would suffer, she says, and seasonal forecasts would become “very, very bad.”
But the parameterizations aren’t good enough to solve the Antarctic cold pole problem. Climate models generate plenty of gravity waves directly over the Andes and the Antarctic Peninsula, but not in the observed hot spot at 60° South, where the fusillade of waves attacks the polar vortex during the Antarctic winter. By summer, the vortex breaks down, and warmer air from higher latitudes mixes in. (This also ends the seasonal ozone hole, which requires cold temperatures for ozone-destroying reactions to occur.)
The SouthTRAC flights showed how the gravity waves get to the hot spot. Taking off from Rio Grande in Argentina, a modified Gulfstream jet flew up to 15 kilometers high, with a belly-mounted infrared instrument to see gravity waves rising from below, and a laser shooting up to trace them higher. The instruments detected the waves from temperature fluctuations they cause, and built a 3D map showing how the waves marched through the stratosphere toward the vortex.
Prior missions had seen hints of the refraction effect, but not as clearly. In December 2019, Nick Mitchell, an atmospheric physicist at the University of Bath, and his colleagues reported using an infrared instrument on NASA’s Aqua satellite to trace the rising waves from above. Moffat-Griffin is leading a related effort to study them from below using radars and weather balloons. But those observations do not show how the refraction evolves over time. “The advantage of the aircraft, of course, is it can loiter over the mountains,” Mitchell says.
The resulting images of the waves show the polar vortex “is almost like a duct dragging them all in,” Moffat-Griffin says. But harnessing that picture to improve climate models won’t be easy. Accounting for refraction would bog down existing models, van Niekerk says. “We wouldn’t be able to run climate models as long.” Until the arrival of supercomputers that can simulate gravity waves without approximations, the cold pole problem might just be left out in the cold.
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