CHAPTER 11
Wind: Global Systems





Consider an idealized model of Earth as a non-rotational sphere with uniform solid surface. Since the sun heats the equatorial regions more intensely than the polar regions, a convection cell will develop in each hemisphere due to meridional temperature gradient on the sunny side of the globe. The planetary-scale air will then circulate between the equator and the poles such that warm (less dense) air rises at the equator, and flows toward the poles aloft, while cold (dense) air sinks at the poles and flows toward the equator at surface. If the idealized Earth begins to rotate, the Coriolis force will shift the surface winds to southwest in the Northern Hemisphere, northwest in the Southern Hemisphere.

Figure 10.1, page #266 (Ahrens)


Subtropical anticyclones: semi-permanent warm core, high pressure systems centered over subtropical latitudes (approximately 30 N and 30 S) of the North and South Atlantic, North and South Pacific, and the Indian Ocean.

The eastern side of each subtropical high is associated with subsiding stable air, which produces low relative humidities and sunny skies, while the atmosphere at far western side of each subtropical high is less stable and can be associated with cloudy, humid weather. Therefore, the world's major deserts are located at the eastern side of the subtropical high such as the Sahara of North Africa and southwest US which is under the influence of Hawaiian high , particularly in winter, while cloudy, humid weather prevails the southeast US which is under the influence of Bermuda-Azores high , particularly in summer.

A subtropical high features a weak horizontal pressure gradient over a broad area at around the system's center. Hence, surface winds are very light or even calm over subtropical oceans.

Horse latitudes: the east-west belt of latitude at about 30 to 35 where winds are predominantly light and the weather is hot and dry.


Subtropical anticyclones exert higher surface pressures in summer than in winter. They shift toward the poles in spring and toward the equator in autumn. The north to south variation of the subtropical anticyclone has a substantial influnce on seasonal climate.

An unexpected nortward shift of Bermuda-Azores anticyclone produces unsusal mild winter weather in the New England, known as January thaw. The westward displacement of Bermuda-Azores high in Ocotober or November is associated with unsual warm weather over the eastern US, known as Indian summer. The nortward shift of North Pacific subtropical anticyclone extends the influence of Bermuda-Azores high westward toward southwestern US where it brings a rainy end to dry spring in late June.

Subpolar lows: semi-permanent cyclone where surface midlatitude southwesterlies converge with the polar northeasterlies. Subpolar lows, the Aleutian low over the North Pacific Ocean and the Icelandic low over the North Atlantic, are well-developed in winter, but weaken or disappear in summer. In the Southern hemisphere, the subpolar low forms a continuous trough that completely encircles the globe.


Inter-tropical convergence zone (ITCZ): an discontinuous belt of thunderstorms parallel to equator. It is the boundary zone separating the northeast trade winds of the Northern hemisphere from the southeast trade winds of the Southern Hemisphere. Its average location is just north of the geographic equator, so-called heat equator where the Earth's highest mean surface temperature exists.

The north to south variation of the ITCZ over the oceans is only 4 through the year, considerable less than over the continents since the oceans exhibit greater thermal stability. The northward migration of the ITCZ in spring triggers summer monsoon rains in Central America, North Africa, India, and Southeast Asia.

Because of seasonal temperature contrasts, the continents are dominated by relatively high pressure in winter and relatively low pressure in summer. The cold anticyclones that form over northwestern North America and over interior Asia, Siberian high, in winter are called thermal high and the warm cyclones that form over central-south Asia, and over southwestern US are called thermal low.

Figure 10.4, pages #269 (Ahrens)

Figure 10.5, pages #270 (Ahrens)


Trade winds: the persistent planetary-scale surface winds that blow from northeast in the Northern Hemisphere and from southeast in the Southern Hemisphere between the equator and 30 latitude. At the same time, the middle- and upper-tropospheric winds blows opposite to the trade winds resembling a huge convection cell, namely Hadley cell, with rising air over the equator and sinking air in the subtropical anticyclones.

A similar but opposite in direction convection cell exists between subtropical anticyclones and subpolar lows, known as Ferrel cell. Another convection cell, but weak in magnitude with respect to Hadley cell exists between subpolar lows and the poles, called Polar cell.

Doldrums: an east-west belt of light and variable surface winds near the equator where the trade winds of the two hemispheres converge.


Midlatitude westerlies: the highly variable planetary-scale winds that blow from southwest in the Northern Hemisphere and from northwest in the Southern Hemisphere between 30 and 60 latitude. In middle- and upper-troposphere, midlatitude westerlies blow west to east in a wavelike pattern of ridges and troughs. These winds are responsible for the development and displacement of the weather systems (highs, lows, and air masses) and for the poleward heat transfer.

The tropopause is not continuous from poles to equator but occurs in discrete segments. The altitude of the tropopause is directly proportional to the mean temperature of the troposphere; therefore, it decreases with latitude.

Figure 10.11, page #275 (Ahrens)


Polar easterlies: the planetary-scale winds that blow from northeast in the Northern Hemisphere and from southeast in the Southern Hemisphere between 60 latitude and the poles.

Polar front: the transition zone between cold polar easterlies and mild midlatitude westerlies. The polar front is not continuous around the globe; it is well-defined in the presence of sharp temperature gradient.

Figure 10.2, page #267 (Ahrens)

Figure 10.7, page #271 (Ahrens)


Atmospheric waves: the pattern of ridges and troughs generated by middle to upper tropospheric winds. Winds exhibit a clockwise (anticyclonic) curvature in the ridges and counter-clockwise (cyclonic) curvature in the troughs in the Northern Hemisphere.

Atmospheric waves are characterized by wavelength, distance between two successive troughs or two successive ridges, amplitude, north-south extent, and the number of waves encircling the hemisphere. There are two types of atmospheric waves: i) long waves and ii) short waves.


Long waves in the midlatitudes are weaving westerlies that consist of north-south (meridional) and east-west (zonal) component. They are also called Rossby waves that are characterized by a long length (thousand of kilometers) and significant amplitude. Between two and five Rossby waves typically encircle the hemispheres.

Rossby waves are more vigorous in winter than in summer. They exhibit fewer waves of longer length and greater amplitudes in winter than in summer. This is because of sharp temperature contrast, and therefore strong north-south pressure gradient in winter.

Figure 10.10, pages #273 (Ahrens)


Rossby waves shift their pattern back and forth between dominantly zonal and dominantly meridional flow that affect north-south air mass exchange, poleward heat transfer, and storm development and movement. When Rossby waves to the north have different wave configuration than that to the south, their pattern, known as split flow pattern, is quite complicated. For example, winds may be zonal over Canada and meridional over the United States.


Short waves are relatively small ridges and troughs superimposed on long Rossby waves. They travel rapidly through the Rossby waves. There may be a dozen or more short waves present at a given time in contrast to five or less Robby waves encircling the hemispheres.

Figure 12.9, page #328 (Ahrens)


Both long and short waves can contribute to cyclone development. For the same horizontal pressure gradient, anticyclonic gradient winds are stronger than geostrophic winds, and cyclonic gradient winds are weaker than geostrophic winds. Therefore, midlatitude westerlies tend to strengthen in a ridge and weaken in a trough. This induces horizontal convergence of air to the east of a ridge and horizontal divergence to the west of a ridge.

Figures 2-3, page #327 (Ahrens)


Blocking system: a cutoff cyclone or anticyclone that blocks the usual west-to-east progression of weather systems. The pool of cold air rotating counter-clockwise is a blocking cyclone, while the pool of warm air rotating clockwise is a blocking anticyclone, also referred to a omega block.

A blocking system is observed at middle to upper troposphere in high-latitudes. It persists usually several weeks or longer and is associated with extreme weather conditions such as anomalous heat (summer of 1976 in western Europe), anomalous cold (winter of 1972 throughout former Soviet Union), drought (spring and early summer of 1986 in the southeast US and spring and summer of 1988 in midwestern US), and flooding (summer of 1993 in midwestern US).


Jet stream: the relatively strong winds concentrated within a narrow band in the atmosphere. It is not always a continuous air stream, but may splinter into separate segments. Jet stream is usually thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Jet streams are usually found at elevations between 10 and 15 km, even though they may occur at both higher and lower altitudes.

Jet streak (jet maximum): the region of high wind speed that moves through the axis of a jet stream. Wind speeds in the center of jet stream often exceed 100 knots. The strongest jet maxima develop in winter along the east coasts of North America and Asia where the land-to-sea temperature contrast is particularly great. A typical jet maximum is 160 km wide, 2 to 3 km thick, and several hundred kilometers in length.

A jet maximum is outlined by lines of equal wind speed, isotachs, and is divided into four quadrants; left rear, left front, right rear, and right front. Horizontal convergence occurs in the left rear and right front, while horizontal divergence occurs in the right rear and left front.


Polar front jet stream (polar jet): the zone of strong winds situated in the upper troposphere (9 - 12 km) between the midlatitude tropopause and the polar tropopause and directly over the polar front.

Subtropical jet stream: the zone of strong winds situated in the upper troposphere between the tropical tropopause and the midlatitude tropopause and on the poleward side of the Hadley cell. Subtropical jet is stronger and less variable with latitude than the polar jet.

Figure 10.12, page #275 (Ahrens)

Jet streams contribute the development and maintenance of cyclones. A jet stream typically progresses from west to east at a faster pace than does the west to east displacement of the troughs and ridges in the westerlies. The strongest divergence occurs when a jet maximum is on the eastern side of trough where a cyclone typically develops.


Tropical easterly jet stream: the zone of strong winds situated near the tropopause on the equatorward side of the upper-level subtropical high above southeast Asia, India and Africa. It forms in summer having easterly winds that peak near 15 N.

Stratospheric polar night jet stream: the zone of strong winds situated in polar regions at an elevation near 50 km. It forms during dark polar winter.

Low-level jet: the zone of strong winds situated several hundred meters above the surface where winds blow from south or southwest reaching occasionally 60 knots. Low-level jet typically form at night above the temperature inversion.

During the summer, strong southerly winds carry moist air from the Gulf of Mexico into the Central Plains, where then is coupled with converging, rising air of the low-level jet, enhances thunderstorm formation. Therefore, on warm, moist, summer nights, when low-level jet is present, it is common to have nighttime thunderstorms over the plains.


Ekman spiral: the turning of ocean water with depth. At some depth, usually 100 m, the water can move in the direction opposite to the flow of water at the surface. This is because surface ocean currents are coupled with air motion at the surface, while the deep ocean currents does not affected by the air motion just above the sea surface.

Figure 10.19, page #281 (Ahrens)

Surface ocean currents: the patterns of surface water generated by the wind just above the sea surface. Because of large larger frictional drag in water, ocean currents move more slowly than the prevailing wind. Typically, they range in speed from several kilometers per day to several kilometers per hour. Major ocean currents do not follow the wind pattern exactly; rather, they spiral in semiclosed circular whirls, called gyres.

Figure 10.16, page #279 (Ahrens)


Oceanic front: a boundary that separates masses of water with different temperatures and densities. It develops, for example, off the east coast of the US when warm Gulf Stream meets cold water to the north.

Upwelling: the upward circulation of cold, nutrient-rich bottom water from depths of 200 to 1000 m (660 to 3280 ft.) to the ocean surface. It occurs along southwest coast of US, Mexico and northwest coast of South America when the ocean drives surface water westward off the coast as it couples with northeast trade winds in the Northern Hemisphere or southeast trade winds in the Southern Hemisphere. Upwelling produces good fishing, but swimming is only for the hardiest of souls.

Figure 10.20, page #281 (Ahrens)


El Nino: a period of anomalous warming of surface ocean waters in the central and eastern equatorial Pacific. It is accompanied by suppression of upwelling off coasts of Equator and northern Peru as warm surface water moves eastward into the eastern equatorial Pacific in December. A typical El Nino lasts only a few months, but occasionally, an intense El Nino persists for a year or longer, collapsing anchovy production along Peruvian coast, initiating weather extremes in widely separated regions of the world.

Southern oscillation: air pressure gradient between the eastern and western tropical Pacific. It is discovered by Gilbert Walker, who noted that the pressure at Darwin, Australia is inversely proportional to the pressure in Tahiti, a south Pacific island. El Nino begins when the air pressure gradient between the eastern and western tropical Pacific starts to weaken.

Figure 10.21, page #282 (Ahrens)

La Nina: a period of strong trade winds and unusually low sea surface temperatures in the central and east equatorial Pacific. An intense La Nina is accompanied by weather extremes that are usually opposite those of an El Nino. For example, the US drought of 1988 coincided with an strong La Nina.


During an intense El Nino, the southeast trades shift direction and become equatorial westerlies that drive warm surface waters eastward over a larger area of the Pacific. This process will then alter the planetary-scale circulation. The prevailing winds over western equatorial Pacific become offshore (westerly) and the weather is generally dry, while heavy rainfall is observed off the northwest coast of South America.

In midlatitudes, a poleward shift of polar jet brings relatively mild winter to the northern US and southern Canada, while a northward shift of subtropical jet brings abundant rainfall to the Gulf Coast.

Figure 10.25, page #284 (Ahrens)

Figure 10.26, page #285 (Ahrens)


Teleconnection: a linkage between weahter changes occuring in widely different regions of the world. The interaction between El Nino and Southern Oscillation (ENSO) and it s impact on varies part of the world is the best known example of atmospheric teleconnection.

Figure 10.24, page #284 (Ahrens)

Since the beginning of the 20th century, there were 25 ENSO events, two of which were noted strong, 1982-1983, and 1997-1998.


Pacific Decadal Oscillation (PDO): a reversal of atmospheric pressure over the Pacific Ocean that influences the weather over US. During its warm phase, the Aleutian low becomes stronger, resulting in more Pacific stroms into Alaska and California. Winters are warmer and drier in northwest US, drier over Great Lakes, and cooler and wetter in the southern US. The salmon production increases in Alaska, but diminish along the Pacific Northwest coast. The conditions reverse during cold phase.

Figure 10.27, page #286 (Ahrens)

Figure 10.28, page #288 (Ahrens)


North Atlantic Oscillation (NAO): a reversal of atmospheric pressure over the Atlantic Ocean that influences the weather over Europe and eastern North America. During its positive phase, the pressure gradient between Icelandic low and Bermuda high becomes stronger, resulting in stronger and numerous storms over the Atlantic, wetter and warmer northern Europe, milfer and drier southern Europe, wetter and milder eastern US, and cold and dry Canada and Greenland. During its negative phase, the pressure gradient is weaker, resulting in weaker and less storms over the Atlantic, cold and dry northern Europe, milder and wetter in Spain and around Mediterranean sea, cold and snowy eastern US, and mild Greenland.

Figure 10.29, page #288 (Ahrens)


Arctic Oscillation (AO): a reversal of atmospheric pressure over the Arctic that influences the weather over Europe and eastern North America. During its warm phase, the relatively lower pressure over the Arctic is coupled with relatively higher pressure in the south, resulting in strong upper-level westerlies, which in turn prevents the cold air to penetrate to the US, but eastern Canada and Greenland are colder than normal. The Northern Europe is hit by strong storms, resulting in mild and wet winters, while southern Europe remains dry. During its cold phase, cold Arctic air penetrates to the US in the presence of weak westerlies. The Europe and Asia also receives cold air, but Greenland and easthern Canada experiences relatively mild winters.

Figure 10.30, page #289 (Ahrens)