Joe G,
It's the same concept. In the winter, you are taking the denser, warmer water on the bottom and mixing it with the cooler water near the surface; hence, super cooling. In the summer, you are taking the denser, cooler water from the bottom and mixing it with the warmer water near the surface.
You might ask how can this be possible to be mixing denser water that is warmer in the winter than the surface but then taking denser, cooler water in the summer and mixing with the surface. Logic would suggest that have denser/warmer and denser/cooler doesn't add up. But remember, water is a unique compound. It is most dense at 39 degrees. This is why ice floats at 32 degrees. Most compunds become most dense at their solid state, but not water. It gets denser the cooler it becomes until it reaches it's max density at about 39 F and then becomes less dense again below 39 F.
