Weather school: Oscillations and acronyms

If Rossby wave patterns were predictable, weather would be easy to predict

If the North Atlantic Oscillation is in a positive phase, the Arctic Oscillation is said to be in its warm phase, trapping cold air in polar regions.

Before diving into this issue’s topic, I just wanted to pipe in about the recent snowfall across a large portion of southern Manitoba and the predictions that were being made by at least a couple of weather personalities. These predictions were that the snow would likely stick around since the forecast was for continued cold weather. These predictions drove me crazy. No, the snow was not going to stick around, for two fairly obvious reasons. First, the snow cover just was not deep enough. If there is less than about 10 cm of snow on the ground, the sun’s energy can make it to the ground, speeding up melting — even when cloudy. Secondly, the long-range forecast was not favourable for snow to stick around. The weather models (check the forecast) have consistently been leaning toward milder weather moving in later this month and into early November.

Air battle

OK, moving on. In last week’s article we took a look at Rossby waves — you know, the long-term wave patterns that slowly undulate across our part of the world, bringing with them much of our ever-changing weather. Yes, I did say much of our weather, as everything cannot be blamed or explained just by looking at Rossby waves — otherwise, our weather would be simple to forecast!

Before we start going into some of the details that make our weather so unique, I thought we should examine the big picture of Rossby waves and how there are general overall patterns as to how they behave. These patterns of behaviour are known as oscillations.

If we look up the term oscillation, we find a lot of definitions that state something about a periodic movement around a mean, or a swaying of data around a central point. While these definitions work for what we want to talk about this week, they do not really hit the weather nail on the head. The best definition I found that really hit the meteorological point is: a single complete execution of a periodically repeated phenomenon, such as the changing of seasons as the sun “oscillates” between the Northern and Southern Hemispheres.

Now that we have a definition of oscillation I think we can all work with, back to the weather. As we have learned, the cold and warm air on our planet is in constant battle, sending globs of itself either northward or southward in a slow, endless dance that we call Rossby waves. If you are still uncertain of how these waves work, simply think of a big glob of cold goo sitting on top of the planet. This goo wants to sway southward and the only thing keeping it from doing so is the warm air. Now, picture your hands as the warm air: you can only hold the cold goo from sagging southward in some locations, and as you do so, more wants to sag southward in other locations. Now, start to spin the whole thing and you have just a little idea of how complicated the Rossby waves can get — just a little.

When we look at the different patterns that Rossby waves can set up, we end up seeing some general long-term patterns emerge. This does not mean Rossby wave patterns are predictable; otherwise, weather would be easy to predict. Rather, we find Rossby waves tend to go through periods where they favour a particular pattern. This is where we get the term oscillation.

There are four major oscillations we will examine. Probably the most famous is the El Niño-Southern Oscillation (ENSO). This is an alternating pattern of high and low pressure across the Pacific Ocean, which then creates differences of ocean and atmospheric temperatures that can greatly influence the weather we experience across much of North and South America. The ENSO typically lasts between six and 36 months but can last much longer.

The second oscillation is what is known as the North Atlantic Oscillation (NAO). This oscillation is expressed as a difference between the strength of the Icelandic low, which is a common Rossby wave feature, and the strength of the subtropical high to the south (something we talked about earlier this year). When the area of low pressure is weak and the subtropical high pressure is strong, the NAO is said to be in its positive phase. Under these conditions the northeastern part of North America, along with the Mediterranean, tends to be dry, while northern Europe is wet. This oscillation tends to be unpredictable, lasting for very short periods of time or hanging around for years.

A third oscillation is known as the Arctic Oscillation (AO). As with the NAO, the AO has two phases, warm and cold, and they tend to be associated with the phases of the NAO, especially in winter. When the NAO is in its positive phase, the AO is said to be in its warm phase. Strong high pressure to the south means the polar regions will have relatively low pressure. Cold air then tends to get trapped up north and we will have warmer-than-average winters. The main question with these two oscillations is what triggers them, and which oscillation controls the other, or are they mutually independent? So far, we just do not know.

Finally, our last oscillation is the Pacific Decadal Oscillation, or, you guessed it, the PDO. This is one of the longer-lived oscillations, lasting up to 30 years (thus the term “decadal” or decade). This pattern is not very well understood but researchers hope it will help to better predict ENSO events. Alas, I am running out of acronyms, so I guess it is time to Call It An End or CIAE.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



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