Winter is warming without getting warmer?

What's that you say?

You may have noticed that recent winters in the Upper Midwest have been warmer than less recent ones. Winter is warming, and nobody disputes that. Here are the average December-through-February temperatures at Minneapolis, back to 1900.



Impressive. You can see a clear winter warming signal, especially since the late 1970s. I'd also like to call your attention to the up-down-up pattern: warming in the first few decades, then cooling, then warming again for the last few decades. You will see a lot more of this pattern in other posts, and I mentioned it at the bottom of my post about wet winter days.

As a climatologist, I am fascinated by how this wintertime warming is composed. The wintertime average temperature for any given year, which you see above on any one of the stops along the blue trace, is essentially the average of all the average daily temperatures. And each average daily temperature is the average of the official high and the official low temperature for that day. So, are the highs and the lows getting warmer? Just a subset of them perhaps? Is the whole of winter warming? How uniform is this phenomenon across the season?

The image below, and feel free to mouse over it to get annual values, shows the single highest temperature each winter (red/upper trace), and the average of the 15 warmest high temperatures from each winter (orange/lower trace). I tossed in the 10-year averages, calculated on the tenth year (so the value for 1997 is the average of 1988-1997).

When compared with the top graph, this one makes it clear that the increase in average winter temperatures has nothing to do with the behavior of the warmest days of the season, which are essentially unchanged, plus and minus normal variability. You do not see the dramatic, multi-decade increase you see in the top graph. The warmest days are not getting warmer.

Nor are the warmest nights are getting warmer. The graph below shows the warmest daily low temperature(s) of each winter. Again, you'll find nothing resembling the major increase we see in the last few decades of the first graph in this post.


Now, I cannot project these trends into the future with much confidence. They really only show you what has happened so far. And so far, while our winters have warmed dramatically, our warmest winter temperatures have not. It's almost counter-intuitive, I know. But it's true.

So how can our average winter temperatures have increased so much without our warmest days budging at all? The answer appears to be in our coldest days. They're simply not that cold. Anymore.
 
Note: this information was gathered in support of research conducted with and for Hennepin County Emergency Management