UMD-LLO/NOAA
Average Lake Temps
Konnie LeMay
Lake Superior hit a 31-year high mark in the daily average temperature of its surface waters on August 11 when the average for that day on several monitoring buoys on the open lake tallied 68.7°F (20.4°C). The previous high for daily average surface water temperature was set on August 14, 1998, at 67.5°F (19.7C).
The daily average temperature, while not the high temperature for the day, better reflects the warming situation on the lake, according to researchers. The lake’s surface water temperatures have been monitored for 31 years on a series of buoys placed around the lake by the National Oceanic Atmospheric Administration (NOAA).
The high temperature so far for the summer has not exceeded the 74.3°F (23.5°C) record, falling just short at 73°F (22.8°C). Such temperature highs may last only an hour or two, according to Jay Austin, associate professor of physics at the Large Lakes Observatory, University of Minnesota Duluth.
Lake Superior’s warm waters have made news across the country from the New York Times to the Chicago Tribune to the L.A. Times.
The 1998 record was preceded – just like this year – with an El Niño-influenced winter in which heated water in the eastern Pacific Ocean impacted world weather patterns. A warmer Lake Superior in turn affected regional weather, creating warmer and sometimes stronger winds.
The warmer lake caused stratification of hot and cold waters by June this year, a month or so earlier than usual. The deepest water stays at 39° F (4° C), about the lake’s year-round average. In summer, this colder water sinks as warmer water stays near the surface. The early warming this year relates to ice cover in winter. With little ice cover, the lake, just like a giant solar panel, absorbed energy (and heat) from the sun.
While researchers know what happened with the water temperatures, they don’t know exactly how it will influence the lake’s complex eco-systems and water circulation.
“Temperature is one of those sort of master variables,” says Jay, who with Steve Colman, head of the LLO, first released information about dramatic water temperature increases in Lake Superior in 2007 using current data and 100-year-old records from the power plant at Sault Ste. Marie, Michigan. Researchers already noted long-term increases in average air temperatures, but this study indicated that the Lake was heating up at a rate almost double that of the air.
Warmer waters may drive some fish, like lake trout, deeper while certain invasive species, such a zebra mussels, kept in check by the lake’s usually chilly temperatures, may have get a better foothold, Jay says.
Lake Superior was not the only large lake in hot water, Steve says. Lake Michigan exceeded its all-time record temperature and Lake Huron tied its previous record. Lakes Ontario and Erie did not exceed previous records.
“It does appear that most of the Great Lakes, which is a huge regional area,” he says, “is behaving more or less the same way.”
Winter ice cover is just as important on the lake’s surface water temperatures as summer air temperatures. This summer’s top regional temperatures did not come near the record highs set before testing of lake water temperatures began. In 1936, Duluth peaked at 106 degrees, its highest ever temperature and recorded on July 13. August is often thought of as the hottest month around the lake, but says Dean Packingham of the National Weather Service’s Duluth station, “We’ve never cracked 100 in August.”
On the eastern side of the lake, the highest air temperature recorded in Sault Ste. Marie, Ontario, was 36.8°C (98.2°F), recorded in July both in 1988 and 2007.
Conditions can change quickly on Lake Superior. Jay noted that on August 14 and 15, “the strongest sustained summer wind event on record occurred, mixing the warm surface waters with cooler waters below, dramatically cooling off the surface of the lake, which dropped to 59°F (15°C) over just two days.” The established average water surface temperature for Lake Superior this time of year is 56°F (13.4°C).