Chapter 8
Atmospheric Circulation
and
Pressure 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 and
diverge 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 northern
South America into the Pacific on this satellite image.
At about 20° to 30° latitude, air in the Hadley cell sinks toward
the surface to form the subtropical highs, large bands of
high surface pressure. Cloud formation is greatly suppressed
and desert conditions are common in the subtropics.
In the Northern Hemisphere, as the pressure gradient
force directs surface air from the subtropical highs to
the ITCZ, the weak Coriolis force deflects the air slightly
to the right to form the northeast trade winds.
In the Southern Hemisphere, the northward-moving air from
the subtropical high is deflected to the left
to create the southeast trade winds.
Immediately flanking the Hadley cell in each hemisphere is the
Ferrel 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 the
air 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 moves
from 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 create
high surface pressure and low-level motion
toward the equator. In both hemispheres,
the Coriolis force turns the air to form
a zone of polar easterlies in the lower atmosphere.
The real world is not covered by a series of belts that completely encircle
the globe. Instead, we find a number of alternating semi-permanent cells
of high and low pressure. Among the most prominent features in the
Northern 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 the
tropopause largely because pressure gradient force is typically stronger at
high altitudes. The surfaces representing the 900, 800, and 700 mb levels
all slant downward to the north, but not by the same amount. Higher surfaces
slope 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 km
above 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 ridges
and 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 waves
circling 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 sometimes
drag the warmer surface waters seaward, which
draws up cooler waters from below to take their place.
This process, called upwelling,
greatly influences sea surface temperatures
over the eastern portions of the major oceans.
Features such as the Intertropical Convergence Zone, the
westerlies, 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 square
kilometers 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 the
southern part of the continent.
Foehn is the generic name for synoptic scale winds that flow down
mountain 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 highelevation 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 overall
reduction 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 lowpressure 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 when
daytime heating causes the
mountain surface to become
warmer than nearby air at the
same altitude. The air expands
upward and the air flows from
the valley to replace it. Nocturnal
cooling makes the air dense
over the mountain and initiates
a mountain breeze (b).
El Niño is a recurrent event in the tropical eastern Pacific in which
sea-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 examines
air masses and fronts.