Sunday, 26 April 2015

Polar ice

Giant Waves Quickly Destroy Arctic Ocean Ice and Ecosystems
The biggest waves seen in northern sea ice show how this vital cover can be crushed much faster than expected


21 April, 2015

The chance encounter of a Norwegian research vessel with the largest waves ever recorded amid floating packs of Arctic ice shows how such rollers could reroute shipping, damage oil platforms and threaten coastal communities with erosion. In a March report in Geophysical Research Letters scientists at the U.S. Naval Research Laboratory (NRL) describe how large waves can penetrate more deeply into ice cover and break it up faster and more completely than anyone had suspected.

Less ice means more open water to generate large waves—creating a feedback loop that could doom the ice cap. (This dangerous cycle is illustrated in “Waves of Destruction” in the May issue of Scientific American.)

Every year Aleksey Marchenko of The University Center in Svalbard, a Norwegian archipelago north of Scandinavia, leads students across the chilly waters of the Barents Sea to study the seasonal ice pack. Near its edges pack ice is composed of pieces loosely drifting on the water. Farther inside the pack there are kilometers-wide chunks that have been blown together into a nearly solid mass. Toughened ships like Marchenko’s converted Arctic fishing vessel, the RV Lance, can usually pick their way slowly through it. When the Lance left port in May 2010, Marchenko was expecting two or three days of leisurely fieldwork. In previous years the group had even camped out on large floes.

The Lance sailed east and around 80 kilometers from the small island of Hopen moored next to a large expanse of pack ice on May 2. Marchenko prepared to lead his class out onto the floe. “We were ready to go but when I went out, I discovered many cracks around,” he remembers. He decided to move the Lance deeper into the pack for safety. As he did so, the ship encountered small waves that grew in size over time—a surprise as even a little ice near the pack edges usually damps out waves. These waves then rapidly broke up the ice around the ship into thousands of smaller pieces.

Within an hour there was a four-meter swell. The Lance’s navigation system ultimately recorded occasional waves more than six meters in height, the largest ever measured amid Arctic ice. “And we could see even bigger waves higher than the deck of the ship—30 feet [nine meters] or more,” Marchenko says.

This incident marks the first time that scientists had recorded any waves over three meters high amid Arctic pack ice. Marchenko later gave his measurements to Clarence Collins and his colleagues at the NRL in Mississippi, who analyzed the interaction of the waves and ice. It turned out that although the ice damped incoming waves, it also contributed to its own destruction.

Ice near the outer edge of the pack absorbed some energy from arriving waves but also focused the remaining energy into pulses that could strike deeper into the pack, lifting it as the waves rolled beneath. The rise and fall strained ice to the breaking point. Once broken, the smaller ice chunks allowed the largest waves to pass almost unhindered and attack solid ice farther in. The ice went from blocking almost all the wave energy to none at all within just one hour. The process happened so fast, in fact, that Collins calculated waves were destroying the pack at a rate of over 16 kilometers of ice an hour.

Scientists had never imagined that Arctic waves could break up pack ice so quickly. Historically, the region’s extensive ice cover left no large expanses of open water needed by storms to whip up really big rollers. But climate change has brought milder winters, warmer sea temperatures and bigger storms, which create a vicious cycle that promises less sea ice and more wind and open water to generate ice-crushing waves.

The waves’ unexpected speed and ferocity makes them impossible to predict with current low-resolution computer models, based on ice observations that are typically updated only daily. That could spell disaster for mariners, oil companies and native communities who are unprepared for large waves or rely on sea ice to protect them. And that is to say nothing of wildlife like polar bears and walruses that rely on abundant sea ice to survive.

Collins does not expect the record waves of 2010 to stand for long. As the ice-breaking feedback loop accelerates in years to come, more and more towering waves are likely to batter the shrinking ice cap. For the Arctic Ocean, there are stormy times ahead.

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