12.4: Global Atmospheric Circulation

We now turn our attention to global atmospheric circulation, the three-dimensional motions of air within the troposphere. Global atmospheric circulation transports momentum (i.e., mass in motion), heat, gases, water (as water vapor and as a liquid or solid in clouds), suspended particles, and even microscopic organisms on a journey around the globe. Most important for ocean dwellers, global atmospheric circulation gives rise to the surface winds that “stir” the ocean; accelerate air–sea transfer of energy, materials, and gases; generate ocean currents and up- and downwellings; and create waves. We’ll explore some of these topics in the chapters ahead. But for now, let’s get a general sense of the patterns of atmospheric motions and the forces that cause them.

A One-Cell Model

The simplest model of atmospheric circulation assumes a nonrotating Earth. As you know from our discussion above, the Coriolis force (caused by Earth’s rotation on its axis) plays a role in the direction of fluids, winds, and currents moving across Earth’s surface. Absent Earth’s rotation and the Coriolis force, the circulation of the atmosphere works much like the circulation of a room with a space heater on the floor of one side. The air above the heater warms and rises vertically. Reaching the ceiling, the warmed air flows horizontally toward the opposite wall. As it moves along the ceiling, the air loses heat and cools, causing it to sink. Once it reaches the floor, the cooled air is drawn toward the heater as the rising air above creates an area of low pressure. The circulation of the air from heater to ceiling to floor back to heater describes the journey of air in a convection current—the heat-driven, circular motion of fluids (air and water) from one place to another.

On a global scale, the movement of air in the atmosphere traces a similar three-dimensional path. Just as a space heater supplies the energy that causes the air in a room to move, the Sun’s rays supply the energy that causes the atmosphere (and, indirectly, the ocean) to move. You may recall that the Sun’s rays strike Earth’s surface most directly in the tropics, the region between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). This radiant energy raises the temperature of the ocean and land. The warmed ocean and land surfaces in turn heat the atmosphere. The heated tropical air rises. In this sense the tropics serve as the “firebox” of the atmosphere, acting as the main source of heat that drives its motions (Garstang and Fitzjarrald 1999, after Malkus 1962).

We can also see how this works in a simple model of the radiation balance from the equator to the poles. Using our simple reservoir model, we know that the total energy at any location on Earth depends on how much energy that location receives (from the Sun) versus how much it loses (ultimately, to outer space). Mathematically, we can state Earth’s radiation balance as:

E_total = E_in − E_out

(Eq. 15.2)

It should make sense to you that if E_in > E_out, then the location gains energy. If, on the other hand, E_in < E_out, then that location will lose energy. Viewed in this way, we can see that between 40°N/S—a region that includes the tropics—there is a net gain of energy: E_in > E_out. But from 40°N/S to the poles, there is a net loss of energy: E_in < E_out (see Garstang and Fitzjarrald 1999, p. 4).

Heating of Earth’s surface by the Sun at equatorial regions causes the air in this region to rise. The entire column of air above the equator expands upward (because, as you know, heating causes expansion of air). Aloft, at the top of the warmed and expanded column of air, the air pressure of the heated column is greater than that of the unheated air on either side of it. Because winds blow from regions of high pressure to regions of low pressure, the air at the top of the warmed column begins to flow outward. Visualizing this process in three dimensions, the air at the top of the warmed column flows to higher latitudes, or toward the poles. The flow of air aloft causes a reduction in air pressure at the surface because there is now less air above the surface. The low air pressure at the surface around the equator draws in air from surrounding regions—that is, from nearby latitudes. Winds flow toward the equator, drawn there by the low pressure caused by the rising air at the equator.

The vertical and horizontal flows of air in the atmosphere constitute an atmospheric cell, the large-scale convection of a part of the atmosphere. Air rises at the equator and moves poleward aloft. Surface air from the poles rushes toward the equator to replace the rising air. The poleward-moving air cools, sinks to the surface, and replaces the air that moved toward the equator. It’s a giant cell of moving air, and this pattern describes the three-dimensional circulation of air in Earth’s atmosphere. In our simple model of an Earth that is not turning, we observe one atmospheric cell in each hemisphere.

The temperature imbalance between the tropics and subtropics and the polar and subpolar regions drives atmospheric circulation. Differences in temperature create differences in pressure, and when that happens, winds occur. Global winds act to compensate for the imbalances in pressure that result from temperature differences across the globe. They try their best to bring Earth’s atmosphere into balance, but because temperature and pressure differences always exist somewhere on Earth, the winds are constantly blowing. Such is the dynamic nature of our atmosphere. But what happens when we allow Earth to rotate as it normally does? Enter the Coriolis force.

A Three-Cell Model

The Coriolis force causes winds to deflect to the right in the Northern Hemisphere and left in the Southern Hemisphere. As a result, the warm equatorial air moving poleward and the cool, higher-latitude air moving equatorward in our one-cell model never make it to their destination. Our one-cell model transforms into a three-cell model—three cells in each hemisphere—when we add the effects of the Coriolis force. Because they were discovered independently, each of the three cells has a different name. The atmospheric cell positioned in the tropics is called the Hadley cell, named after English lawyer and amateur meteorologist George Hadley (1685–1768), the man who first developed a mathematical description of tropical circulation. The middle-latitude atmospheric cell is called the Ferrel cell, named after American meteorologist and Tennessee school teacher William Ferrel (1817–1891; Persson 2006). The polar-region atmospheric cell is simply called the polar cell; apparently, no single discoverer can claim credit for it.

In the three-cell model, poleward air aloft (the air that rose over the heated tropics in the one-cell model) sinks, descending to the surface at about 30°N/S. This motion of air from the equator to 30°N/S aloft and back again at the surface belongs to the Hadley cell. But this equatorward surface wind also bends due to the Coriolis force. This gives rise to surface winds that move northeast to southwest in the Northern Hemisphere and southeast to northwest in the Southern Hemisphere. Because these winds are constant, they became known as the trade winds (from the Middle Dutch/German meaning “course,” “track,” or “habitual,” according to etymonline.com 2023). Note that in the Northern Hemisphere, these trade winds are referred to as the northeast trade winds. In the Southern Hemisphere, they are called the southeast trade winds.

In the absence of a connected single cell, as would occur on a nonrotating Earth, the poles develop their own cells—the polar cells. Here cold air descends to the surface and moves toward the equator. As it moves poleward, the Coriolis force deflects it right (Northern Hemisphere) or left (Southern Hemisphere). The net effect is a surface flow that moves from the east in either hemisphere. This gives rise to a wind pattern at high latitudes called the polar easterlies.

As polar air moves toward the equator, it warms, and as it warms, it rises. As a result, there is another limb of ascending air at about 60°N/S similar to, but not as strong as, the rising limb of air at the equator. The rising limb of air at 60°N/S and the descending limb of air at 30°N/S act like gears to create a third atmospheric cell between the Hadley cell and the polar cell. This circulation cell is the Ferrel cell, which operates intermittently and, at times, doesn’t even exist. Nevertheless, it provides a useful part of the global atmospheric circulation model because it links the Hadley and polar cells.

The Ferrel cell features its own set of winds—the westerlies—which flow west to east at latitudes between 30° and 60°N/S, that is, at mid-latitudes. Though not as dependable as the trade winds, the westerlies provide a convenient return route for sailing ships. Of course, their higher latitude often brings more severe weather. Winds here go by names like the Roaring Forties, the Furious Fifties, and the Screaming Sixties, a reference to the latitudes where they occur. No doubt these names helped sailors weave more dramatic stories when (if) they returned home.

One other consequence of the rising and descending limbs of the atmospheric cells is that at latitudes where air is rising or descending, the surface winds are very light, if not downright calm. The best known are the equatorial doldrums, a region of calm rising air near the equator. A similar region—the horse latitudes—occurs as a region of sinking calm air at about 30°N/S. Sailors on sailing craft dreaded these regions—the calm winds and stifling heat drove men mad. The days and weeks of additional time at sea often proved deadly.

A popular mythology of the horse latitudes is that they derive their name from the practice of Spanish sailors casting dead horses overboard in these regions. The animals died due to lack of water or food or both while the ship drifted. In some cases the horses may not have been dead yet, and with no Monty Python crew to check the status of their deceasedness, it may well be true. Lest this upset you, let me assure you that every horse—dead or alive—once entering the ocean turned into a seahorse and may swim the ocean to this very day.

Though both calm regions feature light and variable winds, the equatorial doldrums and the horse latitudes differ markedly in their precipitation patterns. Air rising from the equator carries with it a lot of moisture in the form of water vapor. When that air cools, the water vapor condenses and forms clouds. Towering thunderheads develop and generate copious amounts of rainfall. We can see this in satellite images of equatorial regions. The dense band of clouds that develops over the equator is known as the Intertropical Convergence Zone, or ITCZ. This is the region where northeast trade winds meet southeast trade winds—they converge. The high amounts of rainfall in the ITCZ actually lower the salinity of surface waters along the equator. The ITCZ is also responsible for producing the world’s most diverse ecosystems, the tropical rainforests, which contain at least half of all of the plant and animal species in the world. I like to call them the “coral reefs of the land.”

At the other end of the Hadley cell, at the horse latitudes, the opposite process happens. Sinking air warms as it moves to lower altitudes. The increase in pressure that accompanies a decrease in altitude causes compressional heating, the same phenomenon created by the Santa Ana winds. The increase in temperature lowers the relative humidity of the air and makes it much more difficult for clouds and precipitation to form. Take a look at any world map, and you will see that the great deserts—the Sahara, the Kalahari, the Arabian, the Gobi, the Great Victoria, the Patagonian, the Mojave, and the Sonoran, among others—generally occur in the subtropics at the latitude of the descending limb of the Hadley cell. The global pattern of atmospheric circulation does not favor these regions with precipitation like it does the equatorial regions. Less than 10 inches of rain falls on these places annually—the very definition of a desert. Sinking air also creates polar deserts. And while we don’t usually think of polar regions as deserts, Antarctica qualifies as the biggest desert in the world, receiving less than 10 inches of precipitation annually. The snow covering Antarctica has accumulated very slowly over time because it doesn’t melt. Someday I hope to ride a camel across the Antarctic desert.

At times the winds over the subtropical deserts blow so hard that they lift sand and small grains of sediments into the air. These sandstorms can be quite intense and bring blackout conditions. While sand is rarely transported very high, dust particles 1 to 100 micrometers in size may be carried 20,000 feet into the air. At this height these particles get picked up by upper-level winds and may be distributed around the globe. Saharan dust storms are the most famous. Dust from the Sahara actually settles over Florida and Texas in the days and weeks after a large sandstorm. Other desert locations generate similar dust storms.

A More Realistic Model

While the three-cell model of global atmospheric circulation does a decent job of explaining surface wind and pressure observations, it is inadequate to describe the wind patterns that meteorologists observe in the upper levels of the atmosphere. Observations from weather balloons reveal a more complex wind environment in the upper troposphere. While the Hadley cell model closely approximates observations, the Ferrel and polar cell models do not match what is observed. Instead, swift currents of air known as jet streams interrupt the idealized flow of the atmosphere. At subpolar latitudes at an altitude of 5–9 miles (8–14 km), we find the polar jet streams, high-altitude flows of cold air that meander like rivers around the North and South Poles (e.g., Lindsey 2021). Farther south (between 20° and 30° latitude) and at higher altitudes (6–10 miles high), the subtropical jet stream can be found. Both jet streams occur at the boundaries between air masses with different temperatures and reach their highest speeds—at times greater than 200 miles per hour—during winter when the temperature difference between their air masses is greatest.

The strongest and most pronounced atmospheric jet stream is the polar jet stream, which is the major weather maker across most of the United States, especially the Midwest. The polar jet stream often bends and dips across the middle part of the United States due to a phenomenon known as a Rossby wave, a large-scale, wave-like motion of the upper troposphere. Rossby waves are one of the reasons the weather in the Midwest can be so unpredictable and deadly. As the westward-traveling wave meanders and dips toward the south, cold polar air behind it rushes in. Westward-propagating Rossby waves in the polar jet stream cause changes in the position of cold and warm air masses, or fronts, that generate weather. Tornado Alley in the Midwest is a good example of what happens when cold and warm air masses collide. Some studies suggest that the propagation of Rossby waves in the polar jet stream has slowed down so that weather patterns associated with the movements of Rossby waves persist for longer than normal (e.g., Francis and Vavrus 2012). Other studies find no evidence for this (e.g., Blackport and Screen 2020). The now-popular “polar vortex” and its odd behavior suggest some relationship to global warming, but meteorologists have yet to identify the underlying processes. In any case, we can expect Earth’s weather to continue to change in unexpected ways as we add heat to the system.

The subtropical jet stream can spawn the Pineapple Express, a stream of warm, humid subtropical air that can bring heavy rains to the West Coast of the US. The Pineapple Express is now included in a category of extremely wet upper-air currents dubbed atmospheric rivers. Extending across thousands of miles, they may contain more water than a dozen Mississippi Rivers!

Winds also distribute heat, but they are not sufficient in and of themselves to balance the temperature inequality that exists from the equator to the poles. To more adequately distribute heat across the globe, Mother Nature uses a stronger force—storms. The most powerful storms on Earth, hurricanes, are one way to transfer heat from the tropics to higher latitudes. But those are topics beyond the scope of our discussion here. Consult a meteorology text to learn more.