In our last weather school lesson 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 can’t be blamed or explained by just 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 don’t 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.
OK, now we have a definition of oscillation that I think we can all work with, now back to the weather. As we have learned, the cold and warm air on our planet is in constant battle with each other; sending globs of itself either northwards or southwards 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 southwards in some locations, and as you do so, more wants to sag southwards 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 the Rossby Waves can set up, we end up seeing some general long-term patterns emerge. This does not mean that Rossby Wave patterns are predictable, otherwise weather would be easy to predict. Rather, we find that 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 that we will examine. Probably the most famous is the El Nińo-Southern Oscillation or 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 ENS Oscillation typically lasts between six and 36 months, but can last much longer.
The second oscillation is what is known as the North Atlantic Oscillation or 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, then the NAO is said to be in its positive phase. Under these conditions the northeastern part of North America, along with the Mediterranean, tend 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 or 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 don’t 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 particular pattern is not very well understood, but researchers hope that it will help to better predict ENSO events. But alas, I am running out of acronyms, so I guess it is time to Call It An End or CIAE.