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Violent storms may shatter sea ice

Tall waves’ impact on frozen ocean hints at trouble in tempestuous future

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1:41pm, May 28, 2014

FRAGILE ICE  Strong waves around Antarctica, unleashed by distant ocean storms, may break up packs of sea ice critical to global climate.

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Towering waves that rise from cyclones can pummel the frigid waters around Antarctica, potentially wrecking sea ice critical to maintaining global climate. Because researchers predict climate change will bring more and stronger storms in the future, the thrashing swells could help ferry in an ice-free future.

Around Antarctica, sea ice is forming in some places and disappearing in others, says sea ice researcher Alison Kohout of the National Institute of Water and Atmospheric Research in Christchurch, New Zealand. Because the ice reflects the sun’s rays, thereby shielding Earth from solar heat, and because the ice also insulates the ocean below, the frozen rafts influence global temperatures, storms and ocean circulation. But, Kohout says, scientists don’t know enough about sea ice to predict its changes.

With colleagues, Kohout collected data that suggest that waves flung from ocean storms may be particularly damaging to sea ice. By collecting a ship's observations of ice thickness around Antarctica and wave height measurements from five ice-bound sensors off the coast of the southernmost continent, the researchers estimated wave energy. Using those numbers, the scientists calculated that tall waves fueled by distant cyclones could plow through frozen waters, packing enough energy to break ice for hundreds of kilometers. Their results appear in the May 29 Nature.

The waves, Kohout says, “can travel further than previous theory expected.” Scientists thought that waves diminish exponentially as they slam into floating ice. In other words, ice closer to the continent experiences a small fraction of the wave’s original ramming power. But the new data suggest that storm-generated waves taller than 3 meters weaken linearly as they move across ice, preserving their smashing power for much greater distances.

“The fact that storms have an effect on ice breakup has been known for a long time,” says climate scientist Claire Parkinson of NASA in Greenbelt, Md. But she says the finding that these larger waves could punch deep into ice fields is important. “Storms could have a bigger impact on the ice cover than had been thought,” she says. Many scientists expect global warming to boost cyclone activity, Parkinson adds, suggesting that ice-breaking waves will also increase and potentially crush sections of ice.

But ice-breaking waves need more study, says sea ice researcher Julienne Stroeve of the National Snow and Ice Data Center in Boulder, Colo. The thickness of the chunks of ice and how closely they’re packed together will influence how much damage waves can cause to them, she says. However, she says, the study focused on young, one-year-old ice 0.5 to 1.0 meters thick, which is common on the edges of sea ice. So she adds, the results probably indicate how strong waves will affect sea ice in the future. 

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