Circulation of the Atmosphere, the Coriolis Effect, and Climate
(Reading #4 for my course on Climate Change, Alan Holyoak, PhD)
1. Be able to explain why there is a difference in the net radiation at different latitudes.
2. Be able to explain what latent energy is and how the ITCZ is formed and distributes latent heat directly to latitudes as high as 30oN and S.
3. Be able to list the three types of cylindrical cells of moving air in each hemisphere, as well as how they are formed.
4. Be able to describe the difference between an absolute and a relative frame of reference, and how the Coriolis Effect acts on an object moving over a relative frame of reference.
5. Be able to explain why wind currents moving over the Earth in the northern hemisphere always deflect to the right, and to the left in the southern hemisphere.
6. Be able to explain what jet streams are, and how they are indirect transporters of energy across middle latitudes.
Now that you know about solar radiation and Earth’s energy budget you are ready to look at how the atmosphere affects climate. You already know that the unequal heating of the Earth results in an unequal distribution of heat, with the tropics being warmer than higher latitudes. Interestingly, tropical heat doesn’t stay there, a variety of processes move it around the planet.
We will address three main questions in this reading:
1. What is the pattern of atmospheric circulation, and how are winds formed?
2. What role does the Coriolis Effect have on the movement of air masses?
3. How does circulation of the atmosphere move heat and impact climates at different latitudes on Earth?
Unequal Heating of the Earth Gets Things Started
Both the ocean and the atmosphere are involved in the transport of heat, and the movement of heat influences and creates climate.
As you already know, the orbit and tilt of the Earth produce large differences in the amount of solar radiation received at different latitudes. The tropics receive much more solar radiation than higher latitudes. At the same time IR is emitted by the surface. Recall that net radiation is the difference between absorbed solar radiation and emitted IR. If we calculate the average values of absorbed and emitted radiation for all latitudes over an entire year, we will see results that look like those in Fig. 1. There is always a surplus of net radiation in the tropics. At the same time, middle and high latitudes emit more infrared radiation than they receive, so they experience chronic deficits in net radiation.
The crossover point between net gain and net loss of radiation is at about 37.5oN and 37.5oS. If no other process mitigated this difference in differences in net radiation by latitude, the tropics would become unbearably hot while the rest of the world would experience extreme cold. Of course, this does not occur. Such a situation does not and cannot exist, because heat always flows from areas of higher heat to lower heat. There are also mechanisms that transport heat from tropical regions to higher latitudes. Such transport is conducted through both atmospheric and oceanic processes. The atmosphere is the focus of this reading.
Figure 1. The annual average of the influx of solar radiation (solid blue line), and emission of infrared radiation (dashed red line) by latitude. (Image courtesy of the Univ. of Wisconsin.)
There is always a surplus of net radiation in the tropics. What happens to this excess energy? You already know that most direct solar energy is used to evaporate water, regardless of latitude. The remainder of direct solar energy that is not reflected warms the land and oceans. If you recall, the First Law of Thermodynamics states that energy cannot be created or destroyed, so the energy used in evaporation of water is not lost, but where does it go? It is stored in the water vapor that is produced in a form called latent heat. When water vapor condenses back onto liquid, that latent heat is released into the atmosphere.
A huge amount of latent heat moves around the planet in water vapor. How can water carry so much heat? Water has an extremely high specific heat. The specific heat of a substance determines how much energy it can absorb before it changes temperature. Water’s specific heat is extremely high, so it can absorb a LOT of energy before its temperature changes (we won’t get into the nitty gritty of the details of how we know this, but suffice it to say, it’s a LOT), and water absorbs even more energy when it changes from liquid to vapor.
Since a lot of energy is required to evaporate water, the process of converting solar radiation into latent heat and then back into infrared radiation is extremely important to climate. The main stages involved in the processes of energy exchanges at the surface in tropical oceans are shown in Fig. 2.
Figure 2. The fate of positive net radiation at the ocean surface in the tropics. (Image courtesy of Dr. Hipps, Utah State University.)
Positive net radiation in the tropics creates warm, moist air near the ocean surface that is unstable, causing it to rise. Interestingly, the low density of air in the tropics is not only related to warm temperatures, but to its high water vapor content; moist air is less dense than dry air. This may be counterintuitive, yet it is the way air works. So, adding water vapor to warm air reduces its density, and increases its tendency to rise.
Rising air expands as it rises (due to decreasing air pressure) and consequently cools. When warm moist air becomes cool enough, water vapor condenses and clouds form. As condensation occurs, latent heat held in water vapor is released, warming the air around it and causing further vertical motion. This air that rises in the tropics is replaced by air moving in from the north and the south. This region where surface air converges as warm air rises and forms clouds is called the Intertropical Convergence Zone or ITCZ. This zone is characterized by an abundance of clouds and rainfall. The vertical and horizontal movement of air and moisture in the ITCZ is shown in Fig. 3.
Figure 3. The Intertropical Convergence Zone pulls air from the north and south to replace warm moist air that rises. (Image: ARH)
Latent heat is transported vertically in the ITCZ by two mechanisms: 1) warm moist air moves upward, and 2) latent heat moves vertically with the water vapor and is released when it condenses in the clouds, causing the air to move even higher. The second process is by far the more important. This movement of latent heat is vital to maintaining Earth’s climate system.
The ITCZ moves north and south with the seasons along with the latitude of maximum solar radiation. Now do you see why I had you spend all that time reading about the equator, the Tropics of Cancer and Capricorn, sunlight angle, etc.? If not, go back and read that again and it should make more sense in light of what you now know about the ITCZ. Anyway, as the ITCZ slides north and south it brings rain, and the ITCZ is thereby responsible for the seasonal wet climates of the tropics. In some tropical locations however it is dry except for the period when the ITCZ passes. For example, the seasonal north-south movement of the ITCZ provides rains that drive the massive migrations of large mammals of the African savannah. Figure 4 shows the approximate locations of the ITCZ in January and July.
Figure 4. Generalized location of the ITCZ in January (blue line) and in July (red line). Note how the northern limit of the ITZC in July lines up almost perfectly with the lower boundary of the Sahara Desert, and in January falls along the northern boundary of the Kalahari Desert in southern Africa. (Image courtesy of Dr. Hipps, USU.)
In reality, the location of the ITCZ varies from year to year. Sometimes it moves farther north or south than shown above, while in other years it does not move far from the equator. This variation in seasonal movement can have huge impacts on regions that depend on seasonal rains brought by the ITCZ. In some regions near the limits of the ITCZ’s range, people depend on it arriving each year to bring rain. In these cases the appearance of the ITCZ, or not, can be the difference between life and death.
Research by Shanahan, et al (2009) shows that multiple effects can influence climate conditions within the range of the ITCZ, particularly in the Sahel – the semi-arid region just south of the Sahara Desert. These factors include interactions between increasing surface seawater temperatures due to climate change and the effects of the Atlantic Multidecadal Oscillation, and have produced drought conditions that affect millions of people in the Sahel.
As the rising air of the ITCZ reaches the top of the troposphere it spreads out horizontally and moves to the north and south (see Fig. 3). By this point the air has already produced clouds by condensation, released much of its latent heat, and reached an altitude of several miles. The air cools further by the loss of infrared radiation that is released into space, and subsequently sinks; it is now cold, dry, and energy-depleted. The air returns to the Earth’s surface at about 30oN and S. The sinking air warms and forms regional high pressure cells as it descends. The air flows away from these high-pressure cells; some of it flows back toward the ITCZ and some of it flows to higher latitudes. This cyclic pattern of air movement driven by the ITCZ defines Hadley Cells (Fig. 5). Hadley Cells, like the ITCZ, also move north and south with the seasons.
Figure 5. Hadley Cells are located on both sides of the ITCZ, and movement of air in these cells is driven by the evaporation of water in the Intertropical Convergence Zone. (Image courtesy of Dr. Hipps, USU.)
Air that flows back toward the ITCZ across ocean surfaces warms and picks up water vapor as it goes. It does not, however, move directly back to the ITCZ. If it is moving from the north the Coriolis effect will deflect it to the right and if it is moving from the south it will be deflected to the left. A more detailed description of the Coriolis effect and is provided later in this reading. This deflection produces persistent surface air currents around these large, persistent, subtropical large high-pressure cells (Fig. 6).
Figure 7. This map shows the locations of the world’s major deserts. (Image: ARH, modified from Wikimedia Commons.)
The ITCZ and Hadley Cells are the key atmospheric features of climate in the tropics. They explain tropical wet climates, tropical seasonal wet and dry climates, and tropical regional deserts. They also move heat toward the poles, at least as far as 30oN and 30oS. If these cells were to change in size, or experience any shift in their north-south ranges, there would be significant changes in climate in a number of regions. Imagine that Hadley Cells were to grow larger in their latitudinal extent. How might this affect climate in various locations?
The Coriolis Effect, as mentioned above, is the tendency of moving objects that are not attached to the Earth’s surface, such as air or water currents, to experience an apparent deflection to the right in the northern hemisphere, and to the left in the southern hemisphere. We observe this apparent deflection because of our frame of reference.
There are two classes of reference frames. One is an absolute reference frame. This is an observation point that is fixed in space and does not move, and to which we compare the movement of a non-fixed object – like a car moving in the foreground with a mountain in the background. The mountain provides the absolute reference frame. The other class is a relative reference frame. This is a reference frame or point that is itself in motion, and from this kind of frame of reference we moving objects in relation to the moving frame of reference. It is important to understand what a relative reference frame is, because this is the kind of frame of reference that is most important when we consider the movement of atmosphere over a rotating Earth. An example of a relative reference frame is when you observe a car moving in the foreground, and there is a large semi tractor-trailer just on the other side of the car. Both are moving, but we notice the movement of the car relative to the moving truck. This might not make a lot of sense right now, but maybe another example will help.
Imagine that you have a flat disc on a table. The disc is spinning, but the table is stationary. A ball is rolled from the middle of the spinning disc toward the edge of the disc. If you observe the motion of the ball against the non-moving background of the table (an absolute reference frame) the ball appears to travel in a straight line. The ball’s path in this case is called the absolute path, since it viewed in relation to a non-moving background. OK, next imagine that you dip the ball in water or paint so that it leaves a mark showing the trail it followed from the center to the edge of the disc. When you do this you will see something very different – you as the ball rolls it will leave a mark showing its path. The trail is curved, not straight. This curved path is called the object’s apparent path. An example of each of these paths is shown in Figure 8.
All right, which of the two paths, the absolute path or the apparent path, is the ball’s true path? This might make your head hurt a little, but the true path depends entirely on which frame of reference is chosen. The fixed table results in a true straight path, and the spinning disc results in a true curved path. This means that motions are observed based upon the reference frame that we have decided to use.
There are a number of visual examples of this effect. An animation of the Coriolis Effect as an object moves over the rotating Earth can be seen at:
Watch this video as many times as needed to get the idea that while the rocket is actually flying in a straight line in relation to the fixed frame of reference of space behind it, when it flies over a moving frame of reference – the spinning Earth – the path of the rocket deflects to the right relative to that frame of reference. This happens because the Earth actually rotates under the rocket’s path. If you were to observe the rocket flying from the equator toward the North Pole we would still see deflection because the rocket has momentum when it takes off. By the way, the farther the rocket flies, the greater the deflection is, no matter its starting point or initial direction. The Coriolis Effect plays a major role in Earths’ climate because the movement of air over the spinning Earth is also deflected.
It took some pretty serious thinking to figure out the mathematics that explains why this deflection happens. We won’t worry about mathematical proofs for the purposes of this course (whew!), but what a 19th century mathematician, Gustav Gaspard de Coriolis, discovered was that the size of the deflection effect is proportional to the relative velocity of motion between the moving object and the relative frame of reference. This means that the faster the rotation of the relative reference frame, and the longer the object moves over the frame of reference, the greater the amount of deflection from a straight line there is. He also discovered that the amount of deflection is also dependent upon the latitude. Think about that for a minute…why would latitude matter?
Imagine someone standing on the North Pole, and they have their arms extended straight out. Over a 24-hour period, what motion will that person experience? The left-hand image in Fig. 9 shows that there will be one complete rotation of their body with the axis of the Earth extending through the center of their body, but their body would not have actually gone anywhere – they just spin in place. This is 100% rotational motion. Now imagine someone standing on the equator, facing north. After 24 hours, what motion will they have experienced? The right-hand image in Fig. 9 shows that they have changed location in space, but they will not have spun at all! This is 100% translational motion. An object experiences increasing amounts of rotational motion and decreasing amounts of translational motion the farther it gets from the equator. And, because the Coriolis Effect is an effect of rotation, the higher the latitude, the stronger the Coriolis Effect!
Figure 9. An example of 100% rotational motion in the upper image, and an example of 100% translational motion in the lower image. (Figures: ARH.)
It is also important to know that the Coriolis Effect changes only the apparent direction of an object, not its velocity. This means that this is not a true physical force like gravity or centrifugal force. The deflection is only an apparent acceleration that is simply due to the relative frame of reference.
It takes a long period of time for the Coriolis Effect to become significant. We don’t notice it in our daily lives, since we usually observe motions that are of only short duration. Coriolis effect does, for example, affect the path of a baseball thrown from the outfield to the infield, but its flight duration is sufficiently short that the effect is minimal. We, however, feel the effects of the Coriolis Effect all the time in the form of prevailing surface winds. OK, now you are ready to consider what happens with the atmosphere at the middle and high latitudes.
The Middle and High Latitudes
Now it’s time to take a look at air movement at middle and high latitudes. To start with, to make things easier, let’s imagine only what is happening in the northern hemisphere of Earth and imagine that Earth is not spinning (i.e., no Coriolis Effect).
Sinking air forms subtropical high-pressure regions at about 30o N, and air from this high-pressure region flows north and south when it reaches the surface (see the orange arrows in Fig. 10). At the same time, cold, dense air slides moves south, away from high-pressure cells at the pole (see the blue arrows in Fig. 10). This cold polar air collides with warmer subtropical air someplace in the middle latitudes, usually around 60oN.
Figure 10. Movement of air across the surface of a non-spinning Earth, showing only the northern hemisphere – the same thing happens in the south. (Image by ARH.)
Figure 11 shows a simplified view of the three-dimensional motion of air on the planet. Notice the low pressure zones at the equator and 60oN, and the high pressure zones at 30oN and the North Pole.
Figure 11. The three global cells of moving air: Hadley Cell, Ferrel Cell, and Polar Cell. Red arrows represent warmer air, and blue arrows represent cooler air. Clouds represent regions of condensation and precipitation (Image: ARH.)
Ok, let’s view the Coriolis Effect produced by a spinning Earth and see how it affects surface winds. As you should recall, the Coriolis Effect causes moving air to deflect to the right in the Northern Hemisphere. This means that air moving to the south away from 30oN will deflect to the west. Air moving north away from 30oN will deflect to the east, and so on (see Fig. 12).
In the middle latitudes (30-60oN) the average surface winds blow from the west to the east. These surface winds are commonly called the Westerlies. This is why weather fronts blow from west to east at these latitudes (e.g., across the lower 48 states). The air moving south away from the poles, however, deflects toward the east and creates the Polar Easterlies.
Figure 12. The Coriolis Effect on northern hemisphere surface winds. Remember that all wind deflects to the right in the northern hemisphere. (Image: ARH)
This pattern of surface winds exists around the world, but we need to consider what this means in terms of climate. In the tropics, the trade winds blow consistently toward the west along both sides of the equator. In the middle latitudes, the west to east flow of wind is only a statistical average. Wind direction and strength there are often variable from day to day due to the passage of weather systems. The tendency for flow to be westerly is strong, and the average values reflect this, but flow is not as consistent as we observe along the equator.
Once air rises at the equator upper altitude winds (not surface winds) blow away from the equator and carries latent heat as far as 30oN and S. In the higher latitudes, however, something different happens. As air rises at 60oN and S heat is also carried toward the poles, but this movement of heat is less direct than in lower latitudes. How does this indirect transfer of heat happen?
There is a region in the middle latitudes where warmer masses of air collide with colder masses of air. This turbulent collision, called the clash zone, is most common and strongest in the winter when cold polar air moves further toward the equator than it does during other seasons. A clash zone exists in northern and southern hemispheres. The clash zone has a large change in temperature over a short latitudinal (Fig. 13). This is a common situation during the winter over much of the USA. You may have personally observed situations where temperature changes of 30-40oF over distances of 100-200 latitudinal miles occur.
Middle Latitude Heat Transfer: Jet Streams
Large changes in temperature at the surface in the middle latitudes can produce powerful upper level winds. When conditions are right, abrupt temperature changes can create a narrow ribbon of high velocity wind in the upper troposphere. These are jet streams. The strongest and most commonly observed one is the polar jet stream. Sometimes there is a weaker jet stream farther south. This is the subtropical jet stream. These jet streams move north and south with the seasons. During the northern hemisphere winter the polar jet stream often moves south and enters the airspace over the USA. During summer, the polar jet stream is generally located north Canada-USA border.
The strength of the jet stream is proportional to the size of temperature change at the surface. Often the largest latitudinal change in temperature exists over the southern ocean around Antarctica. This is where very cold Antarctic polar air often collides with much warmer subtropical air. The southern hemisphere therefore has an extremely powerful polar jet stream. The strength of the southern polar jet stream is also aided by lack of land in this region. The lack of land means that winds encounter less friction, and can blow at greater velocities. There are names from sailing and maritime history for strong winds at higher latitudes: “the roaring forties”, “the furious fifties”, and “the screaming sixties”. Indeed, the most routinely violent ocean waters are observed between the southern tip of South America and the Antarctic Peninsula. This passage of ocean is particularly dangerous, and is called Drake’s Passage. Historically ships had to choose whether to run the risks of Drake’s Passage or the difficult path of the Straits of Magellan (Fig. 14). Today most ships get from the Atlantic to the Pacific via the Panama Canal.
Jet streams and winds in the upper atmosphere appear in wave patterns. It is similar to waves on water as there are large ones and small ones. Figure 15 shows a map of a jet stream. The jest stream has a trough in the jet stream over southern Idaho, and a ridge as it swoops up over the Great Lakes region.
Figure 15. A top-down view of a jet stream including a ridge and a trough. (Image: weather.com)
It turns out that in order to conform to physical laws, the air just ahead of a ridge is pulled together or converges, causing it to become denser, and therefore sinks. In front of a trough, however, air is pulling apart or diverging, which causes air to rise from below. Figure 16 shows a side-view of a jet stream with a ridge and a trough together with the vertical motion of air.
Figure 16. Side view of a jet stream. (Image: Dr. Hipps, USU.)
High and low pressure cells are formed at the Earth’s surface below the ridges and troughs of jet streams. A temporary high-pressure cell forms as air descends from a ridge, and a low-pressure cell forms as air is drawn up into a ridge. Jet streams thereby produce temporary centers of high and low pressure at the Earth’s surface. Low-pressure systems often create storms, and high-pressure systems typically bring clear weather.
The key to energy flow across the middle latitudes toward the poles is that during their short lifetimes airflow around temporary high- and low-pressure cells moves warm air toward the poles and cold air toward the equator. Jet streams do not show up in the prevailing average winds that are produced directly and consistently by Hadley, Ferrel, and Polar Cells. Jet streams are, nevertheless, what move heat across the middle latitudes.
Large changes in temperature due to the collision of warm and cold air masses, and the resulting jet streams and surface pressure systems they produce are a key to the climate of the middle latitudes. This is a critical part of the global climate system. Winds, however, are not the only thing moving heat around the planet, ocean currents also play a significant role, but that will have to wait until the next reading.
Hipps, LE. 2010. Personal communication and readings produced by Dr. Hipps. Professor of Atmospheric Science, Department of Plants, Soils, and Climate. Utah State University.
Shanahan, TM., et. al. 2009. Atlantic Forcing of Persistent Drought in West Africa. Science 324: 377-380.