Hello, my name is David Schultz. Welcome to Our Earth, Its Climate, History, And Processes. In this lecture, I want to talk about the polar jet streams. These are regions of strong winds, five to ten kilometers above the surface of the Earth. That control much of the weather we experience in the middle latitudes of both hemispheres. To do this we are going to use the Build Your Own Earth application and specifically the current day 2015 simulation. We're going to look at the average January pattern of the winds at 250 hectopascals. Remember this is equivalent to about 250 millibars, or about 10 kilometers above the surface of the Earth, at the top of the troposphere or in the lower stratosphere. This means that there's about 75% of the mass of the atmosphere below this level. The color shading on this map represents the wind speed in meters per second and the line segments indicate the wind direction. If you think of an arrow in flight then the tail feathers on these line segments represent the feathers on the arrow, so the arrow is flying generally from west to east on this map. The two bands of high wind speeds, one in each hemisphere, are the two jet streams. The jet stream in the northern hemisphere is stronger than it is in the southern hemisphere, because it's January, it's winter in the northern hemisphere and there's a greater difference in temperature between the pole and the equator. Here I've drawn a blue line approximately tracing the jet stream. As we go to the next map, which is a map of the air temperature at 500 hectopascals or about five and a half kilometers above the surface, we see a close correspondence between the gradient and temperature in the middle troposphere and the jet stream. If we go back to the winds at 250 hectopascals, we see a prominent feature of the northern jet stream being these undulations in the jet that we call Rossby waves these undulations will extend pole-ward and equator-ward. Where the jet stream dips equator-ward, cold air from the polar regions is moving equator-ward. Where the jet stream moves polar-ward in one of these undulations, then that is bringing warm air towards the pole. Now if we dab a little bit more Into what this graph is showing us. We see three ridges in the jet stream. These are stationary Rosby waves that occur over three major mountain regions in the northern hemisphere that the jet stream passes over. The Rockies, the European Alps, and the Tibetan Plateau. Following these ridges we the opposite a trough, a little bit digging southward of the jet stream, followed by an amplification of the wind speed in the jet stream downstream to the east of these ridges. Now it's significant to follow a line of constant latitude. In other words, a location that would get the same amount of solar radiation throughout the year. We can see that the jet stream deviates from the this line of constant latitude. Sometimes it's from the north. Sometimes it is further south. Specifically, you can see, that in eastern north America, it dips south of the line. And in western Europe, it dips to the north of this line. This ridge over western Europe, is one of the reasons why the average temperature in western Europe is higher compared to say Eastern Europe or Eastern North America. When we go to the southern hemisphere winter, which happens in July, and look at the position of the jet stream, we can see that without these same scales of mountain ranges we have in the Northern Hemisphere. The jet stream has fueled meanders, and the meanders are simply not as big as they are in the northern hemisphere. In the southern hemisphere, the Andes are the biggest chain that affects the jet stream, but these mountains despite their quite impressive height, are quite thin in the east west direction compare to the much broader Rocky Mountains. That's why the Rockies do more to alter the path of the jet stream in the northern hemisphere than in the Andes do to alter the path of the jet stream in the southern hemisphere. And so, we see a straighter jet stream around the southern hemisphere with fewer stationary Rossby Waves. And again, here is the map of 500 Hectopascal temperature in July, showing that the jet stream lies along this gradient and temperature. One of the things that I wanted to impress upon you is the role that these mountains play, not only in the climate by affecting the path of the jet stream, but also in terms of the day to day weather. Specifically, when moist air flows up the side of mountains, it can enhance precipitation on the windward side of the mountains. This enchanced precipitation can lead to heavy rains, flash flooding and landslides on the weather scales. On the climate scales, we have enhanced erosion leading to steeper slopes, steeper river valleys. The formation of glaciers and increased vegetation. Thus, the lithosphere itself alters the atmospheric circulation, the hydrological cycle, and the biosphere. Again, showing how intimately linked the components of the earth's climate system really are. So to summarize today's lecture, we talked about the role of the jet stream and how it controls the weather in the mid latitudes. We talked about how the jet stream separates cold, polar air from relatively warm, tropical air. We also talked about how the jet stream is stronger in the winter hemisphere, and that the flow is diverted polar over these major mountain ranges, producing the ridges and downstream troughs that we see. The stationary Rossby waves. And we talked about how the ascent of moist air up a mountain can enhance precipitation on the windward side of the mountain.
