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Atmospheric circulation
Atmospheric circulation is the large-scale movement of air, and
together with
ocean circulation is the means by which
thermal
energy is redistributed on the surface of the
Earth.
The Earth's atmospheric circulation varies from year to year, but the large
scale structure of its circulation remains fairly constant. The smaller scale
weather systems – mid-latitude depressions, or tropical convective cells –
occur "randomly", and long range weather predictions of those cannot
be made beyond ten days in practice, or a month in theory (see
Chaos
theory and
Butterfly effect).
The Earth's
weather
is a consequence of its illumination by the
Sun, and the laws of
thermodynamics.
The atmospheric circulation can be viewed as a heat engine driven by the Sun's
energy, and whose
energy sink, ultimately, is the blackness of space. The
work produced by that engine causes the motion of the masses of air and in that
process it redistributes the energy absorbed by the Earth's surface near the
tropics to space and incidentally to the latitudes nearer the poles.
The large scale atmospheric circulation "cells" shift polewards in
warmer periods (for example,
interglacials
compared to
glacials), but remain largely constant as they are,
fundamentally, a property of the Earth's size, rotation rate, heating and
atmospheric depth, all of which change little. Over very long time periods
(hundreds of millions of years), a
tectonic
uplift can significantly alter their major elements, such as the
jet stream,
and
plate tectonics may shift
ocean
currents. During the extremely hot climates of the
Mesozoic, a
third
desert
belt may have existed at the
Equator.
Latitudinal
circulation features
The wind belts girdling the planet
are organised into three cells in each hemisphere: the Hadley
cell, the Ferrel cell, and the Polar cell. Those cells exist in both the
northern and southern hemispheres. The vast bulk of the atmospheric motion
occurs in the Hadley cell. The high pressure systems acting on the Earth's
surface are balanced by the low pressure systems elsewhere. As a result, there
is a balance of forces acting on the Earth's surface.
Hadley
cell
The atmospheric circulation pattern
that George
Hadley described was an attempt to explain the trade winds.
The Hadley cell is a closed circulation loop which begins at the equator.
There, moist air is warmed by the Earth's surface, decreases in density and
rises. A similar air mass rising on the other side of the equator forces those
rising air masses to move poleward. The rising air creates a low pressure zone
near the equator. As the air moves poleward, it cools, becomes more dense, and
descends at about the 30th parallel, creating a high-pressure area. The descended air then
travels toward the equator along the surface, replacing the air that rose from
the equatorial zone, closing the loop of the Hadley cell. The poleward movement
of the air in the upper part of the troposphere deviates toward the east,
caused by the coriolis acceleration (a manifestation of
conservation of energy). At the ground level however, the movement of the air
toward the equator in the lower troposphere deviates toward the west, producing
a wind from the east. The winds that flow to the west (from the east, easterly
wind) at the ground level in the Hadley cell are called the Trade Winds.
Though the Hadley cell is described
as located at the equator, in the northern hemisphere it shifts to higher
latitudes in June and July and toward lower latitudes in December and January,
which is the result of the Sun's heating of the surface. The zone where the
greatest heating takes place is called the "thermal
equator". As the southern hemisphere summer is December to March, the
movement of the thermal equator to higher southern latitudes takes place then.
The Hadley system provides an
example of a thermally direct circulation. The thermodynamic efficiency and power of the
Hadley system, considered as a heat engine, is estimated at 200 terawatts.[1]
Polar
cell
The Polar cell, likewise, is
a simple system. Though cool and dry relative to equatorial air, the air masses
at the 60th parallel are still sufficiently
warm and moist to undergo convection and drive a thermal
loop. At the 60th parallel, the air rises to the tropopause (about
8 km at this latitude) and moves poleward. As it does so, the upper level
air mass deviates toward the east. When the air reaches the polar areas, it has
cooled and is considerably denser than the underlying air. It descends,
creating a cold, dry high-pressure area. At the polar surface level, the mass
of air is driven toward the 60th parallel, replacing the air that rose there,
and the polar circulation cell is complete. As the air at the surface moves
toward the equator, it deviates toward the west. Again, the deviations of the
air masses are the result of the Coriolis
effect. The air flows at the surface are called the polar easterlies.
The outflow of air mass from the
cell creates harmonic
waves in the atmosphere known as Rossby
waves. These ultra-long waves determine the path of the polar jet stream,
which travels within the transitional zone between the tropopause
and the Ferrel
cell. By acting as a heat sink, the polar cell moves the abundant heat from
the equator toward the polar regions.
The Hadley cell and the polar cell
are similar in that they are thermally direct; in other words, they exist as a
direct consequence of surface temperatures. Their thermal characteristics drive
the weather in their domain. The sheer volume of energy that the Hadley cell
transports, and the depth of the heat sink that is the polar cell, ensures that
the effects of transient weather phenomena are not only not felt by the system
as a whole, but — except under unusual circumstances — do not form. The endless
chain of passing highs and lows which is part of everyday life for mid-latitude
dwellers, at latitudes between 30 and 60° latitude, is unknown above the 60th
and below the 30th parallels. There are some notable exceptions to this rule.
In Europe, unstable weather extends to at least the 70th parallel north.
These atmospheric features are
stable. Even though they may strengthen or weaken regionally over time, they do
not vanish entirely.
Ferrel
cell
Part of the air rising at 60°
latitude diverges at high altitude toward the poles and creates the polar cell.
The rest moves toward the equator where it collides at 30° latitude with the
high-level air of the Hadley cell. There it subsides and strengthens the high
pressure ridges beneath. A large part of the energy that drives the Ferrel cell
is provided by the polar and Hadley cells circulating on either side and that
drag the Ferrel cell with it.[5]
The Ferrel cell, theorized by William
Ferrel (1817–1891), is therefore a secondary circulation feature, whose
existence depends upon the Hadley and polar cells on either side of it. It
might be thought of as an eddy created by the Hadley and polar cells.
The Ferrel cell is weak, and the air flow and temperatures within it are
variable. For this reason, the mid-latitudes are sometimes known as the "zone
of mixing." At high altitudes, the Ferrel cell overrides the Hadley
and Polar cells. The air of the Ferrel cell that descends at 30° latitude
returns poleward at the ground level, and as it does so it deviates toward the
east. In the upper atmosphere of the Ferrel cell, the air moving toward the
equator deviates toward the west. Both of those deviations, as in the case of
the Hadley and polar cells, are driven by conservation of energy. As a result,
just as the easterly Trade Winds are found below the Hadley cell, the Westerlies
are found beneath the Ferrel cell. The forces driving the flow in the Ferrel
cell are weak, and so the weather in that zone is variable. Thus, strong
high-pressure areas which divert the prevailing westerlies, such as a Siberian
high, can override the Ferrel cell, making it discontinuous.
While the Hadley and polar cells are
truly closed loops, the Ferrel cell is not, and the telling point is in the
Westerlies, which are more formally known as "the Prevailing
Westerlies." The easterly Trade Winds and the polar easterlies have
nothing over which to prevail, as their parent circulation cells are strong
enough and face few obstacles either in the form of massive terrain features or
high pressure zones. The weaker Westerlies of the Ferrel cell, however, can be
disrupted. The local passage of a cold front may change that in a matter of
minutes, and frequently does. As a result, at the surface, winds can vary
abruptly in direction. But the winds above the surface, where they are less
disrupted by terrain, are essentially westerly. A low pressure zone at 60°
latitude that moves toward the equator, or a high pressure zone at 30° latitude
that moves poleward, will accelerate the Westerlies of the Ferrel cell. A
strong high, moving polewards may bring westerly winds for days.
The Ferrel cell is driven by the
Hadley and Polar cells. It has neither a strong source of heat nor a strong
sink to drive convection. As a result, the weather within the Ferrel cell is
highly variable and is influenced by changes to the Hadley and Polar cells. The
base of the Ferrel cell is characterized by the movement of air masses, and the
location of those air masses is influenced in part by the location of the jet
stream, even though it flows near the tropopause. Overall, the movement of
surface air is from the 30th latitude to the 60th. However, the upper flow of
the Ferrel cell is weak and not well defined.
In contrast to the Hadley and Polar
systems, the Ferrel system provides an example of a thermally indirect
circulation. The Ferrel system acts as a heat pump
with a coefficient of performance of 12.1, consuming kinetic energy at an approximate
rate of 275 terrawatts.[1]
Longitudinal
circulation features
While the Hadley, Ferrel, and polar
cells (whose axes are oriented along parallels or latitudes) are the major
features of global heat transport, they do not act alone. Temperature
differences also drive a set of circulation cells, whose axes of circulation
are longitudinally oriented. This atmospheric motion is known as zonal
overturning circulation.
Latitudinal circulation is a result
of the highest solar radiation per unit area (solar intensity) falling on the
tropics. The solar intensity decreases as the latitude increases, reaching
essentially zero at the poles. Longitudinal circulation, however, is a result
of the heat capacity of water, its absorptivity, and its mixing. Water absorbs
more heat than does the land, but its temperature does not rise as greatly as
does the land. As a result, temperature variations on land are greater than on
water. The Hadley, Ferrel, and polar cells operate at the largest scale of
thousands of kilometers (synoptic scale). But, even at mesoscales (a horizontal range of 5 to
several hundred kilometres), this effect is noticeable. During the day, air
warmed by the relatively hotter land rises, and as it does so it draws a cool
breeze from the sea that replaces the risen air. At night, the relatively
warmer water and cooler land reverses the process, and a breeze from the land,
of air cooled by the land, is carried offshore by night. This described effect
is daily (diurnal).
At the larger, synoptic, scale of
oceans and continents, this effect is seasonal or even decadal. Warm air
rises over the equatorial, continental, and western Pacific Ocean regions. When
it reaches the tropopause, it cools and subsides in a region of relatively
cooler water mass.
The Pacific Ocean cell plays a
particularly important role in Earth's weather. This entirely ocean-based cell
comes about as the result of a marked difference in the surface temperatures of
the western and eastern Pacific. Under ordinary circumstances, the western
Pacific waters are warm, and the eastern waters are cool. The process begins
when strong convective activity over equatorial East Asia
and subsiding cool air off South America's west coast creates a wind pattern
which pushes Pacific water westward and piles it up in the western Pacific.
(Water levels in the western Pacific are about 60 cm higher than in the
eastern Pacific.)[6][7][8][9]
Walker
circulation
The Pacific cell is of such importance
that it has been named the Walker circulation after Sir Gilbert Walker, an early-20th-century
director of British observatories in India, who sought a
means of predicting when the monsoon winds of India would fail. While he was never
successful in doing so, his work led him to the discovery of a link between the
periodic pressure variations in the Indian
Ocean, and those between the eastern and western Pacific, which he termed
the "Southern Oscillation".
The movement of air in the Walker
circulation affects the loops on either side. Under normal circumstances, the
weather behaves as expected. But every few years, the winters become unusually
warm or unusually cold, or the frequency of hurricanes
increases or decreases, and the pattern sets in for an indeterminate period.
The Walker Cell plays a key role in
this and in the El Niño phenomenon. If convective activity slows in the
Western Pacific for some reason (this reason is not currently known), the
climates of areas adjacent to the Western Pacific are affected. First, the
upper-level westerly winds fail. This cuts off the source of returning, cool
air that would normally subside at about 30° north latitude, and therefore the
air returning as surface easterlies ceases. The consequence of this is twofold.
Warm water ceases to surge into the eastern Pacific from the west (it was
"piled" by past easterly winds) since there is no longer a surface
wind to push it into the area of the west pacific. This and the corresponding effects
of the Southern Oscillation result in long-term unseasonable temperatures and
precipitation patterns in North and South America, Australia, and Southeast
Africa, and the disruption of ocean currents.
Meanwhile, in the Atlantic,
fast-blowing upper level Westerlies of the Hadley cell form, which would
ordinarily be blocked by the Walker circulation and unable to reach such
intensities. These winds disrupt the tops of nascent hurricanes and greatly
diminish the number which are able to reach full strength.
Pertanyaan:
1. Apa itu atmosfir, lapiasan atmosfir serta gas penyusunnya
2. Apa yang dimaksudkan dengan sirkulasi atmosfir
3. Jelaskan perbedaan pola sirkulasi atmosfir secara vertikal dan horizontal secara global
4. Kira-kira apa makna kalimat yang ada dalam postingan di atas berikut ini
The Walker Cell plays a key role in
this and in the El Niño phenomenon.
Selamat mengerjakan tugas, be blessed
4.
