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Semi-permanent highs and lows

By the time you read this article it will already be February and you will probably be expecting the usual monthly weather summary and looking forward to seeing what the long-range forecasts call for in the next month. Unfortunately, because I write these articles in advance to meet publishing deadlines, the long-range outlook will have to wait until next week. Instead, this week I am going to try and tackle a subject on which I’ve received the most questions over the last couple of weeks, and that is just why there are semi-permanent areas of low pressure around Iceland and semi-permanent areas of high pressure over the Azores region of the Atlantic Ocean.

For much of this winter, the talk across the Prairies — at least most of the agricultural part — has been about the record-warm temperatures (hint for next week’s article). The explanation for these warm temperatures has centred on a weather pattern known as the Arctic Oscillation and its close cousin, the North Atlantic Oscillation. Early in January I wrote an article that discussed how these oscillating pressure patterns over the Arctic and Atlantic Ocean can impact the weather over our region of the world. Now the question I am getting is, why are these particular patterns in place to begin with?

Hopefully I will have enough room to fully discuss this topic this week, as it is a fairly broad topic. Several years ago I wrote a series of articles discussing the hows and whys of the Earth’s general atmospheric circulation patterns, so now it’s time to see how good your memory is!

The single cell

The fundamental driving force behind Earth’s weather is the fact that the equatorial regions receive an excess amount of energy (heat) while the polar regions have an energy deficit (more energy goes out than comes in). The second thing we have to keep in mind is that in general, warm air rises and cold air sinks. Now, if we put these two points together, we’ll see that all the extra heat around the equator wants to rise up and all the cold air around the poles wants to sink.

Now, in your mind’s eye you can picture the cold air sinking at the poles. It hits the ground and the only direction it can flow is toward the equator. At the same time, the warm air at the equator is rising up and then hitting the top of the troposphere (the part of the atmosphere where almost all weather takes place). When this warm air hits this point (known as the tropopause) it can’t rise any farther, so now it has to spread out horizontally. This means that the only direction it can flow is toward the poles. We now have a simple circulation pattern where warm air rises at the equator and then flows toward the poles, while cold air sinks at the poles and flows toward the equator. This is known as the single cell model of the atmosphere. If only it were this simple!

The trouble with this model is that the Earth rotates and this creates all sorts of problems. The rising air at the tropics, flowing toward the poles, begins to curve, primarily due to the conservation of angular momentum. By the time this air reaches about 20-30 degrees latitude, it is flowing from west to east. With more air continuing to rise up and move into this region, the air begins to pile up and the only place for it to go is downward. Since downward-moving air creates regions of high pressure, this creates a band of high pressure around the Earth at these latitudes.

The same thing happens with the air at the poles moving toward the equator. As it travels toward the equator, it is curved, and ends up flowing from east to west. This curvature takes place near 60 degrees latitude. Again, air flowing toward the equator will pile up in this region, but now the only place for it to go is upward, and upward movement of air is associated with areas of low pressure. This gives us a region of low pressure circling the globe around this latitude.

So we now have an equatorial cell of low pressure and rising air at the equator, sinking air and high pressure around 20-30 degrees latitude, and another region of sinking air and high pressure at the poles, with rising air and low pressure around 60 degrees latitude. This now gives us a basic understanding of why we have what is known as a semi-permanent area of low pressure known as the Icelandic low, and the semi-permanent Azores high. The reason they are called “semi-permanent” is that they undergo seasonal changes in both intensity and position. The position and strength of these features is controlled by the amount of energy entering and exiting each part of the system we have described. If a change occurs to one part of this system, it will have an effect on the whole system — and it gets even more complicated. Heck, we haven’t even talked about the area in between these two cells, namely the areas between 30 degrees and 60 degrees, which just happens to be our own neck of the woods.

See? I knew I would run out of space.

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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