It’s best to hope these ‘blocking patterns’ don’t become the norm
This is one of those weeks when I’m just not sure what to write about, so I thought I’d discuss some ongoing weather research that just might help us understand how we could go from a record-warm March to a record-cold April in just one year’s time.
Whether you want to believe it or not, the Arctic is warming, and for this article I’m not going to discuss why. Research and weather records show the Arctic warming two to three times faster than the rest of the northern hemisphere, owing primarily to sea-ice loss, earlier snowmelt on Arctic land in spring, and an increase in the northward transport of moisture into the Arctic. All of these conditions coming together to increase the amount of warming in the Arctic are known as Arctic amplification. Usually when I write about these topics I try to take current research and write it in such a way that you don’t have to be a university scholar to understand it. In most cases I don’t like to take a big piece of someone else’s work and simply reprint it — but occasionally something is so well written that it is pretty hard to come up with something better.
This next paragraph explains why it appears that the warming of the Arctic is helping to drive more extreme weather patterns across our part of the world. It is written by Dr. Jennifer Francis, a research professor at the Institute of Marine and Coastal Sciences at Rutgers University, where she studies Arctic climate change and the link between Arctic and global climates. This is one paragraph of a much longer article entitled “The Changing Face of Mother Nature.”
“This so-called ‘Arctic amplification’ means that the temperature difference between the Arctic and mid-latitudes is weakening. This is important because the west-to-east winds of the jet stream are driven by that temperature difference. The jet stream is a fast river of wind high in the atmosphere that takes on a wavy path as it encircles the northern hemisphere, forming the boundary between warm air to the south and cold air to the north. As its westerly flow weakens, the waves in its trajectory tend to take larger north-south swings. These waves control weather systems on the surface: conditions tend to be clear and dry in the part of the wave where winds blow from the northwest, and it’s generally stormy where winds come from the southwest. As the waves increase in size because of Arctic amplification, they are expected to progress eastward more slowly, which means that the weather associated with those waves lasts longer in any particular location. Larger waves are also more likely to form ‘blocks,’ which are like back-eddies in a stream that tend to prevent the jet-stream waves on either side — and the weather associated with them — from moving at all.”
So far over the last couple of months the waves in the jet stream have become very large and have created a general blocking pattern over North America. The jet stream has created a large ridge to our west as it surges northward. It then carved out a large trough over the central part of North America before surging northward again off the East Coast, developing a ridge in that region. This places our region on the northern side, or cold side, of the jet stream, with winds generally coming out of the northwest. In fact, looking at the predominant wind direction over the winter, the last four months have all experienced predominantly north-northwest winds.
As Dr. Francis pointed out, this type of pattern can cause the jet stream to block up or not move at all, keeping a region in the same type of weather pattern for much longer periods of time than usual. With predominantly northwest winds, that means we were in clear and dry but cold conditions.
So why all the snow, then? This has to do with the fact that this type of jet stream pattern tends to generate large and strong storm systems. These storm systems tend to form near the bottom end of the troughs and then move east or northeast following the jet up and over the next ridge. As we saw this winter, several large storm systems did develop in the trough sitting over central North America. Where these storm systems went depended on the exact position and strength of the ridges and troughs. If they were centred in the middle of the continent then the storms stayed to our south and east. If the ridges and troughs slid a little to the west then we were under the gun. It all depended on the timing and the overall position of the jet stream and when a storm developed.
The important thing to understand about all of this is not that it is miserable to be stuck on the wrong side of these blocking patterns, but rather to worry that this type of pattern is going to become the “norm.” It’s not fun having one of the latest, coldest springs on record — but it’s also not fun having the warmest, driest summer ever!
— Daniel Bezte is a teacher by profession with a B.A. (Hon.) in geography, specializing in climatology. He operates a computerized weather station near Birds Hill Park, Man.