7.4: Climatic Winds

Winds are extremely variable from one day to the next at any location. The day-to-day variations are part of what we call “weather.” Weather events may include calm, thunderstorms, hurricanes, fog, and the other phenomena that a weather forecaster tries to predict hours or days before they happen. If we average the day-to-day variation over relatively large areas and over periods of several days or weeks, we find patterns of average winds, temperatures, rainfall, and so on that are relatively repeatable from one year to the next in each area. These long-term average patterns are called “climate.” This chapter is concerned primarily with climatic winds, temperatures, and rainfall, although some aspects of various weather events are also examined.

Climatic Winds on a Nonrotating Earth

On a nonrotating Earth with no continents or Coriolis effect (CC12), wind patterns would be simple (Fig. 7-9). More heat is transferred to the atmosphere from the oceans than is absorbed by the atmosphere directly from the sun (Fig. 7-6). Solar heating of the oceans is greatest at the equator. Thus, ocean water is warmer and the rate of heat transfer by evaporation of water from ocean to atmosphere is greater than at higher latitudes. The equatorial air mass would rise through the troposphere, creating a low-pressure zone and horizontal pressure gradient in the atmosphere at sea level. The rising air would be replaced by air flowing into the region from higher latitudes. Once it reached its equilibrium density level (CC1), the rising air would spread toward the poles. As the air mass moved north or south in the upper troposphere, it would progressively cool and release its water vapor as rain or snow.

Figure 7-9. Atmospheric convection cells and winds on a hypothetical water-covered, nonrotating Earth.

Once the air mass reached the pole, it would have sufficiently cooled and lost water vapor by condensation or deposition to become dense enough to sink and create a high-pressure zone in the atmosphere at sea level. The air would then move toward the equator along the horizontal atmospheric pressure gradient between the polar high-pressure zone and the equatorial low-pressure zone. As the air moved back toward the equator, it would be warmed and would gain water vapor from evaporation of ocean surface water. This is a simple convection cell motion (CC3). There would be a convergence at sea level at the equator, and divergence at sea level at the poles. Surface winds would blow from the poles toward the equator in each hemisphere. This simple two-convection-cell pattern (Fig. 7-8) illustrates the basic principles of atmospheric circulation, but it does not exist on the Earth, because we have so far not accounted for the fact that the planet rotates and moving air is deflected by the Coriolis Effect. The presence of land is also not yet accounted for, but we can best understand the complications that land introduces if we first look at a hypothetical water-covered Earth that is rotating.

Climatic Winds and the Coriolis Effect

The deflection of air movements by the Coriolis Effect, caused by Earth’s rotation, results in an atmospheric circulation system that consists of three convection cells arranged latitudinally in each hemisphere (Fig. 7-10). To some extent, this real-world system has the same net result as the simple two-convection cell system: air rises at the equator and sinks at the poles, and heat is transported from the equator to the poles.

Figure 7-10. Atmospheric circulation cells and winds on a hypothetical water-covered, rotating Earth are arranged in latitudinal bands. There are three cells in each hemisphere. Upwelling occurs at the equator and the polar fronts, and downwelling at the poles and in mid latitudes. Note how the Coriolis Effect deflects moving air masses: air masses moving away from the equator are deflected to the east, and air masses moving toward the equator are deflected to the west. Atmospheric convection cells are arranged in this general pattern on the Earth, but they are modified substantially by the influence of the land masses.

To understand how a three-cell per hemisphere circulation pattern develops, consider the movements of an air mass initially located at sea level on or near the equator. The air mass is heated by solar radiation and has a high water vapor content because of the relatively high evaporation rate from the ocean’s surface. It rises through the troposphere to its density equilibrium level and spreads out toward the poles, just as it would on a nonrotating Earth. As the air mass moves away from the equator, where there is no deflection by the Coriolis Effect, it is not deflected significantly until it reaches 5° to 10°N or S (Fig. 7-10). As it continues to move toward the poles, the air mass is deflected increasingly toward the east because the Coriolis Effect increases with latitude.

At about 30°N or 30°S, the now eastward-moving air has risen, expanded, cooled, and lost most of its water vapor. The cooler, dry air sinks to the surface, where some air moves toward the pole and some moves away from it, forming a high-pressure zone and atmospheric divergence at ground level. Air that moves toward the pole in this region enters the “Ferrel cell” circulation (Fig. 7-10). Air that moves back toward the equator is deflected toward the west. The deflection decreases as the air mass moves nearer the equator. Consequently, surface winds in the subtropical regions blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere (Fig. 7-10). These are the trade winds, and the convection cell air movements that occur between the equator and about 30°N and 30°S are called “Hadley cells.”

Surface winds in the Ferrel cells (Fig. 7-10) are also deflected by the Coriolis Effect, but to the east because they are moving away from the equator. Therefore, this is a region of persistent surface winds from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere, commonly called the westerlies. Winds are named for the direction from which they are blowing from.

Although the predominant winds are from the east in subpolar regions, air movements in the polar cells are complex because of the stronger Coriolis deflection and the presence of the jet streams. Jet streams are swiftly moving west-to-east air currents high in the troposphere. They are located over the boundaries between the Ferrel and polar cells, called the “polar fronts” (or “Antarctic front” in the Southern Hemisphere). The jet streams undergo complex meanders associated with movements of the polar fronts. Because storms often form along the jet stream, and because meanders in the jet stream can bring cold polar air to lower latitudes and warm air to higher latitudes, the orientation and trajectory of the jet stream are routinely reported in many weather forecasts. 

In any one region, the predominant wind direction and its persistence, the average extent of cloud cover, and the average rainfall are determined largely by the region’s location with respect to the atmospheric convection cells (Fig. 7-10). In the equatorial region known as the Doldrums, where trade winds converge and atmospheric upwelling occurs, sea-level winds are light and variable. In addition, because the rising moist air is cooled, cloud cover is persistent and rainfall is high. This region is called the intertropical convergence zone because it is where two wind systems converge. Air rising from the sea surface through the troposphere causes low atmospheric pressure at sea level in the intertropical convergence zone. Air also rises through the troposphere at the polar front. However, unlike the intertropical convergence zone that migrates seasonally but otherwise remains generally invariable, the polar front oscillates in wavelike motions, called “Rossby waves” (Chap. 11), which have timescales of days or weeks. These waves can spawn massive storms called extratropical cyclones.

In the atmospheric downwelling (or subsidence) regions between the Hadley cells and Ferrel cells and at the poles, atmospheric pressure is high, winds are light, skies are usually cloudless, and rainfall is low. The subtropical high-pressure zones between the Hadley and Ferrel cells are called the “horse latitudes.” Sailing ships were often becalmed for long periods in these regions, and the horses on board were killed to conserve water. Because of the high-pressure zone at the South Pole, much of Antarctica is a desert with very low annual precipitation. In fact, there are dry desert valleys in Antarctica with no snow cover, and the interior of Antarctica receives only about 50 mm of precipitation per year (desert climates are defined as having less than 254 mm of rain per year). The massive amounts of ice and snow in the thick ice sheet that covers other parts of Antarctica took millennia to accumulate, and the ice sheet exists only because Antarctic temperatures are too low for snow to melt during the summer.

Under the centers of the atmospheric cells (between convergences and divergences), winds are generally persistent, particularly in the trade wind zones. Rainfall and cloud cover are variable, and the zones are affected by periodic strong storms, especially in the higher-latitude areas of the westerly wind zones, where storms form on the polar front. Westerly surface winds under the Ferrel cells can be particularly strong, especially in the Southern Hemisphere, where few continents disturb the circulation. Sailors call the area of strong westerly winds at about 40°S the “roaring forties.”

The atmospheric convection cell system described here is a simplified view of the climatic wind patterns on the Earth’s surface. Complications of this simplified pattern occur because the interactions between land and atmosphere differ from the interactions between ocean and atmosphere. The pattern of high- and low-pressure zones and winds described by the six-cell convection system model generally holds true over the oceans but is much modified over the large land masses of the continents. The strong Coriolis Effect and shallow troposphere of the polar cell regions favor horizontal transport of air masses by weather systems and poor development of vertical convection cell motion. Complications in the six-cell system also occur because of the seasonal movement of the Earth around the sun and certain longer-term oscillations of the ocean-atmosphere interaction, such as El Niño, which is described later in this chapter. Finally, local wind patterns are highly variable because weather events operate at much smaller geographic scales than do the global climatic wind patterns discussed in this section.

Seasonal Variations 

Because the Earth’s axis is tilted in relation to the plane of the Earth’s orbit around the sun, the latitude at which the sun is directly overhead at noon changes progressively during the year. At the Northern Hemisphere summer solstice (June 20 or 21), the sun is directly overhead at 23.5°N, the Tropic of Cancer. At the autumnal equinox (September 22 or 23) and the spring equinox (March 20 or 21), the sun is directly overhead at the equator. At the Northern Hemisphere winter solstice (December 21 or 22), the sun is directly overhead at 23.5°S, the Tropic of Capricorn (Fig. 7-8a).

As the Earth moves around the sun, the latitude of greatest solar intensity migrates north and south, resulting in a corresponding northward or southward displacement of the atmospheric convection cells. The displacement of atmospheric convection cells causes seasonal climate variations. The seasonal movement of convection cells is shown by a seasonal shift in wind bands (Fig. 7-11) and in surface-level atmospheric high- and low-pressure zones (Fig. 7-12). 

Figure 7-11. Climatic winds over the ocean surface in (a) January and (b) July. Winds generally blow from higher pressure toward lower pressure. Note the seasonal migration of the subtropical high-pressure zones, the intertropical convergence zone, and the associated trade winds. The migration is more pronounced over the major continental landmasses than over the oceans. Over the Indian Ocean, the seasonal migration is enhanced by the strong monsoonal interaction between Asia and the Indian Ocean.

Figure 7-12. Mean atmospheric pressures at sea level in (a) January and (b) July. The greater complexity of the geographical distribution and seasonal change of atmospheric pressure in the Northern Hemisphere is due to the concentration of the landmasses in this hemisphere. The ocean moderates regional coastal climates, leading to larger seasonal changes in pressure over large landmasses such as Asia.

Because the latitude at which tropospheric air temperature is highest changes seasonally, the convection cells also migrate seasonally. Tropical tropospheric air temperature is controlled primarily by radiation and evaporative heat inputs from the oceans and land. Because water has a high heat capacity, a great amount of heat energy is necessary to warm the ocean surface waters (CC5). Therefore, water temperature, evaporation rate, and air temperature do not increase in step with changes in solar intensity as the latitude of greatest solar intensity shifts north and south. Several weeks of increased (or decreased) solar heating must elapse before these parameters change enough to cause the atmospheric convection cells to move. Consequently, there is a time lag between the sun’s seasonal movement and the movement of the atmospheric convection cells. The time lag is why many northern countries have their warmest weather in August and their coldest weather in February, approximately 2 months after the summer and winter solstices occur in June and December, respectively.

Heat captured from solar radiation is transferred to the atmosphere more rapidly by land than by oceans, primarily because of the high heat capacity of water. As a result, the time lag between seasonal changes in solar radiation and air temperature is shorter over the continents than over the oceans. Because the continents are concentrated in the Northern Hemisphere, seasonal migration of convection cells has a shorter time lag north of the equator than south of the equator. Furthermore, the center of the latitudinal wind bands, the intertropical convergence zone, migrates farther north of the equator in the Northern Hemisphere summer (especially over the continents) than it does south of the equator during the Southern Hemisphere summer (Fig. 7-12).

Seasonal changes in climate are greater in areas that are under an atmospheric convergence or divergence zone in one season and under the middle of a convection cell in another. Seasonal climate changes are also greater in higher latitudes than near the equator. The reason is that the total amount of solar heat reaching the Earth’s surface per day depends on both the sun’s angle and the amount of time during which the sun is above the horizon on a given day. Both the sun’s intensity and the day length vary more between summer and winter at high latitudes than at low latitudes.

Monsoons 

In the Northern Hemisphere summer, the intertropical convergence zone moves farther north over the landmasses of Asia and Africa than it does over the oceans. Warm, moist tropical air moves north onto Asia from the Indian and western Pacific Oceans in the trade wind zone under the southern Hadley cell (Fig. 7-13a). Thus, in summer the southern Hadley cell trade winds extend into the Northern Hemisphere, where the Coriolis deflection is to the right. These winds, called monsoon winds, are deflected by the Coriolis Effect in the Northern Hemisphere, so they blow from the southwest, whereas trade winds in the Southern Hemisphere blow from the southeast.

Figure 7-13. Indian Ocean monsoon winds reverse seasonally. (a) In summer, an intense low-pressure zone forms over Asia as the land is heated by the sun and loses this heat to the atmosphere more quickly than does the ocean. The heated air rises to form the low-pressure zone at a latitude where, if the Earth were covered by ocean, the Hadley and Ferrel cells would intersect and atmospheric pressure would be high. The low-pressure zone causes the atmospheric convection cells to shift to the north and bring warm, wet, monsoon winds from the southwest to parts of Asia. A similar, but less pronounced effect occurs over North Africa. (b) In winter, the Asian landmass loses heat more quickly than does the ocean, and an intense high-pressure zone is formed by the sinking cold, dry air mass. As this air mass spreads to the south, it shifts the convection cells south, which creates the northeast monsoons and brings dry weather to much of Asia.

The warm, moist air carried by monsoon winds causes storms and torrential rains in India and Southeast Asia. This densely populated region depends on the rains to support agriculture. However, the area is plagued by alternating floods and droughts. When monsoons are strong, devastating floods occur. When monsoons are weak or absent for a year or two, drought and famine occur. The nature of the multiyear, or “interannual,” climate variations that cause prolonged famine or drought is just beginning to be understood. We discuss some of these variations later in this chapter.

In the northern winter, the intertropical convergence zone moves south over the Indian and Pacific Oceans and Africa. In addition, the divergence between the Hadley and Ferrel cells moves far to the south over Asia in winter for the same reason that the intertropical convergence zone moves far north in summer. Thus, in winter this downwelling zone lies partly south of the Himalayan Mountains. The cool, dry downwelling air flows south over India, producing dry winters (Fig. 7-13b). Similar, but weaker and more complicated, seasonal monsoon wind reversals occur throughout East Asia, tropical Africa, and northern Australia (Fig. 7-13).

Sailors have taken advantage of the monsoons for thousands of years. Coastal traders sail on winter trade winds from India and Southeast Asia to East Africa and Madagascar and returned on the summer monsoon winds.