Climate Change Science: A Primer for Sustainable Development

Some places on Earth persistently receive more rainfall than others, and some are much windier than others. Water is essential for almost all life functions, and even the most technologically advanced agricultural regions in the world today rely on rainfall in the right amount at the right time for successful crop yields. In some regions, winds blow consistently from east to west; in others, just the opposite. These features cannot be explained by patterns of temperature alone, our first-order approximation (described in chapter 1). This chapter provides the next refinement to understanding the climate system, our second-order approximation, in which we account for patterns of wind and rainfall. These refinements can be explained using classical Newtonian mechanics.

The pattern of sea level pressure (SLP) is also mostly zonal in overall form, but some features have a distinct east-west variation. For example, the low pressure system in the western equatorial Pacific and high pressure near the west coast of North America and South America have a distinct east-west variation. Figure 2-3 shows SLP averages over one year.

The most striking feature of this map is the narrow band of high-intensity rainfall across the equatorial Pacific in the Indonesian region and through the Indian Ocean. This band was seen in chapter 1 as the strong albedo effect of clouds in the same region (see figure 1-12). Wind direction and strength, rainfall patterns, pressure, and temperature fields are all interconnected. In fact, these characteristics of the atmosphere, taken together, are what is usually referred to as Earth’s climate.

The strong equator-to-pole temperature gradient—around 90°C—gives rise to a global pressure gradient that creates a driving force for global airflow. This happens in both hemispheres, so global airflow resulting from this gradient alone might look something like the illustration in figure 2-6. This diagram is completely imaginary, however, because it assumes that Earth is not rotating and remains hot around the entire equatorial region. It is a way of asking what the pattern of airflow would look like were the equator-to-pole pressure gradient the only force involved in causing air to move. Only on a very slowly rotating Earth could such a pattern develop. The length of a day on Venus is equivalent to 243 Earth days, and Venus rotates around the Sun in 224.7 days, so a day on Venus is longer than its year. Venus does rotate very slowly, and it exhibits a single cell system of atmospheric motion much like that shown in figure 2-6.

Rotational effects are the second factor affecting Earth’s climate. In 1735, George Hadley, an English lawyer and amateur meteorologist, first proposed the circulation pattern shown in figure 2-5 as a way to explain the trade winds. But it clearly cannot explain the observations in figures 2.2, 2.3, and 2.4, in which we observe strong alternating bands of airflow oriented east to west. North-south airflow is rare, with the exception of some local effects, so another force must act to move air east to west and west to east. That force is provided by Earth’s rotation, which gives rise to the Coriolis effect (box 2.1). Named for Gaspard-Gustave Coriolis, who published a comprehensive paper on the subject in 1835, the effect had been recognized and described as early as the 1650s.² However, it was not developed in a mathematical form until the work of Coriolis. Hadley’s model does not include the Coriolis effect, and he cannot be faulted for that because it was proposed one hundred years before Coriolis published his work.

Box 2.1 The Coriolis Effect

The Coriolis effect refers to the deviation experienced by objects moving on a rotating body. In the figure, the object at A is attempting to go to B by moving due south. Viewed from directly over the North Pole, Earth rotates anticlockwise at a rate of around 15° longitude per hour.

Box Figure 2.1.1 The Coriolis effect.

The formula in scalar form for the Coriolis effect is C = 2mVΩsinΦ where Φ is the angular latitude and Ω is Earth’s angular velocity and m is the mass of the moving object moving at a velocity V. The latitude at the equator is zero, so the Coriolis effect is zero at the equator as well. The Coriolis effect alters the apparent direction of motion of the object, but not its speed of motion.

Imagine an object that starts out at the pole and moves relentlessly toward the equator. This would describe the intended motion of surface winds driven by an equator-to-pole temperature/pressure gradient. Because Earth rotates, the point being aimed toward on the equator moves to the left (relatively) as Earth rotates and the object continues on its straight line toward the equator. This happens because the wind is not well connected to Earth. If instead we considered an object being dragged along Earth’s surface, this effect would not be felt. Earth essentially moves beneath the wind as the wind flows toward the equator. (In fact, there is some friction between surface winds and Earth’s surface, so the wind’s apparent path is not perfectly described by the Coriolis effect.) Aircraft need to account for the Coriolis effect when making long-distance flight plans because they, too, are not strongly coupled to Earth’s surface. Trying to hit an object at a great distance using a rifle also requires some consideration of the Coriolis effect. But this effect is weak when compared with gravitation or centrifugal forces and has no noticeable effect on most human activities. To be noticeable, Earth would need to be rotating at least 10 times faster than it does today.

Box Figure 2.1.2 The Coriolis effect on a moving object.

For instance, think of an airplane flying due south from London, England, to Accra, Ghana, which is almost due south and about 5.5° N. If the pilot flies above the clouds so there is no ground reference, and navigates by keeping the compass pointing due south, Accra would have moved east with the rotating Earth before the plane arrived, and the plane might land in Sierra Leone instead (depending on the speed of the plane). If the flight were long, the plane might end in the Atlantic. Coriolis is a virtual or fictitious force, described more correctly as an “effect,” because no force actually deflects the plane’s path. On a rotating disc (a common way to explain the effect), it acts horizontally, but on a sphere it has horizontal and vertical vector components.

The Coriolis effect is not to be confused with the Eötvös effect, which is also due to Earth’s rotation, and in this case is a true force experienced in the perceived change in the gravitational force on a body traveling east to west or west to east, i.e., with or opposed to the rotation of Earth and so either increasing or decreasing the angular velocity of the body.

The Coriolis effect causes deflection of moving air masses to the right in the northern hemisphere and to the left in the southern hemisphere.

Consider just the cells that meet at the equator, referred to as Hadley cells. These cells can be thought of as a very large version of a pressure-driven sea breeze circulation (see figure 2-5) that extends to about 30° N and about the same extent south. The northern and southern hemisphere Hadley cells meet at the equator, where air rises due to strong heating (recall figure 1-10). Because it is very hot at the equator, extensive evaporation and evapotranspiration³ take place, so the rising air contains large amounts of water vapor. As this air rises, it cools and forms clouds of condensation, which create a band of rainfall across the equator. The condensation process itself also heats the atmosphere due to the release of latent heat when water vapor (a gas) changes phase to liquid water. The equatorial low pressures, the band of intense rain, and the band of high cloud albedo at the equator (see figure 1-12) are all associated with the upwelling of very warm, moist air at the junction of the two Hadley cells. The region where the two Hadley cells meet and air rises strongly is referred to as the Intertropical Convergence Zone (ITCZ). Figure 2-8 shows a satellite image of the cloud band associated with ITCZ storm systems.

The three-cell pattern of circulation comes about as a result of competition between the Coriolis effect and pressure-driven forces. Their relative size on our planet means that a body of air moving directly toward the pole from the equator in response to thermal stresses is deflected to the east-west after it has traveled through about 30° of latitude, or one-third of the full equator-to-pole distance. If Earth rotated more slowly or the pressure gradient were stronger, there might only be two cells. Under a different balance of forces, there might be four, or only one. Jupiter has five circulating cells in each hemisphere, and the balance is quite different. The Great Red Spot is a storm in one of the cells. As previously noted, Venus is rotating so slowly that it has only one cell.

The return flow in the upper atmosphere is also shown in figure 2-7, and the flow is a motion away from the equator. The Coriolis effect, therefore, causes a right-directed deflection of the return flow as well (see box 2.1). The surface flow is toward the equator, so the deflection is to the west, and the return flow deflection is to the east. This gives rise to an overall circulation pattern that is oblique to lines of latitude.

In addition to the three circulating cells in each hemisphere, two jet streams (channels of very strong high-level winds) are shown at the frontal areas in figure 2-9, where two circulation cells meet within the troposphere. These are regions of very high pressure and temperature gradients that drive strong winds approaching two hundred miles per hour, which is stronger than typical cyclonic winds. Weather on either side of the polar jet stream can be profoundly different.

The jets have oscillatory paths around Earth that are constantly changing. When a lobe of the polar jet dips south from Canada, it can bring very cold conditions to much of North America. The polar jet is important to commercial air travel across the United States and the Atlantic. A plane flying with the advantage of the jet stream can save more than an hour of flying time.

Looking back at figures 1.1 and 1.2, together with figure 2-7, we see the broad correspondence between circulation cells in the atmosphere and climate zones. Within the three-cell pattern in any hemisphere, there are two regions of upwelling—one at the equator and one at about 60° N/S, which includes traditionally rainy places such as Great Britain, northern Europe, Iceland, and Washington State in the United States, and in the south Tasmania and Terra del Fuego. The upwelling region between the polar cell and the Ferrel cell is often described as the polar front. It is similar to the upwelling in the ITCZ, but it is much colder.

At 30° N/S, and at the North and South Pole, the opposite occurs. These regions are at the down-welling limbs of circulation cells. The great deserts of the world are located at around 30° N/S. In these areas, rainfall in the ITCZ has removed most of the moisture from the air. As the air moves over the top of the Hadley cell and then descends at around 30°, it becomes very dry and, because air is descending, atmospheric pressure is high. Thus the deserts owe their location to the three-cell pattern of atmospheric circulation, and their dryness to loss of moisture that “rained out” in the tropics.

Regions affected by the ITCZ have two distinct seasons—a rainy season and a dry season—whereas temperate middle latitudes experience four distinct seasons. It is not hard to imagine that agricultural practices differ between the tropics and temperate zones. Regions under the influence of the ITCZ experience a band of heavy rainfall moving from north to south and then reversing with the seasons. In July the band of heavy rainfall is north of the equator, and in January it is south of the equator (figure 2-10).

This apparent motion of the ITCZ also accounts for the Indian monsoon, an extremely important weather phenomenon Indian farmers rely on to recharge aquifers, which provide water essential for crop irrigation. The position of the ITCZ is shown at the right in figure 2-11 in summer during the monsoon and is at the left during the winter. During summer, the ITCZ is well north of its winter position, lying just south of the Himalayan Mountains. Just as described for sea breezes (see figure 2-5), the landmass retains much more heat than the ocean, causing a pressure differential that drives a landward circulation of very moist air off the ocean in the summer.

The monsoon rains in India are strengthened by the orographic effect (figure 2-12). Moist air coming off the Indian Ocean is forced to rise over the Himalayan Mountains, and as it rises it cools and water vapor condenses, causing rainfall on the windward side of the mountains. A region of dry conditions, often referred to as a rain shadow, is created on the leeward side of the mountains due to this dry air. The coastal ranges in central California also exhibit this pattern of precipitation. West Africa and many other regions experience a summer monsoon as well, but the monsoons are more intense in India than elsewhere because of the dominating effect of the Himalayas. The orographic effect also can be seen in the way many mountaintops and islands are often cloud covered.

What makes the ITCZ move like this? In fact it doesn’t move, but it appears to do so to an observer on Earth. This is another case (like the Coriolis effect) in which the frame of reference is important. The clue here is that the perceived motion is seasonal and relates to the position of Earth with respect to the Sun at different times of the year (figure 2-13).

Earth rotates on an axis that is not perfectly at right angles but is tilted about 23.5° to the plane of the ecliptic in which it rotates around the Sun (see figure 1-6). This tilt is the reason Earth experiences opposite seasons in the northern and southern hemispheres. In the northern summer, the Sun’s rays warm more of Earth’s northern hemisphere. The opposite is true in the northern winter, which is the southern summer. (In figure 2-13, you can see that the Sun’s energy strikes Earth north of the equator on June 21.) In spring and fall, both hemispheres are heated equally. Fall and spring have similar meteorological conditions in both hemispheres, although one is transitioning from cold to warm and the other from warm to cold.

The ITCZ forms where the Sun’s energy is most intense, sometimes referred to as the climatological equator, not at the geographic equator. The arrows in figure 2-13 point directly from the Sun toward the point on Earth where the ITCZ develops. The ITCZ can be thought of as following the Sun. It is located at the equator in spring and the fall, and is north of the equator in the northern summer and south of the equator in the northern winter. The ITCZ, in effect, remains fixed within the plane of the ecliptic. To an observer attached to the plane of the ecliptic, Earth would appear to wobble back and forth beneath a fixed ITCZ. From a fixed vantage point on Earth in the tropics, however, the ITCZ appears to oscillate north to south with the seasons.

Although north-south variations dominate the climate system, important east-west variations occur in a number of places (see figures 1.2 and 1.4). North America is climatologically divided almost down the middle: the western half is dry and the eastern half is much wetter. The dividing line where the Great Plains begins is close to the 100th meridian west. South America shows similar variations east to west at a different longitude.

The key to understanding what causes these variations is to include the effect of ocean circulation. The pattern of surface water movements is shown in figure 2-14. It is important to emphasize that this is the motion of water at the surface. At depth, waters may flow in the opposite direction to water at the surface, primarily because surface water flow is strongly influenced by the global pattern of winds. This is somewhat analogous to circulation in the atmosphere, although the oceans are much denser and are less subject to the Coriolis effect.

Heat in the surface ocean is easily imparted to the air above it and is, in effect, carried along with the water as it slowly moves. Water, having once gained warmth, releases heat much more slowly than air, and thus warm air can be carried into regions where one might expect much cooler air from atmospheric circulation alone. The reverse is also true: cooler water can bring cool air toward the equator.

In addition, the density of water masses is governed by both temperature and salinity of the ocean water and circulation in the oceans, which is referred to as thermohaline. Of course, ocean currents are profoundly influenced by landmasses that deflect them from where they might have traveled had the landmass not been present. This is not strictly the oceanographic equivalent of orographic effects on rainfall described previously, but it does bear some similarities. The only place on Earth where currents flow unimpeded by landmasses is in the Antarctic Circumpolar Current, which, as its name implies, sweeps around the entire continent of Antarctica without intersecting any obstacles.

Surface water motion is characterized by circulating gyres, which are much larger in scale and move much more slowly than atmospheric circulation patterns. Horizontal and vertical friction of water against the continents and seafloor also play an important role. The North Atlantic Gyre is an almost circular feature with a clockwise rotation that hugs the coast of both North America and northwest Africa and crosses the central equatorial Atlantic much like the trade winds, where the waters are warmed in a region of high solar irradiation. Traveling west, they meet the landmass of North America and are deflected north along the east coast of the continent, carrying warm water northward. The water warms the atmosphere, and air becomes both warm and moist, which leads to additional warmth and precipitation along the east coast of the United States and further north into the North Atlantic.

An additional factor is that the west coast of the United States experiences strong orographic rain effects, whereas the east coast—where mountains are much older, eroded, and lower than in the west—does not experience significant orographic effects. The same is true for the west and east coasts of Latin America. In both cases, these effects contribute considerably to the east-west variation in climate conditions.