Chapter 15: Atmospheric Circulation
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With a wingspan reaching 11 feet, the albatross can glide across thousands of miles of ocean without ever flapping. In flight they reach speeds of over 70 miles per hour and remain in the air for 20 hours or more. The distances they cover are epic. One tagged albatross completed a circumnavigation of Antarctica—a journey of more than 16,000 miles—in just 46 days. The albatross achieves these long flights because its slender, glider-like wings lock in place like snap-together tent poles and enable it to fly without moving a muscle. The albatross’s heart rate while flying barely exceeds its resting heart rate. This bird is a marvel of evolutionary engineering and a paragon of energy-efficient flight.
The albatross can teach us a good deal, and so we take to the air on the wings of an albatross in our exploration of winds, those great currents of air that sweep our globe. Modern technology permits us to tag along and experience the flight of these great birds. Albatross cams have captured these birds gliding across the ocean, soaring next to icebergs, and following killer whales—presumably while they’re hunting, an activity at which the killer whale excels. But we are going to follow an imaginary albatross, a robotic one we’ll call a robotross. Though an actual robotross doesn’t yet exist, engineers are very keen to build one to carry out long-lasting, energy-efficient flights. Because ours is imaginary, it may disobey a few engineering limitations and laws of physics. Our robotross does whatever we ask it to do.
In our robotross trek around the globe, we will explore meteorology, the study of the atmosphere. We’ll soar to the very top of the atmosphere and from the equator to the poles. This global flight will introduce us to the structure of Earth’s atmosphere and the global wind patterns. With these basic concepts, we can better understand the nature of global shifts in Earth’s winds, such as those that occur during cycles of El Niño and La Niña, and one of the most powerful forces of nature on the planet—hurricanes. We’ll use this knowledge in future chapters to understand currents and waves.
15.1 Earth’s Atmosphere
Our atmosphere exists as a shell of gases held close to Earth’s surface by the force of gravity. If our robotross could turn back time and visit Earth soon after it formed some 4.56 billion years ago, we would find an atmosphere much different than the one we experience today. Though the thickness and composition of Earth’s early atmosphere remains controversial (e.g., Emspak 2016), scientists are fairly certain that it lacked oxygen (O2). However, owing to the evolution of oxygenic photosynthesis some 2.5 billion years ago, atmospheric concentrations of O2 began to rise. This momentous occasion in Earth’s history has been named the Great Oxidation Event. Fortunately for us (and every other aerobic organism), the modern-day concentration of atmospheric O2 hovers around 21 percent (by volume).
The most abundant gas in Earth’s modern atmosphere is nitrogen (N2), at a concentration of about 78 percent. This is an important element for agriculture and oceanic phytoplankton—but one that is not easily obtained. Turning nitrogen gas into industrial fertilizer takes tremendous amounts of energy. Natural conversion of N2 into biologically available forms (e.g., ammonium, nitrate, nitrite) is carried out by a special group of organisms: the nitrogen-fixing bacteria. So though N2 is quite abundant in the atmosphere, it tends to be a limiting factor for biological processes, especially in the ocean.
Argon (Ar), a noble gas left over from Earth’s formation that is resupplied through the radioactive decay of potassium (K), ranks third among the gases present in Earth’s atmosphere. Because it is non-reactive, welders use Ar to prevent oxygen from reacting with the metals being welded. Otherwise, most people give it no thought.
The other gases in our atmosphere (possibly hundreds) exist only in trace amounts, but a few of them exert a profound effect on our planet. From Chapter 10, you may recall ozone, naturally abundant in the stratosphere and produced via photochemical reactions with car exhaust at Earth’s surface. Stratospheric ozone acts as a shield to protect organisms from mutation-causing UV radiation. We also learned about manufactured chlorofluorocarbons that eat away at stratospheric ozone and increase the intensity of harmful UV radiation reaching Earth’s surface, especially at Earth’s poles.
Of course, greenhouse gases (Chapter 12) keep Earth warm by absorbing longwave radiation from Earth’s surface. Greenhouse gases, especially water vapor, belong to a category known as variable gases, substances whose concentrations vary in the atmosphere. Among the variable gases, water vapor reigns supreme, accounting for up to 4 percent of the atmospheric gases at times. Of course, invisible water vapor and its visible cousins, liquid water and ice, make up the trio of forms responsible for precipitation and other important processes on our planet.
15.1.1 Earth’s Geophysical Fluids
As every robotross knows (or soon learns), it’s easier to fly through air than water. That’s because the atmosphere is 800 times less dense than the ocean. This difference in density explains the difference in the response of the atmosphere and ocean to forces that set them in motion. Atmospheric flows behave like Aesop’s speedy hare, whereas the ocean moves more like the slow, but steady, tortoise. Light transmits much farther in the atmosphere too, permitting us to see for miles and miles. By contrast, visibility in even the clearest ocean water rarely exceeds a couple hundred feet. Sound, however, can be transmitted for hundreds of miles in the ocean; in the atmosphere only something like a violent volcanic explosion can be heard that far away.
Despite their differences, the atmosphere and ocean belong to what scientists call Earth’s geophysical fluids—the air, water, and molten rock found on our planet. The properties and motions of these fluids occupy the attention of numerous disciplines of science, including meteorology, climatology, oceanography, geophysics, volcanology, engineering, and many more. But their study principally falls to a field of science called geophysical fluid dynamics—the study of fluid flows in nature. Our understanding of geophysical fluid dynamics extends well beyond academics—these fluids determine the habitability of our planet. As Vallis puts it (2016): “Geophysical fluid dynamics plays an enormous role in the development of our understanding of the natural world.”
15.1.2 Weather vs. Climate
A simple explanation for the difference between weather and climate comes from a 19th-century school child (whose name, unfortunately, was never recorded): “Climate lasts all the time and weather only a few times” (Le Row 1887). That’s a pretty good way to think about it. In Chapter 1, you learned weather represents what is happening in the atmosphere right now at a given location. Air temperature, precipitation, humidity, cloud cover, barometric pressure, wind speed and direction, and visibility are a few of the weather conditions that might be observed. Climate, on the other hand, is the long-term average of weather conditions at a given location or globally. Climate scientists typically use a 30-year period to compute the average climate for a location. When you hear the TV weatherperson talk about today’s temperatures being so many degrees above or below normal, the benchmark they are referring to is the 30-year average.
The terms weather and climate apply to the ocean as well (e.g., Carlowicz 2006; Pope 2021). Ocean weather may be expressed as a day when the water is cold or warm, with big waves or none, clear or murky water. Ocean weather, like atmospheric weather, changes day to day. Ocean climate refers to longer timescale changes in the ocean, such as El Niño/La Niña or the Pacific Decadal Oscillation.
The atmosphere and ocean work in concert to orchestrate Earth’s weather and climate. The air–sea boundary exchanges heat, water, gases, and chemicals between the atmosphere and the ocean. The atmosphere connects to the ocean; the ocean connects to the atmosphere. Together they drive much of what we experience on the surface of our planet.
15.1.3 Layers of the Atmosphere
If we send our robotross straight up from Earth’s surface into the sky, it would encounter different layers of the atmosphere. Generally speaking, the layers of the Earth’s atmosphere correspond to changes in temperature that occur with altitude. Meteorologists divide the atmosphere into five layers based on temperature or the presence of certain gases or other kinds of matter. The five layers of the atmosphere include (following NOAA/NESDIS 2016):
- The troposphere—the layer closest to Earth’s surface and the one in which we live. This is where we experience weather directly. Temperature here typically decreases with increasing altitude.
- The stratosphere—the layer above the troposphere and the one that produces the highest weather-related clouds, such as cirrus, cirrostratus, and cirrocumulus. Temperature increases with altitude in the stratosphere.
- The mesosphere—the layer above the stratosphere in the middle of Earth’s atmospheric layers, where meteors become visible as they are heated through friction with the gases present in this layer. The mesosphere produces noctilucent clouds, which are made of ice crystals that mysteriously glow at night. These are the highest clouds. In this layer temperature decreases with altitude.
- The thermosphere—the layer above the mesosphere named for its high temperature. That’s a bit misleading because despite the presence of gas molecules with high kinetic energy—the technical definition of high temperature—the molecules are so thin that you wouldn’t feel warmth were you to step out of an aircraft at this height. Temperature increases with altitude in the thermosphere.
- The exosphere—the layer above the thermosphere, so named because “exo” means outer, and this is the atmosphere’s outermost layer. The exosphere is really, really thin and really, really big. A recent analysis of decades-old data revealed that it extends beyond the Moon (Baliukin et al. 2019).
You may also notice at left a dashed line called the Karman Line. This line represents the official-but-not-quite-scientific boundary between our atmosphere and outer space. (Learn more at astronomy.com.)
15.1.4 Atmospheric Pressure
Like we did for the ocean, we can think of the atmosphere as a column—this time it’s a column of air. Meteorologists define an air column as an undefined cylindrical (or rectangular) volume of the atmosphere that extends from Earth’s surface to a given height. Similar to the concept of a water column, visualizing a column of air provides a convenient way to understand the nature of the forces that act upon the atmosphere.
Like the ocean, the atmosphere exerts pressure, the force exerted by a fluid on an object immersed within it. The weight of the atmosphere acting on a unit area of Earth’s surface (or us) represents atmospheric, or air, pressure. Air pressure at sea level exerts about 14.7 pounds per square inch (psi). Equivalent units in common use include millibars (1,013.25 mb) and inches of mercury (29.92 in. Hg; Ahrens and Henson 2018). Fortunately, gases in our bodies push back with an equal force so we don’t experience this crushing weight.
Unlike the ocean, however, air is compressible. Gravity pulls down on our atmosphere, causing air molecules to be packed more closely (compressed) at the Earth’s surface. Meteorologists refer to the packing of air molecules as air density—the number of molecules in a given volume. Fewer air molecules means less air pressure: the less air above you, the lower the pressure. So, in effect, as we climb higher into the atmosphere, both air density and air pressure diminish—rapidly at first, then slower as altitude increases. In mathematical terms they decrease exponentially.
The rapid decrease in air pressure with altitude explains why your ears pop when you travel into the mountains. Air pressure at higher elevations is less than air pressure at lower elevations, so your ears have to adjust. Your eustachian tube opens to equalize the air pressure inside your middle ear and the surrounding air, which occasionally leads to a pop, like a champagne cork releasing gases.
Changes in temperature also cause changes in pressure and density. Anyone familiar with a hot air balloon (or heating in a home) knows that hot air rises. As air warms, its molecules get farther apart. Fewer molecules in a space results in lower air density and pressure for that volume of warmed air. Because the warmed air has a lower pressure than the surrounding air, it rises. The surrounding air pushes the volume of warmed air upward until it reaches an altitude where the pressure of surrounding air and the pressure inside the balloon are equal. Similarly, cooling a body of air results in an increase in its density and pressure. As we know, cold air sinks. The changes in air pressure with temperature are key to understanding motions in the atmosphere.
15.1.5 Highs and Lows
Differences in the heating of Earth’s surface result from differences in latitude and seasonal variations in solar radiation. This and a number of other factors generate regions in our atmosphere with different air pressures. Meteorologists designate regions of high and low pressure. Places where the pressure in a particular area is higher than that of the surrounding region are called high-pressure regions, or highs. Places where the pressure is lower than the surrounding region are called low-pressure regions, or lows. On weather maps or the local news on TV, you’ll often see these regions symbolized with a red letter L for a low and a blue letter H for a high. Designating highs and lows is a kind of shorthand for illustrating regions with mild and stable weather, which typically occurs in high-pressure regions. Regions with unsettled and unpleasant weather generally occur in low-pressure regions.
Observations of air pressure at different locations (or altitudes) provide the basis for designating high- and low-pressure areas. A surface map of air pressure typically illustrates lines of equal pressure, or isobars. Looking at the center of a high-pressure system on an isobaric map, you can see that the pressure here is higher than the surrounding areas. Similarly, the center of a low-pressure system will have a lower pressure than the surrounding areas. While a complete understanding of isobaric maps is beyond the scope of this text, a general familiarity with them proves useful for understanding wind and ocean currents.
15.2 What Causes Winds?
Winds—movements of air—arise from an imbalance of pressure. They are nature’s way of bringing balance to an unbalanced atmosphere. To achieve that balance, winds blow from regions of high pressure to regions of low pressure. To understand winds, we need to understand something about the forces generated by differences in air pressure between different locations (following Ahrens and Henson 2018).
15.2.1 Hydrostatic Equilibrium, a Balance of Forces
As we learned above, air pressure decreases with altitude. It’s highest at Earth’s surface and lowest at the top of the atmosphere. So if winds blow from high to low pressure, why aren’t winds blowing upward from Earth’s surface? The answer is gravity. The force of gravity counteracts the force generated by the upward difference in pressure. When these two forces are equal, and no net vertical motion of the atmosphere occurs, a hydrostatic equilibrium is maintained. The upward forces are balanced by the downward forces, and the fluid is at rest. The hydrostatic equilibrium concept (i.e., a balance of forces) applies to vertical motions in the ocean as well (as we learned in Chapter 13). We’ll visit this topic again in our chapter on ocean circulation.
Of course, some vertical motions in the atmosphere (and ocean) do occur. Sinking air generally accompanies high-pressure systems, while rising air characterizes low-pressure systems. Thermals—rising columns of warmed air (sought out by birds, hang glider enthusiasts, and presumably robotrosses)—also represent vertical motions of air. Violent vertical air motions occur during thunderstorms and tornadoes. Nevertheless, the strongest motions of air lie with pressure differences across horizontal scales—that is, the force that causes winds.
15.2.2 The Pressure Gradient Force
To express the strength of the pressure differences that cause winds, meteorologists and oceanographers define the pressure gradient force (PGF), which is generated by differences in air pressure between two locations. Mathematically, we represent the PGF between two locations as:
PGF = Pressure difference / Distance
The strength of the PGF determines the strength of the winds. This equation predicts that when the pressure difference between two locations is high, the PGF will be strong. When the pressure difference between those two locations is low, it will be weak. What really matters is how pressure changes over distance—the gradient of pressure changes. In an isobaric map, steep changes appear as closely spaced isobars, while gradual changes in pressure are represented by isobars that are farther apart. Steep changes in pressure over short distances bring about very high winds; weak winds will be present where isobars are widely spaced.
15.2.3 A Description of the Coriolis Force
One other force plays a role in the motions of air (and water) on our planet. Formulated by French mathematician Gaspard Gustave de Coriolis (1792–1843), the Coriolis force refers to the apparent deflection of moving objects across Earth’s surface from the standpoint of an observer on Earth. This deflection occurs because the object is in motion on a rotating frame of reference—the spinning Earth. Though not correct in physical terms, the Coriolis force has been compared to what happens to a ball thrown to the other side of a playground merry-go-round. By the time the ball reaches the other side, the target has moved. From the standpoint of a person on the merry-go-round, it appears as if the path of the ball curves, though, of course, it doesn’t.
If this sounds complicated, it is, and a proper understanding would take many pages and some advanced mathematics. Many valiant attempts have been made in textbooks to explain the Coriolis effect, but they generally confuse and muddy the topic more than they help students understand it (e.g., Kearns 1998; Perrson 1998; Shakur 2014). Simply knowing what happens as a result of the Coriolis force suffices for our purposes here. The Coriolis force underlies the clockwise and counterclockwise motions of winds and currents around centers of high and low pressure, helps to explain the formation and circulation of hurricanes, and a lot more.
As a result of the Coriolis force, winds and currents appear to be pulled sideways, or perpendicular to the direction of their motion. In the Northern Hemisphere, the Coriolis force causes moving objects to deflect toward the right (clockwise). In the Southern Hemisphere, the Coriolis force causes moving objects to deflect toward the left (counterclockwise). In other words, the Coriolis force influences the direction of winds and currents. If that’s all you remember about the Coriolis force, you’ll be fine.
For the sake of completeness, you should be aware that the strength of the Coriolis force—the degree to which it influences direction—increases with latitude and the speed of the object. Horizontal motions along the equator experience no Coriolis force. As well, the Coriolis force acts largely over long distances, on the order of tens of kilometers—scales that are important for the circulation of the atmosphere and the ocean. Despite popular notions, the Coriolis force does not cause water in sinks and bathtubs to swirl clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Compared to the forces within the sink or bathtub (such as the shape of the basin and whether it’s level), the Coriolis force is too small to alter the rotation of water down the drain.
To visualize the effect of the Coriolis force on the direction of winds and currents, I recommend that you employ the three-legged handperson model. You’ll need a right hand (or a reasonable facsimile) and a map or piece of paper representing a map. Let your three middle fingers be the legs of your handperson (yes, this handperson has three legs). Let your thumb represent the left hand of your handperson and your pinky finger represent the right hand. Now walk (using your three legs) across a map to the north of the equator. As the handperson moves toward the North Pole, move it in the direction of your pinky, or toward the right. Turning your handperson to the right in the Northern Hemisphere represents the change in motion that occurs as a mass of air or water moves in the Northern Hemisphere. The direction you travel makes no difference; when you are in the Northern Hemisphere, a moving object will be deflected toward the right. Alternatively, when you walk your handperson into the Southern Hemisphere, you will turn in the direction of your thumb—toward the left. Remember that your knuckles are the eyes of the handperson so you’re always facing in the correct direction as you walk across the map. In this way you can visualize and better understand the consequences of the Coriolis force as it acts on winds or currents.
15.3 Local and Regional Winds
In the sections that follow, we’ll use the concepts of pressure, the PGF, and the Coriolis force to understand local, regional, and global winds. Local winds, such as sea and land breezes, operate over the mesoscale—a few to a hundred kilometers. These winds affect coastal air temperatures and local wave conditions. Regional winds, such as Santa Ana winds and monsoonal circulation, operate over synoptic scales, on the order of hundreds to a thousand kilometers. These winds directly impact weather and coastal ocean processes. Global winds, including trade winds, westerlies, and jet streams, operate over global scales greater than a thousand kilometers. These winds drive the ocean currents.
15.3.1 Sea and Land Breezes
A wind coming onto the shore of a coastline, known as an onshore wind, provides a perfect opportunity for a robotross to take flight. Unlike most birds, albatrosses (and our robotross) cannot just flap their wings and take flight. They require some wind or, at the very least, a good runway so they can trot into the wind. The sea breeze, a thermally driven onshore wind, provides an ideal launching pad.
The sea breeze forms as a result of uneven heating of land and water along a coastline. Land, compared to water, heats quickly, so a temperature gradient forms across the land–water boundary. Heating land surfaces causes the air over the land to rise, creating lower pressure at the surface. A pressure gradient forms, causing higher-pressure air over the ocean to move onto the shore. Aloft, the rising air over land creates a higher pressure than that of the corresponding altitude over water. So air at altitude begins to move offshore, from the land to the ocean. A circulation cell thus develops, with ocean-cooled air coming onto the shore at the surface and land-warmed air moving offshore aloft. Because heating of land generally becomes most intense in the afternoon, the sea breeze generally peaks at that time. At night the opposite situation occurs. Land cools more quickly than water, so an area of lower pressure develops over the water. A pressure gradient forms, directed from land to ocean, and an offshore wind develops—a wind that blows from land to ocean. We call this wind a land breeze, a thermally driven wind that blows from land toward the ocean.
Sea breezes and land breezes can moderate coastal temperatures. Sea breezes cool coastlines during the day, and land breezes warm them at night. Sea and land breezes can also generate local waves. An afternoon wind swell is a familiar feature to Southern California surfers, especially during the summer, when the difference between land and ocean temperatures is greatest. Thermal circulations, such as sea and land breezes, require relatively stable atmospheric conditions to form. They also require a strong temperature gradient between the land and the ocean. A stable atmosphere and strong temperature gradient typically occur during the summer months, so sea and land breezes are most common in the summer.
15.3.2 On- and Offshore Winds
Winds that blow from the ocean to land—onshore winds—and winds that blow from the land to the ocean—offshore winds—can also be caused by pressure gradients over synoptic scales. Pressure gradients develop when air mass movements and other factors cause pressure differences between inland and coastal or offshore locations. These pressure gradients may enhance or overpower thermally driven, local wind patterns. On- and offshore flows exert a strong influence over weather along coastlines and can modify conditions within coastal waters.
Television weather forecasts in coastal California cities such as San Diego, Los Angeles, and San Francisco include a reference to on- or offshore winds. Onshore winds typically bring with them the marine layer, a low-altitude cover of stratus clouds that forms over a cool ocean. In places like San Francisco, onshore winds in summer can bring fog, simply defined as a cloud formed near the ground. Because nighttime ocean temperatures are generally warmer than nighttime land temperatures, an onshore flow brings warm, moist air in contact with the cooler land surface, and fog forms. Like a cat, the fog sweeps quietly inland until warmer daytime temperatures cause it to dissipate. Meteorologists often refer to onshore winds as “nature’s air-conditioner,” especially in summer, when the deck of clouds or fog they bring keeps land temperatures cooler than they otherwise would be.
An offshore flow brings higher temperatures to coastal cities. Without a reflective layer of near-surface clouds and cool air from the ocean, offshore winds allow the land to heat faster. Offshore winds also carry airborne particles and air pollution across the ocean. The particles these winds carry can have impacts on marine life. Nitrogen compounds in the air may stimulate phytoplankton growth. On the other hand, toxic metals may inhibit the growth of some organisms (Mahowald et al. 2018).
15.3.3 Santa Ana Winds
Locally infamous offshore winds, the Santa Ana winds, deserve special attention. Named in 1880 for their occurrence through Santa Ana Canyon (along I-91 between Chino Hills and the Santa Ana Mountains), the Santa Ana winds have become the stuff of Southern California legend. They’ve appeared in books and movies, grabbed the attention of songwriters and artists, and struck fear into anyone living at the edge of the chaparral. Santa Ana winds can bring hot, dry, dust- and smoke-filled, hurricane-strength wind gusts across the Southland. They’ve been responsible for some of the worst wildfires in Southern California history. Perhaps that’s why some locals call them “devil winds” (e.g., Masters 2012).
Scientists define Santa Ana winds as “episodic pulses of easterly, downslope, offshore flows over the coastal topography of Southern California and Northern Baja California” (e.g., Guzman-Morales et al. 2016). Let’s dissect this definition.
First, Santa Ana winds are episodic, meaning they occur at irregular intervals. On average Southern California experiences 32 Santa Ana wind events per year, typically from October to April. However, September and May events are not uncommon.
Second, Santa Ana winds blow from the east or northeast at speeds (generally 10 to 30 mph; 4.5 to 13.5 m s-1) that exceed the local wind field for more than 12 hours. Most Santa Ana wind events last from a few to 6 days; about 10 percent last up to 12 days (e.g., Guzman-Morales et al. 2016). Extreme events can generate gusts up to 80 miles per hour (35 m s-1; Fovell and Gallagher 2018).
Third, and perhaps most important, Santa Ana winds are downslope winds, meaning they blow from higher to lower elevations. The movement of air from a higher to a lower elevation causes it to undergo an increase in pressure (because, as we learned above, air pressure is greatest at sea level). This increase in air pressure results in a process called compressional heating (also known as adiabatic heating). It’s the same as what happens when you add air to a bicycle or car tire. Ever feel the valve stem when adding air? It gets hot because you are compressing a given volume of air and the energy it contains into a smaller volume. Compressional heating (and its opposite, expansional cooling) also happens in the ocean, but because air is more compressible than water, the effect is not as dramatic. Compressional heating causes Santa Ana winds to heat up as they travel from east-northeast toward the coast. In fact, during Santa Ana wind events, coastal cities are hotter than areas a few or even several miles inland by a few degrees or more. A few hundred miles or more inland, air temperatures may be quite chilly. The heat of Santa Ana winds comes from compression, not from blowing across a desert. Compressional heating also results in very low humidity values—a measure of the water vapor in a parcel of air. High temperatures and low humidity values increase fire danger.
Finally, like land breezes, Santa Ana winds blow offshore due to the PGF that develops when inland areas have higher air pressure than along the coast. Specifically, Santa Ana winds form when a ridge of high pressure occupies the Great Basin region, an expanse of basins and mountain ranges that cover parts of California, Idaho, Nevada, Oregon, Utah, and Wyoming. The resultant pressure gradient generates winds that blow from the Great Basin toward Southern California and Baja California Norte (i.e., Northern Baja). Because they encounter a barrier of mountains surrounding Southern California (the San Gabriel and San Bernardino ranges), they accelerate as they funnel through the narrow mountain passes and canyons that open onto the coastal plain.
The elevated heat, single-digit humidity, and near-hurricane-strength winds bring a host of discomforts. The heat and low humidity dry out your skin, irritate your eyes, and, according to one author, “curl your hair and make your nerves jump” (Chandler 1946). The high winds send plumes of dust and debris into the air, making it difficult to drive and even breathe. And, of course, their peak season occurs at the end of summer, when vegetation is driest, and the danger of wildfire is greatest. Some of Southern California’s worst wildfires have been stoked or even caused by Santa Anas and similar downslope winds. High winds can knock down power lines or cause them to spark. Once the vegetation ignites, a wildfire in the presence of Santa Ana winds can be very difficult to stop. As one firefighter puts it, “About the only firebreak that works is the Pacific Ocean” (Wolansky 2016).
Their numerous negative qualities aside, Santa Ana winds do bring some benefits, especially to Southern California’s coastal waters, where trace metals such as iron (Fe) may limit phytoplankton productivity. Offshore winds, and especially Santa Ana winds, deliver dust and biologically important micronutrients (namely metals, such as manganese and iron) in quantities sufficient to enhance the growth of phytoplankton. A study in 2017 suggested that as much as 15 percent of phytoplankton growth during winter and spring could be attributed to additional micronutrients supplied by Santa Ana winds (Felix-Bermudez et al. 2017). A number of studies also suggest that atmospheric dust particles delivered to the ocean can act as “ballast” for particulate organic carbon and enhance the sinking rates of these carbon-rich particles (Bressac et al. 2014; Pabortsava et al. 2017; van der Jagt et al. 2018). Such effects—phytoplankton fertilization and ballasting of carbon to the deep sea—have implications for the ocean carbon cycle and climate change. Next time you’re complaining about the Santa Ana winds, just remember that they are fertilizing phytoplankton that feed fishes and removing carbon dioxide from the atmosphere, a benefit to us, the ocean, and the planet.
15.4 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.
15.4.1 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:
Etotal = Ein − Eout
It should make sense to you that if Ein > Eout, then the location gains energy. If, on the other hand, Ein < Eout, 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: Ein > Eout. But from 40°N/S to the poles, there is a net loss of energy: Ein < Eout (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.
15.4.2 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.
15.4.3 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.
15.5 Final Thoughts
Our whirlwind “robotross” tour in this chapter has connected us with those properties and behaviors of the atmosphere that most affect the ocean. As with all such tours, the story is incomplete. A fuller reckoning of the atmosphere can be found in a semester-long course in meteorology, which I encourage you to pursue. Weather and climate play enormous roles in our daily and future lives. A better understanding of your local weather can help you avoid unpleasant drenchings or bone-chilling cold. An appreciation for Earth’s climate and how it is changing can help you chart a course toward a career and a place to live less affected by the extremes of weather and natural disasters climate change will bring. Perhaps you may even choose to take action to help humanity find a more sustainable and less environmentally destructive way of life. Whatever thoughts this chapter inspires, I hope you’ll maintain a greater appreciation for our atmosphere and the ways it affects our lives. As Sting might say, “Every breath you take, every move you make. The atmosphere will be watching you” (Sting 1983).
15.6 Chapter References
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