Environment

Arctic Cyclone Tears Up Sea Ice

Arctic scientists are watching in awe this week as a raging summer cyclone tears up what could become a record amount of rotting northern sea ice.

Arctic cyclones are driven by low-pressure systems in which winds of up to 100 km/h blow counter-clockwise in a spiral more than 1,000 kilometres across. They occur in both winter and summer, but are usually stronger in winter. Cyclones are not unusual in the Arctic, but seem to be changing in recent years. "These cyclones are not getting more frequent, but they are getting deeper -- which means stronger."

And they're getting harder on sea ice, which they break up through wave action associated with high winds and through rainfall, which darkens the ice and makes it absorb more solar energy. The storms also bring up water from the depths, which is actually warmer than surface water.

Cyclones can destroy large amounts of ice very quickly. "In 2009, we actually documented one of these events in which large, multi-year ice floes - Manhattan-sized - broke up in a matter of minutes." Last year, a particularly powerful cyclone is thought to have wiped out 800,000 square kilometres of ice. That contributed to record low sea-ice levels at the end of the 2012 melt year.

This year's storm over the Beaufort Sea formed about mid-week and is expected to die out on the weekend. It isn't as strong as last year's, but the ice is thinner and weaker. As well, the ice has already been pummelled by earlier storms. "The effects of (the storm) are nowhere near what we saw last August. But because the ice is thinner and it's already been pre-conditioned, and because there's less volume, it's much more vulnerable to impacts from this sort of thing."

The ice is getting so weak that new categories have had to be created for it. "We have a whole new class of sea ice in the Arctic, which we're calling 'decayed ice. We started seeing it in 2009. It's extremely weak." Changing sea-ice cover is increasingly being linked to southern weather patterns. The jet stream, which strongly influences weather at mid-latitudes, is driven by temperature differences between the Arctic and the equator, a difference that shrinks with the sea ice. Ice coverage is slightly about last year's record low but still well below the 30-year average.

Much remains unknown about the role of Arctic cyclones in the annual freeze-thaw cycle. Back when the sea was thick and lasted for years, cyclones tended to spread the ice out and actually increase its extent. Now, when ice gets spread out, it simply breaks up and disappears. "As our ice cover has thinned, some of our old rules are changing...This year has been very stormy. The month of August is definitely one to watch in the Arctic."