In the last issue we started to examine why our part of the world has such changeable weather. We looked at general global circulation patterns, then examined the zone where westerly winds bump up against easterly moving polar winds, creating giant eddies of swirling air.
This week we are going to continue our look at these swirling eddies of air and tie in something known as Rossby waves. Rossby waves are named after the meteorologist Carl Rossby, who in 1938, mathematically described how the boundary between cold arctic air and milder air to the south interacts.
Before we dig into this topic, the Weather Network came out with its winter forecast, so I figured we should take a quick look at that first. Now, just like NOAA’s, forecasts from the Weather Network (a U.S. company) magically stop at the border, but then pick up again up in Alaska. If you know anything about how weather models work, you would know that’s just not how they do things — they would be figuring out all the points across Canada and beyond. But either showing the data across Canada confuses the American reader/viewer, or the company just does not want to share its data with us Canadians. No matter; since southern Manitoba is close to the border, we can, with fairly high confidence, extrapolate the data northward to come up with a fairly reasonable forecast. So, with that said, here is what they are calling for this winter.
December is forecast to be the cold month, with all the north-central and northeastern U.S. showing below-average temperatures. For January they show a bit of warming trend with the below-average temperatures sliding to the west and near- to slightly above-average temperatures moving into our region (or at least across Minnesota, North Dakota and eastern Montana). February has the below-average temperatures pushing eastward a little bit, with Montana and western North Dakota showing near- to below-average temperatures. For us, I would guess that would mean slightly below-average temperatures. As for precipitation, they are going with the average departure from average snowfall during La Niña years — as seen above, in the graphic included with this issue’s forecast — which shows slightly higher-than-average snowfall. Now, on to Rossby waves.
One way to think about Rossby waves is to picture the cold air over the poles or top of our planet as a big glob of goo. If you dropped a pile of goo on top of a ball, it would want to sag down, and the same holds true for the cold air in the polar regions — it wants to sag to the south (in the north). Now, things would be pretty straightforward if this was all we had to worry about, but as we all know, this simple picture is missing one important part, our planet is not just a ball simply sitting in place — it is spinning.
This spinning motion takes the atmosphere along with it, so our pile of cold air, or goo, is spinning around. Now we have a spinning pile of cold air at the top of our planet that is trying to sag toward the south. This sagging creates undulations, or waves, along the boundary of this cold air, and these are what we call Rossby waves. One of the best ways to understand this is with a diagram. If you look at the diagrams I’ve included with this week’s article, you’ll see the development of a Rossby wave in three steps.
In Figure 1 we can see there are some slight undulations along the boundary between the cold and warm air. Over time, these undulations become larger waves like we see in Figure 2. These waves are beginning to develop into eddies and are becoming areas of low pressure. In Figure 3 we see a very strong Rossby wave pattern, where one of the eddies of cold air, or areas of low pressure, is starting to break away from the main area of cold air. When this happens we get what is called a closed upper low, and these can sit over an area and slowly spin down over a fairly long period of time, bringing with them cool, cloudy and generally miserable weather — but more on that in a future article.
At any given time, there will typically be between four and six Rossby waves at different stages of development in the Northern Hemisphere. In our next lesson we will continue to focus in on what causes our weather, because, believe it or not, it is much more complicated than simply watching out for Rossby waves.