Climate Fact: Lake Superior Stratification
During winter, Lake Superior’s cold water is on the surface and warm water is on the bottom; during summer, the opposite is the case. The “switchover” happens in the late spring or early summer and is an important event, as it “stirs” the water. This stirring brings nutrients (which feed the lake’s wildlife) from the depths of the lake to the surface. After the switchover, summertime stratification season begins. This period, when a layer of warm and relatively nutrient poor water covers the lake surface, occurs until another switchover happens in the fall. Over the past 30 years, the temperature around Lake Superior has increased by 2.4 degrees Fahrenheit and the water temperature in Lake Superior has risen by 5.3 degrees Fahrenheit. The relatively higher rate of temperature rise in the water is thought to be due to declining average winter ice cover, which fell by 11.3 percent over the same 30-year period. Open water absorbs more sunlight than ice does, resulting in greater warming. This creates a positive feedback cycle where warming leads to less ice and more warming. This warming has been linked to a longer summer stratification season – now 25 days (17 percent) longer than it was in the early 20th century.
Seasons: Winter, Spring, Summer, Fall
Source: Austin, JA and Colman, SM. “Lake Superior summer water temperatures are increasing more rapidly thanregional air temperatures: A positive ice-albedo feedback.” Geophysical Research Letters 34 (2007): L06604 and Austin, JA and Colman, SM. “A century of temperature variability in Lake Superior.” Limnology and Oceanography 53 (2008): 2724-2730.
