Wednesday, September 19, 2007

The complexity of ice melt

Over the past few months, the science media and those of us in the echo-chamber known as the blogosphere has been following the Arctic sea ice melt season with a fervour normally reserved for only for the hurricane season, and that arguably should also be applied to the summer fire season.

The chattering about this year’s record low may be giving the false impression that the Arctic sea ice dynamics are simple. A recent short paper in Geophysical Research Letters reminds us that the Arctic climate is complicated and the sea ice decline will not be smooth or orderly. The authors Jennifer Francis and Elias Hunter from Rutgers find that the factors influencing sea ice cover in the Bering Sea (that’s the Pacific side) and the Barents Sea (the Atlantic/Norwegian side) are quite different:

Between 1979 and 2005 in the Bering Sea, the ice edge is influenced mainly by anomalies in easterly winds associated with the Aleutian Low, which was particularly strong during the 1980s. The Barents Sea ice edge, in contrast, is driven primarily by two factors: anomalies in sea-surface temperature, particularly close in time to the maximum extent, and by southerly wind (from the south) anomalies integrated back to mid- and early winter. The hemispheric-mean decline in winter ice extent is due in large part to increasing sea-surface temperatures in the Barents Sea and adjoining waters, which are consistent with increased concentrations of greenhouse gases.

If you want a simple climate-ice connection, try a small temperate lake. There, mechanical (wind) action and ice dynamics have a relatively small effect on the areal ice coverage. The net accumulation of below-freezing temperatures in the fall is a very good indicator of the date of “ice on” – basically the inverse of how we predict coral bleaching. So when the late fall and early winter is warm, as happened last year in eastern North America, the lakes either freeze later or not at all. That’s little Muldrew Lake in the photo, with just an inch of slushy ice and snow on Dec. 28th of last year, to the consternation of those of us clutching hockey sticks. And the net accumulation of temperatures above freezing is a decent indicator of the date of “ice off”, though winter snowfall levels, winds and the non-linearity of albedo changes complicate matters a little bit.

Yes, limnologists do say “ice on” and “ice off” as if they are in a Canadian adaptation of the The Karate Kid. These days, it is probably safer for Ralph Macchio to clean the ice by hand. If this winter is as warm, I wouldn’t recommend driving a Zambonie out on the lake.

1 comment:

Unknown said...

To everything there is a season, turn turn turn. The National Snow and Ice Data Center reports sea ice increased by 50,000 km2 in the past few days. See you all again next year.