The New Lake Superior
By Meteorologist Graham Saunders
Lake Superior has many reputations – it is the largest, deepest, coldest and most pristine of the Great Lakes. Is it a “Weather-Maker”? Ask anyone who lives around the Lake. Is it “colder by the Lake”? Yes, in spring and early summer. But “warmer by the Lake” does apply in the fall and winter seasons.
There are fundamental changes happening when we compare modern Lake Superior with the past: Warmer water temperatures and reduced ice cover have been observed in the later 20th century and these trends have continued through the recent two decades. Although all seasons show a warming trend, the warming is most pronounced in fall.
Water temperatures in Lake Superior have been measured with buoys and in recent decades with satellites. Water temperatures are coldest in late February (average 1° C 1995-2021). In March the surface water temperatures begin to warm, although some years this is delayed because of melting ice. Warming takes place slowly until water reaches 4° C [39° F], the threshold where fresh water is at maximum density. The denser water sinks and mixes with deeper water, a process known as the spring overturn. Mixing with the deeper water results in oxygen and nutrients becoming more available at various depths of water. Summer stratification follows, a time when the top layer of warmer water becomes separate from colder and denser water below. The less dense surface water much faster than in previous weeks.
The processes of spring overturn and summer stratification take place in almost all lakes in the mid-latitudes. The volume and surface area (82,100 square kilometres) of Lake Superior result in a time lag compared to smaller lakes.
Temperature data confirm that the lake is getting warmer. In the 20th Century, The average date of the beginning of the spring overturn was June 7, but in recent decades it has started earlier, averaging May 26. Superior’s “average surface temperature in the summer months has risen 2° C over the past 30 years”, said Jay Austin, a professor at the Large Lakes Observatory in Duluth. Other complexities include that Superior is warming faster than almost all other lakes as well as the land surrounding it.
The annual cycle of temperature change in Lake Superior shows that typically the warmest temperatures are in August with an average of 16° C [61° F], although shallow bays are warmer. Then, with cooler conditions and reduced sunlight, the water temperature declines to 4° C, initiating the fall overturn. The denser surface water sinks to lower levels. The trend to warmer lake temperatures and longer duration of summer stratification have shifted the average timing of the fall overturn from early December in the 20th Century to later in the month in the past two decades.
In 2021, lake temperatures were still around 15° C [60° F] in the first half of October, luring in some swimmers. The very slow decline of water temperature in the final months of 2021 was unusual and I wondered if the fall overturn would be delayed until January 2022. On December 30 the temperature finally declined to 4 [39° F] degrees, the latest on record. The fall overturn is beginning later and spring overturn is occurring earlier. One could speculate – what happens if someday there is no fall overturn?
Warmer lake temperatures have impacted ice cover which is declining at an average of 7% per decade since 1973. In the 21st century the maximum annual ice cover varies regularly between 10 and 90% whereas in previous decades ice coverage was usually above 60%.
