Monday, March 12, 2012

I wouldn't try skating across the Great Lakes either

Following upon last week's news about a shrinking "skating" season across Canada, a paper in the Journal of Climate reports that the length and extent of the Great Lakes ice season has decreased rather dramatically since the early 1970s.

You don't need library access to Journal of Climate to get a sense of the data. The Canadian Ice Service has plots of total Great Lakes ice cover going back to the 1980s (right). One things that's striking about the data, and is analysed at length in the paper, is the periodicity, which seems to follow the El Nino cycle; ice cover is lowest during the 1982/83, 1986/87, 1991/2. 1997/98, and 2009/10, all El Nino event.

El Nino, however, does not tell the whole story. Ice cover is also low during much of the past decade, with the exception of two winters, one of which was a strong La Nina winter (2008/9). And this winter, which was not included in the analysis in the paper will go down as one of the most ice-free. The map at right shows the departure in ice cover from normal. The dark reds covering Lake Erie, Lake Superior and parts of Lake Huron/Georgian Bay are regions where there's "normally" ice in March.

Right now, you could almost swim across Lake Superior, something even these guys probably did not dream was possible. This is one part of the planet where we may need to redefine normal.

Unlike the reported changes in the ice season on small lakes, the subject of our video, or the outdoor skating rink season, the change in Great Lakes ice can have a weather and climate effect. One, that's a bit counter-intuitive, and is perhaps the only piece of good news here for skiers, is that less ice on the lakes can lead to more "lake-effect" snows in east-central Ontario and upstate New York (ever wonder why there are big "freak" snowstorms north of Toronto and around Buffalo in late November? It's from a cold north-westerly winds mass passing over the unfrozen lakes an picking up moisture). Of course, that can only happen with sufficiently cold air masses, which were rare this winter.

The other effect, which as far as I know has not been quantified, is the change in "albedo" or reflectivity. We hear about this all the time with the Arctic - less ice means less reflection of incoming solar radiation. A similar positive climate feedback, albeit smaller in magnitude, should occur in this region dominated by freshwater lakes.


Steve L said...

Is the 'memory' of the Great Lakes shorter than that of the Arctic sea ice? That is, I imagine that the albedo feedback in the Arctic results in a long term amplification (heat is stored in the Arctic Ocean and contributes to more blue water in subsequent years), but I wonder if the feedback only exists intra-annually in the Great Lakes. Due to the seasonal mixing, perhaps each spring starts anew from approximately the same thermal starting point?

Simon Donner said...

I've not done that type of analysis myself, but this paper does a nice job quantifying the albedo feedback for Lake Superior.