[MUSIC] Welcome back. In this lesson, we'll learn about the climate of the Arctic and the way the climate interacts with the rest of the Earth's system. Last time, we saw how the Earth receives energy from the Sun and how heat is trapped on our planet through the natural greenhouse effect. Now, we'll discuss how this energy is distributed across the globe and how it is transferred from the tropics to the poles via the atmosphere and the ocean. We'll learn the difference between the natural and the human induced greenhouse effects and discuss what the future holds for Arctic climate. As you will see, the global climate system is tightly linked and climate change in the Arctic also effects temperate latitudes. In the last lesson, we saw that incoming solar radiation is not distributed equally across the Earth's surface with the tropics receiving more annual insolation than the poles. Why is this? Select all the correct answers. A, due to the earth being a sphere. B, due to the Earth's tilt. C, due to the 12-hour Polar Night. D, due to the different albedo across the Earth's surface. Answers A, B and D are all correct. The Earth is a tilted sphere, which means that the rays of the sun arrive at the tropics at a more direct angle than at the poles where the angle becomes more oblique. And thus, less intense. As the Earth orbits around the Sun over a year, either the Northern or Southern Hemisphere is tilted towards the sun. The side that is tilted towards the Sun experiences summer and it's respective pole has 24 hours of daylight. The reflectivity of the Earth's surface will determine whether a surface absorbs or reflect insolation. Light-colored surfaces such as sea ice on the Arctic Ocean or the glacial ice of the Greenland ice sheet will reflect most of the insolation. Darker-colored surfaces, such as the tropical rainforest absorb insolation. The latitudinal variation in incoming energy from the Sun leads to an imbalance between the tropics and the poles. This causes Earth's large-scale circulation systems to transport heat from the equator to the polar regions. This heat exchange occurs through three major mechanisms. First, through the atmosphere. Warm moist air rises in the tropics and carries heat towards the poles. Secondly, via the oceans. Both the wind-driven circulation and the global thermohaline circulation play roles. The thermohaline circulation is driven by temperature and salinity and is sometimes called the ocean conveyor. In its simplest form, warm, salty surface water in the tropics flows towards the poles becoming cooler and denser as heat is lost to the atmosphere along the way. This cool, dense deep water flows back towards the tropics in the deep ocean. Eventually, upwelling to the surface to complete the loop. The third major mechanism is evapotranspiration, which is the total transfer of moisture from liquid form to gaseous form. This includes evaporation from all water bodies, but mostly the oceans. It also encompasses transpiration, the expulsion of water vapor by land plants. Transpiration is a lot like perspiration, which humans and other animals use to stay cool. Once water vapor is in the atmosphere, it is transported towards the poles via the atmospheric circulation when it condenses from gas back to liquid it releases heat. Evapotranspiration is a fundamental part of atmospheric and oceanic circulation with water vapor transporting an enormous amount of heat throughout the globe. As a result, we will discuss evapotranspiration as part of the atmosphere and ocean circulation systems starting with the atmosphere. Atmospheric circulation not only moves heat from the tropics to the poles, but also drives weather patterns and ocean currents. It is also an efficient way of transporting pollutants all across the globe and particularly to the Arctic. Volatile chemicals like persistent organic pollutants or POPs, that include many pesticides are transported by the atmosphere to the Arctic where they concentrate and accumulate. We can divide atmospheric circulation into three main levels. First, there's the very large global scale. Second, there is the medium scale of high and low pressure areas. And third, there's the small local scale of weather and wind patterns. We'll spend most of our time at the global scale, though we'll also look at the medium and small scales in the Arctic. To understand atmospheric circulation, we need to review some atmospheric basics. At the global scale, wind is the mainly horizontal movement of air across the Earth's surface. Locally, wind can also move vertically. Winds are described by the direction they come from. So wind moving from the west to the east is called westerly. Wind is mainly a function of the difference in air pressure between different locations with wind carrying air from higher to lower pressure regions. Four major forces determine the speed and direction that winds take. The first force wind is affected by is gravity. Gravity causes the vertical distribution of mass, density and pressure in the atmosphere. It pull the majority of the mass in the atmosphere close to the Earth's surface. This means the density of the air decreases with increasing altitude. The gravitational force interacts with buoyancy, which is the tendency of fluid to expel less dense material. This causes less dense warm air to rise and denser cold air to sink. This is the principle that hot air balloons are based on. The second force is the pressure gradient. This causes air to move horizontally from more dense, higher pressure regions via atmosphere to less dense, lower pressure regions. Air will move faster with stronger pressure gradients. The third force is the Coriolis force, which is driven by the Earth's counterclockwise rotation. This deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. To understand these Coriolis deflections, it is important to note that the directions of the deflections are from a viewer's perspective, looking in the direction the object is travelling. Lastly, the friction force acts against the Coriolis force due to drag as air moves near the Earthy's surface. This results in wind spinning tangentially away from areas of high pressure and into areas of low pressure. This all may seem complex, but many of you will be familiar with these forces from television weather reports, typically in the different rotation of winds around areas of high pressure compared to areas of low pressure, because winds also affect the ocean. These forces also influence ocean currents, which are responding to the same major forces. We've seen that the Coriolis force causes air to deflect to the right in the Northern Hemisphere and to left in the Southern Hemisphere. To help understand this principle, which of the following statements are true? Select all the statements that are correct. A, air travelling east in the Northern Hemisphere will be deflected towards the south. B, air travelling south in the Southern Hemisphere will be deflected towards the east. C, air travelling north in the Northern Hemisphere will be deflected towards the east. D, air travelling north in the Southern Hemisphere will be deflected towards the west. Each of these statements is true. In the Northern Hemisphere, the deflections are to the right of the initial motion and they are to the left in the Southern Hemisphere. Just remember that the deflection is from the perspective of the direction the air is travelling to begin with. At a planetary scale, atmospheric circulation is driven by pressure gradients leading to three large-scale loops or cells. They operate meridionally, moving north, south in the atmosphere in both the Southern and Northern Hemispheres. In these cells, winds at the lower and upper levels travel horizontally linked by vertically ascending and descending motion. From the equator to the pole, these cells are known as the Hadley cell, the Ferrel cell and the Polar cell. Each cell operates within the Earth's troposphere, the part of the atmosphere between the surface and about ten kilometers up. In each cell, warm air rises from the surface and travels meridionally. As it does, it loses heat and descends as cold dry air towards the surface and then returns meridionally to the starting point. Air within the Hadley cells and the Polar cells rises nearest the equator, whereas Ferrel cell error rises closest to the pole traveling the other way. The results of the cells are global belts of relative high and low surface pressure. At the equator where insolation is at a maximum air in the Hadley cell rises, causing persistent low pressure troughs. Where Hadley cell and Ferrel cell air descends, a belt of persistent subtropical high pressure is found. Similar patterns are seen globally. Low pressure where Ferrel and Polar cell air rises and high pressure where polar cell air descends. Smaller scale, secondary, high and low pressure regions different from the global belts form within the primary high and low pressure belts. These move with the seasons, leading to the changing weather and the development of storms. The Hadley cell operates meridionally from the equator to about 30 to 40 degrees latitude in each hemisphere. The abundance of heat available at the equator causes warm, less stance air to rise. The convergence of surface winds towards the equator rotated toward the west by the Coriolis force, produces the easterly trade winds. The convergent air is full of moisture, this is a result of evapotranspiration. As it continues to rise, the air expands due to decreasing pressure and then cools down. The expansion of air and the condensation of water vapor causes heat release and reduced density as it rises. This means continued rising through a positive feedback loop and plenty of rainfall. This positive feedback loop is due to the fact that as the air rises, pressure decreases and so it expands. This allows the air to cool and if it cools enough, moisture condenses. Condensation releases heat, which lowers the density causing the air to rise some more and so on, creating a feedback loop. The band of low pressure near the equator experiences heavy rainfall from condensing moisture and is called the Intertropical Convergence Zone or ITCZ. The ITCZ moves according to the seasons, shifting to the South in the Northern Hemisphere winter and to the North in the Northern Hemisphere summer. Hadley cell air in the upper troposphere continues to cool and lose moisture. Eventually, descending as dry air between 30 and 40 degrees latitude. This extremely dry air facilitates the global band of high pressure and dry climate at these latitudes. This region includes Africa's Sahara desert, the Arabian Peninsula and the American Southwest. The air circulation of the Ferrel cell is opposite to that of the Hadley cell. Air rises near the poles up to about sixty degrees and descends in the subtropics between 30 and 40 degrees. Unlike the Hadley cell, Ferrel cell circulation weakens during winter and strengthens during summer. Surface air that diverges due to the ascending air of the Hadley and Ferrel cells forms Earth's principal surface winds. These are the easterly trade winds within the Hadley cell and the westerly's within the feral cell. In each case, the direction results from the Coriolis force. Rotating winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Polar cell operates over the polar region of each hemisphere. Relatively warm, moist air rises at about 60 degrees traveling toward the poles. As it does, it descends to the surface as cold, dry air and then moves back towards lower latitudes. The surface's expression of this circulation are the polar easterlies who direction, again is set by the Coriolis force. The polar easterlies are quite variable, but generally weak compared with the trade winds and westerlies. In the Northern Hemisphere, there are two primary regions of persistent low pressure in a belt at about 60 degrees north caused by rising warm moist air entering the feral and polar cells. Why do these low pressure zones grow during Northern Hemisphere winter and shrink during Northern Hemisphere's summer? Select all the answer you think are correct. A, because of the shrinkage of the subtropical high pressure cells in Northern Hemisphere winter. B, because the subtropical high pressure cells grow in Northern Hemisphere winter, along with the low pressure systems around 60 degrees North. C, because of the equal distribution of annual insolation around the Earth's surface. D, because of seasonal differences in insolation. If you chose A and D, you are correct. The primary subpolar low pressure systems in the Northern Hemisphere are the Aleutian Low in the Pacific Ocean and the Icelandic Low in the North Atlantic Ocean. They're especially pronounced during winter, but diminish during summer as the subtropic high pressure cells grow. Ultimately, these large-scale high and low pressure systems are a result of insolation differences between the seasons. The Aleutian and Icelandic lows are sites of persistent cyclonic activity and are primary generators of Northern Hemisphere storm systems, particularly in winter.