Earlier this year, scientists reported mysterious waves appearing in the skies above Antarctica. These enormous ripples occur in the stratosphere and above, like mountainous swells of air that rise and fall over a period of hours.
A new study by Godin and Zabotin suggests an unlikely cause: vibrations in the massive ice shelf below, which buzzes at the same frequency. If true, the finding may also help scientists better understand the ice shelf itself.
That frozen slab, the Ross Ice Shelf, is the size of France and is several hundred meters thick. It bobs, very slowly, atop the Ross Sea, where the Antarctic ice sheet flows off the continent and out across the water for hundreds of kilometers. The atmospheric waves reported earlier this year were observed at McMurdo Station, the U.S. Antarctic base, situated on an island poking through the ice shelf.
Revealed through lidar observations, these waves propagate through the atmosphere, from altitudes of 30 up to 110 kilometers. As they move upward, they cause rivers of air 20 kilometers high to shift back and forth every 3–10 hours. At first, their origins were unclear, but they seemed unique to Antarctica, and they were seen during every lidar observation for 5 years at McMurdo but have never been observed at lower latitudes.
But the authors show that answer may be found in the Ross Ice Shelf below. As it sits on the water, buffeted by wind and waves, oscillations called resonant vibrations course through the ice like the faint ring of a bell lightly rapped. Although they’re barely perceptible and take hours to pass, the sheer size of the slab and the pressure it generates as it pulses against the air above it seem to create waves that propagate into the atmosphere above.
To test this hypothesis, the authors created some simple simulations, treating the ice shelf as a rectangular slab and modeling its interactions with the sea below it and the air above. With this, they found that the ice shelf’s natural resonant frequency and its main harmonics happen to fall neatly within the 3- to 10-hour-period range, the same as the atmospheric waves.
Another piece of evidence comes from the effects of Earth’s rotation, which are amplified close to the South Pole. The simulations showed that there is a natural limit to the frequencies of waves that can persist in the region: If the oscillations are too slow, the waves get swept away by the Coriolis force. And, as it turns out, that upper limit just happens to be around 10 hours, the same as in the lidar observations.
Next, the authors plan to create a more accurate simulation with actual terrain data of the ice shelf and the seafloor. With this improved model, they hope to use the atmospheric waves to probe the dynamics of the ice shelf itself. The properties of its deep, low-frequency resonances are hard to measure directly, but they could be revealed by analyzing the atmospheric waves that they generate. (Journal of Geophysical Research: Space Physics, doi:10.1002/2016JA023226, 2016)
—Mark Zastrow, Freelance Writer