Why there's more Great Lake ice this year than last

Wednesday, January 14, 2015, 7:47 PM -

Last winter was cold enough in Ontario that the frozen surface area of the Great Lakes was 5-10 percentage points above the median. This year it’s even higher.

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Source: Canadian Ice Service

Looking at the top chart, which shows the weekly percent ice coverage for the 2013-14 season), we can see that for this week last year the ice coverage (blue bar) was above the median (green line) and just above 20 per cent coverage.

The 2013-14 season was notable for the drastic increase in ice coverage from Dec. 11-18, 2013. The lower chart shows the weekly ice coverage for this season. Current coverage is roughly 27 per cent, which puts us about 7 percentage points more than where things were at this point last year and is also above the 1980-2010 median.

This year, the ice has steadily increased up to this point, and the past two weeks we have observed a more drastic increase.

Source: NOAA Coast Watch Great Lakes Node

The chart above is a side-by -side comparison of the ice concentration from Dec. 1 to Jan. 13, last season and this season so far. This data is slightly different from the first two graphs, which show ice coverage.

Coverage describes the area of the lakes’ surface covered by ice. Concentration describes the relative amount of area covered by ice compared to a reference area. Think of it like this: Given a square of fixed area, the concentration describes how much of that square is covered. The two measurements are not interchangeable, but can both be used to describe state of Great Lake ice.

An interesting pattern emerges from the ice concentration chart. The concentration began to increase rapidly around Dec. 11, 2013, with another rapid increase from Dec. 31. onward, with some fluctuations in between.

In December, 2014, lake ice remained low and stable, with a little spike Dec. 20-23 and a steady increase starting near Dec. 30, 2014 through to now.

The early ice coverage in December of 2013, is the result of temperatures that were anywhere from a few degrees below seasonal to near 12C below seasonal. Compare that to December of 2014, where temperatures were consistently above seasonal, with a very warm Christmas Eve. Another deep freeze took hold through the end of December 2013 into early January 2014, which lead to another noticeable increase in ice coverage. The Jan. 10-17, 2014 time frame saw a thaw take hold, with temperatures getting near 12C degrees above seasonal leading to an ice coverage decrease.

This season, temperatures have been on a steady decline since the New Year. Most of this month has had below seasonal temperatures. As a result, the amount of lake ice has surpassed what it was at this point last year.

Will this winter go on to have more lake ice than last winter? Temperatures are set to stay near seasonal, with a slight warm up into the weekend. As a result, the NOAA Great Lakes Coastal Forecasting System projects ice concentrations on most lakes will stay as is, with a slight decrease possible. But another deep freeze looks likely toward the end of January, which will make for good ice-growth conditions once again.

Looking back to the top graph in figure 1, we can see that after this week last year, lake ice went back in the increase right through to the week of March 5, 2014, as temperatures from late January, 2014, through to early March, 2014, were at or below seasonal, on average.

Whether or not we will surpass last year’s coverage is a tough call, and will depend on just how warm the weekend gets and how fast the end of month freeze settles in.

Correction: Due to an editing error, an earlier version incorrectly attributed this article to Michael Carter.