In the last issue I looked at possible trends in the amount of melting occurring during the winter across the Prairies. In this issue I’m going to look at another question that has repeatedly been asked of me, and I feel it kind of ties into the winter melting trends. This question has to do with Arctic ice melting and the position/behaviour of the polar jet stream.
Before we look into this, let’s review just what a jet stream is and how they form.
Jet streams form along the boundaries between warm and cold air. Along these boundaries we have a sharp contrast in temperature that results in a rapid change in pressure. This rapid change in pressure sets up a steep pressure gradient, which allows the already strong upper level winds to intensify into a jet stream.
Within the jet stream we can see wind speeds as high as 300-400 km/h, but typically they are between 100 and 200 km/h. Two main jet streams usually form: the polar jet and the subtropical jet. Here in our part of the world we deal mostly with the polar jet. Since these jet streams are partly the result of strong temperature contrasts, the polar jet tends to be strongest in the winter and weakest in the summer. If the polar jet is to our north, then we are in warm air, and if it dives to our south we are in cold air.
The stronger the jet stream is, the more zonal a pattern it tends to have. What this means is that the jet stream will have a flatter pattern with fewer undulations or waves in it. If you remember back to last summer, I wrote an article or two about this. A zonal pattern means quicker-moving weather systems and fewer ridges and troughs. A weaker jet stream usually results in a meridional flow, which means the jet stream is meandering a lot more, making large curves northward and southward. This results in slower-moving weather systems with very pronounced ridges of high pressure and troughs of low pressure.
OK, now let’s think about what’s going on. The jet stream tends to be strongest in the winter due to the large differences in temperature between the polar regions and the tropics. The stronger the jet stream, the more zonal or stable the path or pattern it has. This is the typical setup we would see during the winter. Now let’s see what happens when you remove a large amount of sea ice from the polar region.
The amount of sea ice left in the Arctic after the summer melt season hit a record low this year. Comparing the amount of ice this year to the long-term average, we find summer ice extent is now less than 50 per cent of what it used to be. This means that a huge amount of the Arctic Ocean is now free of ice and able to absorb the sun’s energy rather than reflect it back into space. This extra energy is then released back into the atmosphere during the fall and early winter as the ocean freezes. This results in a warmer atmosphere over the Arctic, meaning the difference in temperatures between the poles and the tropics decreases.
The smaller the difference between these two regions, the weaker the jet stream; the weaker the jet stream, the more it tends to meander or the more meridional it becomes. As I have already pointed out, a meridional flow means more pronounced ridges and troughs, which can lead to more fluctuations in weather or more extreme weather.
To finish off I need to point out a couple of things. First of all, when we discuss these large-scale atmospheric circulation patterns, such as the polar jet stream, we can’t think simply about our own part of the world. That is, this isn’t just a change in the pattern over North America or Europe, but rather it is a change to the whole northern hemisphere’s circulation pattern. We have to be careful to differentiate between this large-scale change and how that change shows or manifests itself in different parts of the world.
The second point is that this is a bit of an oversimplification of how this all works. As we all know, the atmosphere is very dynamic and there are many different things that affect how it behaves and thus the weather. That said, when you actually sit and think about it, I think Dr. Jennifer Francis sums it up the best when she stated, “The question is not whether sea ice loss is affecting the large-scale atmospheric circulation… it’s how can it not?”