Lake Superior has the largest surface area of any freshwater lake in the world. Because of its immense size, it takes a long time for the temperature to respond to the changing seasons. The lake’s average surface temperature only rises about 30 degrees over the course of six months.

However, swimmers along shorelines can be chilled by dramatic temperature changes when conditions are right. The reason for this is something called upwelling, and it’s pretty simple in theory.

The temperature of the lake is typically the highest at the surface and it falls the deeper you go. Deep water stays close to 40°, even in the late summer. When winds push the surface water away from the shoreline, water from the depths rises up to replace it.

So instead of swimming in comfortable waters that have benefited from months of warming, all of a sudden you’re swimming in water that’s hardly above freezing.

UMD has buoys near McQuade Harbor that can be used to track this. Last Thursday, the water temp fell from 58 in the morning to 44 at 6 pm. It then rose rapidly up into the 60s. Back in early September 2020, the temp nearshore fell 20 degrees in just 9 hours! All the while, the buoy further out held steady in the 60s.

The wind required for upwelling isn’t perpendicular to the shoreline as you might think. Due to something known as Ekman transport, net water transport is 90° to the right of the wind direction.

This means that while the direction the water moves in changes as you move down from the surface, the overall movement of water within 100 m of the surface is at a right angle to the surface wind. So, a southwest wind parallel to the North Shore is the most effective for displacing warmer surface water.

Upwelling can make a big difference for swimmers, especially along the North Shore. We expect the big lake to be cold, but with upwelling, you could be swimming in waters from the icy depths.