Clouds hover over a chilly Lake Superior on the Duluth waterfront this morning.
Fishermen have been reliably watching weather patterns for centuries. That’s why I looked into reports from anglers that the water temperature in Lake Superior took a nose dive early this week. The reports were right.
There were reports that late last week surface water temperatures on the western end of Lake Superior near Duluth had warmed into the low 50s. That’s pretty balmy for frigid Gitchi Gumi. By Monday water temperatures had plunged to 38 degrees off of Duluth Harbor. That’s a 15 plus degree drop in less than 72 hours. What could have caused that?
It turns out it’s a pretty common phenomenon when we get a strong persistent northwest wind like last weekend. For three days winds howled between 20 and 45 mph on the big lake. The friction with the water blew the relatively warm water south and east toward Wisconsin. As the surface water moves east, cold water from the depths of Superior rises on the North Shore to take its place. This process is called upwelling.
Since water is heaviest at precisely 39.2 degrees Fahrenheit, that cold water stays at the bottom of the lake until it is moved to the surface by some force. In this case it was wind. The process of replacing cold water from the depths with surface water is called lake turnover. It is common in inland lakes in the spring and fall. It also brings fresh oxygen rich waters from the surface to the depths where they can recharge plant and marine life.
The abrupt temperature changes in Lake Superior water temperatures this week is no doubt a shock to anyone who dares dip in a toe or brave a swim. But there are benefits to colder water.
Water temperatures have recovered a bit along the north shore into the 40s in most areas. As usual in Superior that’s still way too cold for a safe swim, but it’s progress.