Hello, my name is David Schultz. Welcome to Our Earth, Its Climate, History, and Processes. In this lecture, I want to talk about the atmospheric circulation. This is the processes that occur on the scale of the planet that control the winds and the distribution of rain and snow, in other words, precipitation. As you may know, the tropics get more energy than the polar regions and it is this imbalance in the heat obtained by the earth that leads to circulations in the ocean and the atmosphere. But if the earth didn't rotate, then there would be just one giant circulation cell with rising warm air in the tropics, that air rising up and then cooling at the polar regions and then descending at the polar regions and returning back to the equator. Because the Earth rotates however, we end up with a slightly more complicated circulation pattern, shown here. Near the equator, we have the trade winds in each hemisphere that come together, and rise up in what's called the intertropical convergence zone. This air flow, then extends poleward both north and south at high elevations and eventually, descends in the subtropics. These circulations that encompass the tropical to subtropical regions are called the Hadley cells. This region of descent in the subtropics, produces what are called the subtropical high pressured regions at the surface. These regions are also called the horse latitudes. Within this region of descent, we see relatively few clouds and not a lot of precipitation. Many of the world's deserts occur in the subtropics. Moving towards the poles, there's cold air forming at the north pole and south pole due to the lack of solar radiation and the continual emission of heat away from the Earth. However, this cold air meets up with warm air coming out of the southwest in the Northern hemisphere, and out of the northwest in the Southern Hemisphere. These are the Prevailing Westerlies. Where these two air masses meet on average, there's a region of strong temperature gradient between the polar air and the air from the subtropics. We call this the polar front and both hemispheres have one. It's along these polar fronts that higher up in the atmosphere, say 5, 10, 12 kilometers above the surface, we find the polar jet stream. The polar jet stream occurs in the mid-latitudes and is a region of active weather. Along the polar jet stream form a sequence of low pressure systems and high pressure systems that move eastward. We call these lows, cyclones, and the highs, anti-cyclones, so if you live in the mid-latitudes, you may have heard the phrase, that the weather tends to come from the west. It's because the weather is following the polar jet stream from the west. Now, let's see how we build this schematic picture into the context of Build Your Own Earth. I'm going to start with this aquaplanet simulation and what the aquaplanet is, is simply an Earth covered in oceans with pre-industrial level of greenhouse gases. I've selected the aquaplanet because it's going to show us a simplified atmospheric circulation pattern, so you can easily see these schematic patterns on a real climate simulation. Let's first look at the surface winds in the Northern Hemisphere winter. This is going to be January. The color shading here represents the wind speed in meters per second, and the line segments indicate the wind directions. The more barbs that are on these line segments indicate the stronger winds and the line segments themselves, where they're pointing towards, indicate the wind direction. Think of an arrow in flight, the symbol points in the direction that the winds are moving to, with the barbs in the direction the wind is coming from. Now you can see the northeasterly trade winds in the Northern Hemisphere quite strongly and the Prevailing Westerlies, too, although they're a bit weaker. About 10 kilometers above the Earth's surface or 250 hectopascals in atmospheric pressure, we find the polar jet stream. Both hemispheres have one but in this case, the wind speeds are slightly higher in the jet stream that is in the hemisphere experiencing winter, so this is going to be the Northern Hemisphere. Now you can go into Build Your Own Earth and plot the winds above the surface and loop through the months to convince yourself of this fact. So that you can see the winds in July when the Southern Hemisphere is in winter that a polar jet stream is stronger in that hemisphere. Now if we look at the surface pressure pattern for the aquaplanet, we see that the tropics have relatively low pressure, and the subtropics have these relatively high pressure regions, again, the subtropical anticyclones. If we look at the fraction of time that an area on the aquaplanet is covered by clouds, we see that the intertropical convergence zone is a region where the clouds are common and the subtropical anticyclone region is relatively free from clouds, as we would expect because of the descending air in the atmosphere. Now if we look at the precipitation pattern that goes along with this, we see more rain falls along the intertropical conversion zone than in the subtropical anticyclone. Now it turns out that you can see these features on the real earth, too, on any given day. This is a satellite image of the clouds from space. The satellite here is situated over the equator, looking down on the eastern part of Asia and Australia. Where there are clouds, there are largely rising motions in the atmosphere. We have this region of generally cloudy air rising near the surface associated with the Inter Tropical Convergence Zone and here you can see regions of enhanced cloudiness that roughly span the tropical regions. Not continuous,but there are breaks but that's the Inter Tropical Convergence Zone. Remember that there's descent in the subtropics and sure enough, Australia is largely cloud free, as is the Southern Indian Ocean and the Middle East. In the mid-latitudes, low pressure systems, which on the satellite imagery here are going to be comma-shaped cloud patterns, these traverse from west to east. And this is going to be particularly apparent in the Southern Hemisphere where you see several of these comma-shaped cloud patterns in the Southern Hemisphere. We can also see snow in the Himalayas. Those aren't clouds, that's snow and in this case, we can also see a tropical cyclone, in this case, super typhoon Haiyan approaching China. So even though this is an instantaneous picture of the weather at any given time, we see many of these properties of the average global circulation appearing in this satellite image. So to summarize this lecture, it's the imbalance in solar heating between the tropics and the poles that drives the atmospheric circulation. Because of the rotation of the Earth, we have a richer circulation on the planet then we would otherwise. The circulation features the Inter Tropical Convergence Zone, which is a broken but persistent band of clouds that occur in the tropics, that also features corresponding descent in the subtropical anticyclones because what goes up must come down somewhere. These subtropical anticyclones are responsible for the major desert regions of the world. Finally, we have the convergence and rising air along the polar front, the jet stream aloft, and these features are responsible for producing the extra-tropical low pressure systems and high pressure systems that bring much of the weather to those of us living In the mid-latitudes. [BLANK_AUDIO
