Weather: The Great Lakes are heating up

Tuesday, February 16, 2010

Researchers have found that summer air temperatures over Lake Superior have been increasing more than a degree per decade, bringing changes in weather patterns for the surrounding regions

by HENRY HENGEVELD

If you live close to one of the Great Lakes and think that average summer winds in your area have changed in recent decades, you may be right.

Several years ago, researchers at the Milwaukee campus of the University of Wisconsin noted that there had been a significant shift in summer wind directions over the Great Lakes since 1980, dominated by an increase in air flow from the south. They suggested that a change in the dominant track for summer storms was a primary reason for this change.

In November 2009, in a research article published in Nature Geoscience, Dr. Ankur Desai and colleagues at the Madison campus of the same university presented results from a more recent study into summer winds over Lake Superior that added new insight into changing regional winds. They found that, since 1985, Lake Superior surface winds in early and mid-summer have been increasing in average intensity by five percent per decade. Furthermore, the data suggested that this increase has been caused by local climate feedbacks, rather than changes in storm patterns or larger scale atmospheric circulation.

Consistent with trends in global temperatures, average annual air temperatures in the Great Lakes Basin have increased by about 0.6 C since 1948. Summer temperatures rose by 0.5 C over the same period, although this rate of warming has increased in recent decades. However, when Desai and his cohorts examined data from satellites and from three weather buoys moored on Lake Superior, they found that summer air temperatures over the surface of that lake had increased by 1.05 C per decade since 1985. 

More interestingly, summer surface water temperatures in the centre of the lake had risen at an even larger rate of 1.21 C per decade. This is about twice as fast as the concurrent trend in air temperatures of the larger Great Lakes region. 

According to Desai, the key reason for the rapid rise in summer surface water temperatures in Lake Superior appears to be the decrease in winter lake ice cover caused by the large scale warming of the region. This decrease has advanced the average date when ice disappears from the lake surface.

Ice reflects incoming sunlight, while water absorbs it. Thus, the earlier disappearance of lake ice also causes an earlier warm-up of the lake's surface waters and onset of summer lake stratification – when warmer surface waters become stable and stop mixing with cooler waters below.

The longer season for the persistence of this warm surface layer, in turn, means that lake temperatures become progressively warmer throughout the summer, and remain warm later into the fall season. This delays fall freeze-up, helping to further shorten the winter ice season. 

Since Lake Superior summer water temperatures are warming more rapidly than the temperatures in the air above or over adjacent land areas, the air layer directly above the water becomes less stable. Physical processes translate this instability in the air to more intense winds.

That is indeed what the investigators found in the wind data. Since peak winds did not increase significantly more than average winds, they also concluded that increased summer storminess was not a significant factor in the wind speed increase.

Other experts have already begun to speculate on what implications this increase in wind speed may have for lake currents and ecology, and for people living in adjacent land areas. Lake currents are projected to become stronger, while warmer temperatures in the stratified water layer near the lake surfaces are likely to decrease biological productivity.

Although wind speeds over adjacent land areas increase much less, the changes in wind speeds and patterns that do occur there may also alter the transport of air pollution from source regions to downwind areas. 

As summer changes into fall and early winter, air temperatures drop and become significantly colder than the surface lake waters, which are still flush with summer heat. This temperature inversion causes a lot of heat and moisture to escape from the lake surface into the air, sucking water out of the lake and fuelling the snow squalls that can dump large amounts of snow in the lee of the lake.

In a commentary on the Desai study, McMaster University geographer Wayne Rouse notes that the rapid rise in Lake Superior's summer water temperatures will thus increase the magnitude of the water-air temperature difference in the fall season. This can be expected to enhance the intensity and extent of the snow belt region in the lee of the lake. 

The Desai study focused specifically on Lake Superior, which happens to be the world's largest freshwater body. However, researchers note that similar results can be expected for the other Great Lakes, where winter ice cover and ice season duration have also declined significantly.  

Another reminder that a warming global climate is much more than just a rise in average global temperatures! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.