Great Lakes water levels and quality can be affected by forces brewed thousands of kilometers (or miles) away. The unusually cold winter of 2013-14 illustrates this point. Three atmospheric-oceanic factors have combined to create record cold weather for 200 million North Americans from the Rockies to the Atlantic.
However, folks living west of the Rocky Mountains were spared the cold, as warm air from the Pacific Ocean pushed northward along the coast. This resulted in a ridge in the weather-steering jet stream, about 30,000 feet up in the westerlies, with a trough dipping south in mid-continent and, in the long-wave pattern, a ridge pushing northward along the Atlantic coast toward Greenland.
In the media, much has been made of the role of the polar vortex. There are actually two elliptical or circular polar vortices, one usually over the Arctic and one over the Antarctic in their respective winters. The latter is very stable. In the Arctic, however, the vortex is more variable. It is influenced from below by the changing Arctic sea and land, and from above by Sudden Stratospheric Warming events.
This winter, both factors have served to weaken the vortex. From below, greenhouse gas-induced warming has created much more open, dark, ocean waters, which absorb the sun’s energy, in place of very reflective ice. This tends to weaken the usual polar vortex. The strong Sudden Stratospheric Warming this winter added to this weakness, from above. A weak vortex is much more susceptible to disturbance at its periphery by the ridges and troughs, or waves, on the polar jet stream already prominent this year, as noted earlier.
So instead of one consolidated vortex over the Arctic, there developed four lobes with the cold circulations carried south in the troughs of the jet stream. There was one cold lobe over central North America, one in the mid Atlantic, one in Siberia and one in the western Pacific. On the east side of these, strong temperature gradients and storms developed, moving northward producing large snowfalls in eastern North America and rain-induced floods in the U.K. At the same time, Greenland, the Canadian Arctic and Alaska were unusually mild.
An ice cave on Lake Superior, February 2014. Credit: Bjorn Watland.
In the Great Lakes basin, the trend since the 1970s toward reduced winter ice cover, well-documented in the Upper Great Lakes Levels Study for the IJC, was temporarily reversed. Under the polar vortex, ice cover has grown to up to 90 percent on some of the lakes in 2013-14. Before they became mostly ice covered, lake-effect snows in the lee of the lakes were very heavy, due to rapid evaporation of water into the cold dry air blowing rapidly over them.
Once the lakes froze over, this evaporation nearly ceased and lake-effect snows were reduced, except downwind of local areas with open waters. Both the reduced evaporation losses and the melt of the extra snowpack in the spring should increase lake levels in 2014.
This unusual set of winter conditions will only break temporarily the long-term climate-warming trend affecting Great Lakes levels and water quality. This trend produces higher water temperatures and more frequent polluted runoff events, due to heavy rainfall events and snowmelt periods. Reducing the impacts on the lakes from these polluting events must be a major focus of efforts to protect and improve the quality of Great Lakes waters.
A breakdown of historic Great Lakes ice cover. Credit: U.S. National Ice Center.
--- Jim Bruce was Canadian co-chair of the Public Interest Advisory Group to the International Upper Great Lakes Study. He also played a seminal role in helping launch the Intergovernmental Panel on Climate Change (IPCC), which shared the Nobel Peace Prize in 2007 for its efforts.