A positive oscillation and record-setting warmth

Iknow in the last issue I said I’d do a recap of last year’s weather both local and globally, but sometimes Mother Nature has her own ideas and I have no choice but to follow her lead. So you’ll have to wait another week or two for the recap of last year’s weather.

The main weather story so far this winter has been the unusually warm weather we’ve been experiencing, and over the last week or so we’ve seen this warm-weather story hit new highs — literally! All across the Prairies, starting late last week and lasting right through to the beginning of this week, record-high temperatures were reached, broken and, in some cases, shattered.

It would be easy to just list all the records that were broken during this latest warm spell, but that would end up taking up most of this article. Instead, I thought we should take a look at what’s causing all this warm weather and what conditions came together to bring this unprecedented weather to much of central North America.

If you read last week’s article you might remember the part where I briefly described weather pressure patterns known as the Arctic oscillation (AO) and the North Atlantic oscillation (NAO). This is a very well-known interlinked pattern of air pressure across the Atlantic Ocean and has been measured for centuries due to its effects on ocean travel (sailing). In fact, there are fairly accurate measurements dating all the way back to 1845. For those of you who missed last week’s article here is a recap of just what these oscillations are.

Globally, there are regions that tend to be either areas of low or high pressure. The area around Iceland tends to be a region that sees low pressure and we call this low the Icelandic Low. Farther south in the Atlantic, around the Azores Islands, we typically find a large region of high pressure, and this area of high pressure is called the Azores High. The NAO/AO is simply a measure of the pressure differences between the Icelandic Low and the Azores High. If there is a large difference in pressure between the two, the NAO/AO is said to be positive; if the pressure difference is low then the NAO/AO is considered negative.

Since wind speeds are controlled by differences in air pressure, a positive NAO results in strong winds. As the NAO/AO covers a very large area, the impact of these winds impacts a very large area. These strong winds actually help control the strength of the jet stream as it crosses North America. A stronger jet stream tends to keep cold air bottled up north, and allows mild air to flow northward. Also, the configuration of the two pressure patterns allows for stronger southwesterly winds across North America and this also helps to bring warm air northward. So, the positive phase of the NAO/AO usually results in warm winter temperatures across much of central and eastern North America. The opposite phase, or negative phase, of the NAO/AO results in a weak jet stream and weak to non-existent southwesterly winds across North America. In this phase we tend to see cold air having an easy time moving southward, with little northward movement of warm air. This negative phase of the NAO/AO usually results in cold winters across central and eastern North America.

If you remember back to last winter you might recall just how cold and snowy it was across much of the Prairies and eastern North America. Last winter the NAO/AO was in a very negative phase. In fact, it was the most extreme negative phase on record. This year we are seeing the exact opposite. The NAO/AO is in an extreme positive phase and just like last year, it has now officially become the most extreme positive phase of the NAO that has ever been recorded. So it’s not surprising we see record-warm temperatures across a huge part of central North America.

When this year’s winter forecast of cold and snowy weather first came out, that forecast was based on La Nia conditions across the Pacific Ocean. Some of you might be asking, if the NAO/AO can have such a huge impact on our weather, why didn’t forecasters take this into account when they made their forecasts? The reason for this is that while the El Nio/La Nia pressure pattern over the Pacific can be reasonably predicted months in advance, the NAO/AO is much harder to predict, with lead times of only a week or two. What we do know is that the general pattern does tend to stick around for several months, but it can within that time frame, jump quickly back and forth between the two phases. So far this winter it has been mostly positive, whether it will remain mostly positive or become negative is anyone’s guess. If I was a betting man, I would go with more positive than negative, at least for the next month or so, but I’m not a betting man.

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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