A couple of weeks ago I started an article about the North Atlantic Oscillation or NAO. We began our look at this particular weather pattern because all of the talk about the unusually warm winter so far this year seems to be tied to it. To understand just what is going on we had to learn some basics about the general atmospheric patterns of rising air around the equator and sinking air at the poles. We then looked at how these features can lead to a band of high pressure located around 20 degrees latitude and a band of low pressure around 60 degrees latitude.
If we try to picture this we would see that the Earth has two distinctive circulation patterns. We have warm air rising at the equator, flowing towards the poles then sinking back down to the surface around 20 degrees latitude. This air then hits the ground and some flows back towards the equator, completing the loop, while the remainder of the air flows towards the poles along the ground.
Over the poles air is sinking and flowing along the ground towards the equator. Around 60 degrees latitude it begins to rise, creating a region of low pressure. This rising air hits the top of the troposphere (the part of the atmosphere where most of the weather takes place) and at this point it can’t rise any higher. So this air can flow in two directions — either back towards the poles, thus completing that loop, or it can flow back towards the equator where it will eventually hit the region around 20 degrees latitude where the air is sinking. This air will then also sink back to the ground.
Now we have two loops taking place, one over the poles and one around the equator. For those of you who are really good at picturing this, you’re probably noticing that there is a third atmospheric circulation or loop between these two loops. This just happens to be our part of the world. This third loop is basically driven by the two other loops. At the surface, air is flowing towards the poles from the semi-permanent area of high pressure and at the same time air is being pulled northward along the surface by the semi-permanent areas of low pressure. If the Earth didn’t revolve then it wouldn’t get much more complicated than this, but the Earth does revolve, and this causes the air to be deflected or to curve. So instead of this air moving straight northward it curves and becomes westerly, which is why most of our weather systems move from west to east.
Now back to the NAO. When the regions of low and high pressure over the Atlantic are both very strong, the NAO is said to be in a positive phase. This results in stronger-than-usual westerly winds, especially over North America. Think of these two features like a pair of spinning wheels — the faster they spin the quicker they pull air between them. This faster flow of air helps to keep the really cold air up north and any cold air that does slide southwards is quickly pulled off to the east. This is the pattern that we have seen for most of this fall and winter. In fact, during December it was at its strongest positive phase ever recorded.
The opposite phase, or negative phase of the NAO, is when these regions of low and high pressure are weak. This results in a slackening of the westerly winds across North America. Now the cold air building over the poles has an easier time moving southwards and when it does there are no strong westerly winds to push this air out. This results in more cold air outbreaks, and longer-lasting ones as well.
Tied almost directly to this is another atmospheric circulation pattern known as the Arctic Oscillation or AO. This oscillation is a comparison of pressure differences in the upper atmosphere between the Arctic and the Atlantic. During the winter, in the upper atmosphere over the Arctic, there usually develops an area of low pressure known as the Arctic vortex. When this is strong, which means the pressure is lower than usual, and the region of high pressure over the Atlantic is higher than usual, the AO is said to be in a positive phase. Just like with the NAO, a positive phase results in stronger-than-usual westerly winds which mean fewer cold air outbreaks. The negative phase of the AO sees a weaker Arctic vortex than usual, and this results in weaker westerly winds and more and longer outbreaks of cold air.
Now, you would think that if the NAO is positive then the AO would also be positive and vice versa, but this is not always the case. Next time we’ll take a look at this and then try to tie everything together and hopefully come to some understanding of why this winter’s long-range forecasts were so wrong!