Chapter 8Chapter 8

Atmospheric CirculationAtmospheric Circulationandand

Pressure DistributionsPressure Distributions

George Hadley (1685-1768) proposed a simple circulation pattern called the

single-cell model to describe the general movement of the atmosphere.

In the single-cell model, air expands upward, diverges toward the poles,

descends, and flows back toward the equator near the surface. Winds

blowing east-to-west or west-to-east are referred to as zonal winds; those moving north-to-south or

south-to-north are called meridional.

The three-cell model divides the circulation of each hemisphere into

three distinct cells:the heat-driven Hadley cell that circulates air between the Tropics and subtropics,a Ferrel cell in the middle latitudes, and a polar cell.

Along the equator, strong solar heating causes air to expand upward anddiverge toward the poles, creating a zone of low pressure at the

equator called the equatorial low or the Intertropical Convergence Zone. The ITCZ is the rainiest latitude zone in the world and is observable as

the band of convective clouds and showers extending from northernSouth America into the Pacific on this satellite image.

At about 20° to 30° latitude, air in the Hadley cell sinks towardthe surface to form the subtropical highs, large bands of

high surface pressure. Cloud formation is greatly suppressedand desert conditions are common in the subtropics.

In the Northern Hemisphere, as the pressure gradientforce directs surface air from the subtropical highs to

the ITCZ, the weak Coriolis force deflects the air slightlyto the right to form the northeast trade winds.

In the Southern Hemisphere, the northward-moving air fromthe subtropical high is deflected to the left

to create the southeast trade winds.

Immediately flanking the Hadley cell in each hemisphere is theFerrel cell, which circulates air between the subtropical highs

and the subpolar lows, or areas of low pressure.

On the equatorial side of the Ferrel cell, air flowing poleward away from the Northern Hemisphere subtropical high undergoes a substantial

deflection to the right, creating a wind belt called the westerlies.

In the Southern Hemisphere, the pressure gradient force propels theair southward, but the Coriolis force deflects it to the left, producing

a zone of westerlies in that hemisphere as well.

In the polar cells of the three-cell model, surface air movesfrom the polar highs to the subpolar lows. Compared to

the poles, air at subpolar locations is slightly warmer,resulting in low surface pressure and rising air.

Very cold conditions at the poles createhigh surface pressure and low-level motiontoward the equator. In both hemispheres,

the Coriolis force turns the air to forma zone of polar easterlies in the lower atmosphere.

The real world is not covered by a series of belts that completely encirclethe globe. Instead, we find a number of alternating semi-permanent cells

of high and low pressure. Among the most prominent features in theNorthern Hemisphere during winter are the Aleutian and Icelandic Lows

over the Pacific and Atlantic Oceans, and the Siberian High over central Asia.

In summer, the best-developed semi-permanent cells are the Hawaiian and Bermuda-Azores Highs of the Pacific and Atlantic Oceans

and the Tibetan Low of southern Asia.

The Sahel is a region of Africa bordering the southern Sahara Desert. During the summer (left), the ITCZ usually shifts

northward and brings rain to the region. For much of the year, the ITCZ is located south of the Sahel, and the region

receives little or no precipitation (right).

Wind speeds generally increase with height between the surface and thetropopause largely because pressure gradient force is typically stronger athigh altitudes. The surfaces representing the 900, 800, and 700 mb levels

all slant downward to the north, but not by the same amount. Higher surfacesslope more steeply, which means that the pressure gradient force is greater.

The polar front is a strongly sloping boundary between warm mid-latitude air and cold polar air. Within the front, the slope of the pressure surfaces

increases greatly because of the abrupt horizontal change in temperature.With steeply sloping pressure surfaces there is a strong pressure gradient force,

resulting in the polar jet stream situated above the polar front near the tropopause.

Jet streams can be thought of as meandering “rivers” of air, usually 9 to 12 kmabove sea level. Their wind speeds average about 180 km/hr in winter and

about half that in summer, though peak winds can exceed twice these values.Nearer the equator is the subtropical jet stream, associated with the Hadley cell.The subtropical jet stream can bring with it warm, humid conditions as seen above.

This hypothetical drawing of the 500 mb surface reveals that heights decrease from south to north but also rise and fall through the ridgesand troughs. Vertical changes are highly exaggerated in the figure. Actual height changes are small compared to the size of the continent.

Troughs occur in the middle troposphere where the 500 mb height contours dip equatorward. In the top map, positions 1–3 have the 500 mb level at 5610 m.

Farther to the south, at positions 4–6, the 500 mb level is at 5640 m. In the bottom

map, contour lines are in the same position over the East and West Coasts

as they were up top, but shift equatorward over the central portion of the continent. Thus, positions 2 and 5 have lower pressure than the areas east and west of them. The zone of lower pressure over the central part

of the continent is a trough.

There are anywhere from three to seven Rossby waves or long wavescircling the globe, each with a particular wavelength and amplitude.

Rossby waves often remain in fixed positions, but also migrate west to east and are capable of transporting warm air from subtropical regions

to high latitudes, or cold polar air to low latitudes.

Ocean currents are horizontal movements of surface water that have an impact on the exchange of energy and moisture between the oceans and

the lower atmosphere. Among these are the North Equatorial Current and the South Equatorial Current, which converge, creating the Equatorial Countercurrent. The warm Gulf Stream flows northeast to become the

North Atlantic Drift and later the cold Canary Current as it turns southward. The cold Labrador Current flows south along the east coast

of Canada fed by the East and West Greenland Drift.

Ocean currents are driven by winds in the lower atmosphere that exert a drag on the water at an angle 45° to the right (Northern Hemisphere) and continue to shift clockwise as their speed decreases. At a depth of about

100 m, the current approaches the opposite direction of the surface current and begins to die out in a pattern known as the Ekman spiral.

Strong offshore winds along a coastal region sometimesdrag the warmer surface waters seaward, which

draws up cooler waters from below to take their place.This process, called upwelling,

greatly influences sea surface temperaturesover the eastern portions of the major oceans.

Features such as the Intertropical Convergence Zone, thewesterlies, and large Rossby waves exist on a global scale.

Smaller features, such as cyclones, anticyclones, troughs, and ridges, exist at the synoptic scale,

covering hundreds or thousands of square kilometers.

Mesoscale events are on the order of tens of squarekilometers and last for periods as brief as half an hour.

The smallest exchanges of mass and energy operate at the microscale.

Monsoon refers to the climatic pattern in which heavy precipitation alternates with hot, dry conditions on an annual basis due to the seasonal reversal in

surface winds caused by an oscillation between high- and low-pressure cells.

During winter (top), dry air flows southward from the Himalayas. When summer arrives (bottom) moist air is drawn northward from the equatorial

oceans. Surface heating, convergence, and a strong orographic effect

cause heavy rains over thesouthern part of the continent.

Foehn is the generic name for synoptic scale winds that flow downmountain slopes, warm by compression, and introduce hot, dry,

and clear conditions to the adjacent lowlands.

Winds warmed by compression that descend the eastern slopes of the Rocky Mountains are called chinooks.

The Santa Ana winds of California, common in the fall and spring,occur when high pressure develops over the Rocky Mountains.

Katabatic winds originate when air is locally chilled over a high-elevation plateau and warmed by compression as it flows down slope.

Along the coast during the daytime,land surfaces warm more rapidly than

the adjacent water (a), which causes the air column overlying the land to expand and rise upward (b). At a height of about

1 km, the rising air spreads outward (c), which causes an overallreduction in the surface air pressure. Over the adjacent water less warming

takes place, so the air pressure is greater than that over land. The air

over the water moves toward the low-pressure area over the land, which sets up the daytime sea breeze. At night the land surface cools more rapidly than the

water. The air over the land becomes dense and generates a land breeze.

A valley breeze (a) forms whendaytime heating causes themountain surface to becomewarmer than nearby air at the

same altitude. The air expandsupward and the air flows from

the valley to replace it. Nocturnalcooling makes the air dense

over the mountain and initiatesa mountain breeze (b).

El Niño is a recurrent event in the tropical eastern Pacific in whichsea-surface temperatures are significantly above normal.

La Niña is the inverse event (cold sea-surface temperatures).

The Walker circulation is an east-west circulation pattern of the Tropics, characterized by several cells of rising and sinking air connected by horizontal motions along parallel lines of latitude.

The Southern Oscillation is the reversal of surface pressure patterns over the tropical Pacific associated with El Niño events.

The Pacific Decadal Oscillation is an alternating pattern of sea surface temperature in the Pacific that reverses over several decades.

The next chapter examinesThe next chapter examinesair masses and fronts.air masses and fronts.