13.2: The Origin of Deserts

Atmospheric Circulation

Geographic location, atmospheric circulation, and the Earth’s rotation are the primary causal factors of deserts. Solar energy converted to heat is the engine that drives the circulation of air in the atmosphere and water in the oceans. The strength of the circulation is determined by how much energy is absorbed by the Earth’s surface, which in turn is dependent on the average position of the Sun relative to the Earth. In other words, the Earth is heated unevenly depending on latitude and angle of incidence. Latitude is a line circling the Earth parallel to the equator and is measured in degrees. The equator is 0° and the North and South Poles are 90° N and 90° S respectively. Angle of incidence is the angle from perpendicular to the ground surface made by a ray of sunlight shining on the Earth’s surface. Tropical zones are located near the equator, where the latitude and angle of incidence are close to 0°, and receive high amounts of solar energy. The poles, which have latitudes and angles of incidence approaching 90°, receive little or almost no solar energy.

The figure shows the generalized air circulation within the atmosphere. Three cells of circulating air span the space between the equator and poles in both hemispheres, the Hadley cell, the Ferrel or Mid-latitude cell, and the Polar cell. In the Hadley cell located over the tropics and closest to the equatorial belt, the Sun heats the air and causes it to rise. The rising air cools and releases its contained moisture as tropical rain. The rising dried air spreads away from the equator and toward the North and South Poles, where it collides with dry air in the Ferrel cell. The combined dry air sinks back to the Earth at 30° latitude. This sinking drier air creates belts of predominantly high pressure at approximately 30° north and south of the equator, called the horse latitudes. Arid zones between 15^o and 30^o north and south of the equator thus exist within which desert conditions predominate [2]. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called trade winds near the equator, and westerlies in the temperate zone. Note the arrows indicating general directions of winds in these zones.

Figure \(\PageIndex{1}\): Generalized atmospheric circulation. (By Kaidor; CC BY-SA 3.0 via Wikimedia Commons.)

Other deserts, like the Great Basin Desert that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena [3]. The Great Basin Desert, while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As westerly moist air from the Pacific rises over the Sierra Nevada and other mountains, it cools and loses moisture as condensation and precipitation on the upwind or rainy side of the mountains.

Figure \(\PageIndex{2}\): USGS Map of the Great Basin Desert.

One of the driest places on Earth is the Atacama Desert of northern Chile [4]. The Atacama Desert occupies a strip of land along Chile’s coast just north of latitude 30° S, at the southern edge of the trade-wind belt. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing trade winds blowing west. As this warm, moist air crossing the Amazon basin meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the Atacama Desert. Onshore winds from the Pacific are cooled by the Peru (Humboldt) ocean current. This super-cooled air holds almost no moisture and, with these three factors, some locations in the Atacama Desert have received no measured precipitation for several years. [5]. This desert is the driest, non-polar location on Earth.

Figure \(\PageIndex{3}\): Map of the Atacama Desert (yellow) and surrounding related climate areas (orange).

Notice that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the Polar cell [6]. As with the other cells, cold air, which holds much less moisture than warm air, descends to create polar deserts. This is why historically, land near the North and South Poles has always been so dry.

Figure \(\PageIndex{4}\): The polar vortex of mid-November, 2013. This cold, descending air (shown in purple) is characteristic of polar circulation.

Coriolis Effect

In a non-rotating Earth, air would rise at the equator and sink at the poles, creating one circulation cell. However, as noted above, Earth has three atmospheric circulation cells. Why? As objects move on a rotating sphere, an effect called the Coriolis effect occurs which causes a deflection in the motion. In the Northern Hemisphere, this deflection is to the right; in the Southern Hemisphere, it is to the left.

The Earth rotates toward the east where the Sun rises. Think of spinning a weight on a string around your head. The speed of the weight depends on the length of the string. The speed of an object on the rotating Earth depends on its horizontal distance from the Earth’s axis of rotation. Higher latitudes are a smaller distance from the Earth’s rotational axis, and therefore do not travel as fast eastward as lower latitudes that are closer to the equator. When a fluid like air or water moves from a lower latitude to a higher latitude, the fluid maintains its momentum from moving at a higher speed, so it will travel relatively faster eastward than the Earth beneath at the higher latitudes. This factor causes deflection of movements that occur in north-south directions.

Figure \(\PageIndex{5}\): Example of the motion of a ball thrown on a rotating body. In the inertial frame of reference of the top picture (bird's eye view), the ball moves in a straight line. The observer, represented as a red dot, standing in the rotating frame of reference sees the ball following a curved path. This perceived curvature is due to the Coriolis effect and centrifugal forces.

Another factor in the Coriolis effect also causes deflection of east-west movement due to the angle between the centripetal effect of Earth’s spin and gravity pulling toward the Earth’s center. This produces a net deflection toward the equator. The total Coriolis deflection on a mass moving in any direction on the rotating Earth results from a combination of these two factors.

Figure \(\PageIndex{6}\): Forces acting on a mass moving East-West in the Northern Hemisphere on the rotating Earth that produce the Coriolis effect.

The Coriolis effect has two consequences on masses of air (and water) moving on the Earth. In the Northern Hemisphere Hadley cell, the lower altitude air currents are flowing south toward the equator.  These are deflected to the right (or west) by the Coriolis effect.  This deflected air generates the prevailing trade winds that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands in their tall ships.  This air movement is mirrored in the Hadley cell in the Southern Hemisphere; the lower altitude air current flowing equatorward is deflected to the left, creating trade winds that blow to the northwest.

In the northern Mid-latitude or Ferrel cell, surface air currents flow from the horse latitudes (30°) toward the North Pole, and the Coriolis effect deflects them toward the east, or to the right, producing the zone of westerly winds.  In the Southern Hemisphere Mid-latitude or Ferrel cell, the poleward flowing surface air is deflected to the left and flows southeast creating the Southern Hemisphere westerlies.

Another Coriolis-generated deflection produces the Polar cells. At 60° north and south latitude, relatively warmer rising air flows poleward, cooling and converging at the poles, where it sinks in the polar high. This sinking dry air creates the polar deserts, the driest deserts on Earth. Persistence of ice and snow is a result of cold temperatures at these dry locations.

Figure \(\PageIndex{7}\): Inertia of air masses caused by the Coriolis effect in the absence of other forces.

The Coriolis effect operates on all motions on the Earth. Artillerymen must take the Coriolis effect into account on ballistic trajectories when making long-distance targeting calculations. Geologists note how its effect on air and oceanic currents creates deserts in designated zones around the Earth. The Coriolis effect causes the ocean gyres to turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. These currents bring cold water along the west coasts of both North and South America contributing to the drier climates of the Atacama and Central and Southern California. It also affects weather by creating high-altitude, polar jet streams that sometimes push lobes of cold arctic air into the temperate zone, down to as far as latitude 30° from the usual 60°. It also causes low pressure systems and intense tropical storms to rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis effect acting on both the atmosphere and ocean is a major contributor to climate and weather on the Earth.

Figure \(\PageIndex{8}\): Gyres of the Earth's oceans.

Watch a video on the Coriolis effect:

References

2. Hadley, G. Concerning the cause of the general trade-winds: By Geo. Hadley, Esq; FRS. Philosophical Transactions 39, 58–62 (1735).

3. Grayson, D. K. The desert’s past: a natural prehistory of the Great Basin. (Smithsonian Inst Pr, 1993).

4. Hartley, A. J. & Chong, G. Late Pliocene age for the Atacama Desert: Implications for the desertification of western South America. Geology 30, 43–46 (2002).

5. Collado, G. A., Valladares, M. A. & Méndez, M. A. Hidden diversity in spring snails from the Andean Altiplano, the second highest plateau on Earth, and the Atacama Desert, the driest place in the world. Zool. Stud. 52, 50 (2013).

6. Littell, E. & Littell, R. S. Littell’s Living Age. (T.H. Carter & Company, 1846).