In these politically-charged times, we can take some solace in knowing that, what's happening in terms of the weather, is always going to be a safe topic. But one thing we have to remember about weather, our local weather, is it's just really just part of something much bigger. It's part of a grand global pattern of atmospheric circulation. So let's talk a little bit about how the Arctic fits into this whole global weather machine. A key point. Now, how does this whether machine work? Basically, there's less solar heating in the Arctic latitudes than in the lower latitudes. What that does, is it forces a north-to-south-to north temperature gradient in the atmosphere. That is, the atmosphere is warmer to the south than it is to the north. Well, one thing about nature, we can always be sure of is nature abhors gradients. Doesn't want to see that temperature gradient. So what happens is this poleward energy transport that work to try and destroy that temperature gradient, mostly by the atmosphere and our friends, cyclones and anticyclones, play a big part in that. Now also, these transports tend to be bigger in the winter than they do in the summer. Now, this differential solar heating. Here's how it works. Think about the lower latitudes. The sun hits the earth's surface, the atmosphere and the earth's surface at a fairly direct angle. Whereas in the higher latitudes, the sun's rays strike the surface at a more grazing angle, so the energy is spread over a larger area. So not as much heating. So stronger heating to the south, than to the north. Well, it makes sense. It's warmer in Hawaii than it is in Resolute Bay in the Canadian Arctic. Well, this is what sets up a temperature gradient in the atmosphere and remember that nature abhors gradients. Now here's the pattern of solar radiation at the top of the atmosphere averaged over the entire year. What you see is this basically symmetric by longitudes, so it just varies by latitude. The strongest heating is in lower latitudes that shown in the yellows in this figure. It's something like 400 watts per square meter. That's averaging it over the entire year, night and day, just the long-term average. Then you get to those reds as you get into the higher latitudes, and those basically then turn into the white at the highest latitudes where you get the least solar energy coming in. So that's the pattern you have at the top of the atmosphere, and that's just the solar radiation coming in from the sun, right at the top of the atmosphere. Now, that solar radiation that hits the surface reaches the surface, the pattern is a little bit different. Number 1, you have less solar radiation coming in overall and the pattern at the surface is more complex. So you see this isn't the same scale as the previous one. Yeah, more in the lower latitudes, than the higher latitudes. But overall it's a bit of a more complex pattern. Here's a question. Why that more complex pattern at the surface than at the top of the atmosphere? There's several reasons, as it turns out. Reflection by clouds. Some of that solar radiation coming in at the top of the atmosphere, hits a cloud and is reflected back out into space. Also, scattering by nitrogen and oxygen molecules in the atmosphere. Absolutely, we get solar radiation coming in, but scattered by the atmospheric gases and some of that just goes out into space. Variations in atmospheric path length. That's another one. If the sun is low on the horizon, like you do have it generally at the high latitudes, then it effectively passes through a thicker part of the atmosphere because coming in at that more grazing angle that means more absorption in the atmosphere, and that means more scattering as well. So all of these factors can affect what we get at the surface, meaning that the pattern of incoming solar radiation at the surface, looks somewhat different, than at the top of the atmosphere. But the point is, you still get a lot more at the lower latitudes than in the higher latitudes. Now, I mentioned that because of that differential solar heating, we have a temperature gradient in the atmosphere. Higher temperatures in the equatorial regions, lower temperatures in the higher latitudes. Well, that is also then allied or results in a change or a latitudinal pattern in pressure heights above the surface, but the height of certain pressure surfaces. Now, what we have to remember about this, the important thing to remember is that if we have higher pressure heights in the lower latitudes, lower pressure height in the higher latitude, that's setting up a pressure gradient. And a pressure gradient is going to initiate an atmospheric flow from the lower latitudes into the higher latitudes, transporting that atmospheric heat from the low latitudes into the higher latitudes. Now, there's a lot more going on in this. The earth is rotating on its axis, so we have something called the Coriolis force to reckon with. Let's not worry about that right now, because I think we can understand the basics here of what's going on, by understanding this issue that there's differential solar heating, sets up a temperature gradient, sets up a pressure gradient in the atmosphere, nature abhors gradients. What happens? We have an atmospheric flow of the warm air from the higher latitudes down into the lower latitudes. Now lets think about these atmospheric transports. A lot of that transport is by cyclones and anticyclones, our friends the cyclones and anticyclones. Now what they have to do is destroy that equator to pole temperature gradient. That's their job, is to get rid of that temperature gradient, by transporting the warm air pole-ward and the cold air back down equator-ward. This is what we associate with a lot of our weather. When a weather pattern goes through, when an extra tropical cyclone's going through, it's just doing its job of transporting energy pole-ward, and the warm air pole-ward and the cold air equator-ward. Now the oceans also transport energy. We would never forget that. But here we're just talking about the atmosphere. Now how does this transport work? I mentioned that a lot of it is by cyclones. Because what does a cyclone do? Remember that a cyclone, the flow around it is basically counterclockwise. Now in the southern and eastern parts of that cyclone the winds have a component from the south, bringing warm air pole-ward. On the western side of the cyclone the winds have a pattern of winds coming from the North, which transports the cold air equator-ward, warm air pole-ward, cold-air equator-ward, mixing up the air properties and trying to get rid of that temperature gradient. That's the job of your extra tropical cyclone. If you look at a surface weather map across the Arctic, this is one for the North or North Atlantic. Greenland is at the top left. What you'll see is that there's all kinds of cyclones around and anticyclones, because the anticyclones are involved in this whole process too. It's a very chaotic looking pattern. But overall all these cyclones and anticyclones constantly forming, constantly moving, conforming and dying. They're all doing that job of transporting energy pole-ward. Now this is a view of the northward energy transport in petawatts. That's a lot of watts. It's looking at it by month on the x-axis, December through January. On the y-axis its latitude from 70 South up to 70 degrees North. Now let's just look towards the top of it. You see I've got it marked winter in both of these places on the left and on the right. These are the transports in winter, in the middle you see this one marked summer. The bright reds are the winter ones, and you see those are when the transport are biggest. Where those transport's really biggest, around 40 degrees North latitude. That's about the maximum size of the transport, is around 40 degrees North latitude. Now they're bigger in the winter than they are in the summer. Now on the South, you see it's in the blue. The difference here is that we're looking at northward energy transport, and so if it's a blue it's actually negative because it's actually moving to the South. So it's just a sign convention. But let's just think about what's going on in the North, bigger transports in winter than we have in the summer. Here's the question. Why are those atmospheric transports bigger in the winter than they are in the summer? The answer is, that equator to temperature gradient is bigger in the winter than it is in the summer. Because remember what's happening in the summer, the Northern Hemisphere is basically tilted away from the sun so the Arctic latitudes get little or no solar radiation while the lower latitudes are still getting a lot. So we have a big temperature rate, big differential solar heating in winter, and that needs a bigger temperature gradient, than in the summer when the Northern Hemisphere is tilted toward the sun, and so there's a less of a latitudinal change, or there's less of a gradient in the solar heating, and so less of a temperature gradient, and the transports are then smaller. This makes sense. Think about when do we have the real strong cyclones coming through, winter or summer? It's winter. That's why we have nor'easters in the east and big snowstorms associated with storms elsewhere, it's really winter, not so much summer, and it all has to do with the fact that the temperature gradients equator to pole are bigger in the winter because the distribution of solar heating is much more extreme in the winter than it is in the summer. With that I hope we've learned a little bit more about how the Arctic fits into the grand scheme of the global atmospheric circulation. Thank you.
