Transcript No Slide Title

Observations of Winds & Currents
Circulation of the Atmosphere: The Hadley Circulation
Knowledge of global winds comes from two sources: the patterns of pressure and winds observed
worldwide, and theoretical studies of fluid motion.
• One of the first contributions to the classical model of global circulation came from Hadley
(1735). Hadley proposed that the large temperature contrast between the poles and the equator
would create a thermal circulation similar to that of a sea breeze:
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Heated air rises and expands creating a region of low pressure. Cooler air moves in to replace it. Thus a
circulation cell develops.
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As long as Earth’s surface is heated
unequally, air will move to balance the
inequality. Hadley suggested that on a
nonrotating Earth, the air movement would
take the form of one large convection cell in
each hemisphere.
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In this way the Atmosphere transports heat
towards the Poles.
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Although correct in principle, Hadley’s
model did not fit the observed pressure
distribution that was subsequently established
for Earth.
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Three-cell circulation model
In the 1920s a three-cell circulation model (for each hemisphere) was proposed to explain how
Earth’s heat balance is maintained.
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Note that the surface flow has a much greater east-west component than a north-south
component.
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In the zones between the equator and roughly 30 degrees latitude north and south, the
circulation closely resembles the model used by Hadley for the whole Earth. Consequently,
the name Hadley cell is generally applied. Note that ‘Hadley cell’ refers only to the northsouth component of the circulation.
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Three-cell circulation model….continued
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Near the equator, the warm rising air that releases latent heat during the formation of
cumulus towers is believed to provide the energy to drive this cell. These clouds also
provide the rainfall that maintains the lush vegetation of the equatorial rain forests.
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As the upper flow in this cell moves poleward, it begins to subside in a zone between 20 and
35 latitude. Two factors are thought to contribute to this general subsidence:
(1) As this flow moves away from the stormy equatorial region where the release of latent
heat of condensation keeps the air warm and buoyant, radiation cooling increases the
density of air aloft.
(2) Because the Coriolis force becomes stronger with increasing distance from the
equator, winds that initially were poleward directed are deflected into a nearly west-toeast flow by the time they reach 25 latitude. Stated another way, the Coriolis force
causes a general pileup of air (convergence) aloft.
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The subsiding air is relatively dry, for it has released its moisture near the equator. In
addition, the effect of adiabatic heating during descent further reduces the relative humidity
of the air. Consequently, this dry subsidence zone is the site of the world’s subtropical
deserts.
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Winds are generally weak and variable near the centre of this zone of descending air.
Because of this, the region was popularly called the horse latitudes.
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Continued….
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From the centre of the horse latitudes, the surface flow splits into a poleward branch and
an equatorward branch. The equatorward flow is deflected by the Coriolis force to form
the reliable trade winds. In the Southern Hemisphere, the trades are from the southeast.
The trade winds from both hemispheres meet near the equator in a region that has a
weak pressure gradient. This region is called the doldrums. Here light winds and
humid conditions provide the monotonous weather this is the basis for the expression
“down in the doldrums”.
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The circulation between 30 and 60 latitude (north and south) has a net surface flow that
is poleward. Because of the Coriolis force, the winds have a strong westerly component.
These prevailing westerlies were known to Benjamin Franklin who noted that storms
migrated from west to east across the colonies.
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Franklin also observed that the westerlies were much more sporadic and unreliable than the
trades for sail power. We now know that it is the migration of cyclones and anticyclones across
the midlatitudes that disrupts the general westerly flow at the surface. Because of the
importance of the midlatitude circulation in maintaining Earth’s heat balance, the westerlies
will be considered in more detail later.
Relatively little is known about the circulation in high (polar) latitudes. It is generally
believed that subsidence near the poles produces a surface flow that moves equatorward
and is deflected into the polar easterlies of both hemispheres. As these cold polar
winds move equatorward, they eventually encounter the warmer westerly flow of the
midlatitudes. The region where these contrasting flows clash has been named the polar
front.
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Observed Distribution of Surface Pressure and Winds
The planetary circulation considered earlier is accompanied by a distinct distribution of
surface pressure. We now consider the relationship between the surface winds and this
pressure distribution.
• First examine the idealized pressure distribution that would be expected if Earth’s
surface were uniform (all sea or all smooth land): LT 7-6a.
– Under such a condition, four belts of high and low pressure would exist. Near the
equator, the rising, converging air from both hemispheres is associated with the
pressure zone known as the equatorial low, a region marked by abundant
precipitation. Because it is the region where the trade winds converge, it is also
referred to as the Inter-tropical convergence zone (ITCZ).
– In the belts about 20 to 35 degrees on either side of the equator, where the
westerlies and trade winds originate and go their separate ways, are located the
subtropical high pressure zones. These are regions of subsidence and divergent
flow. Yet another low-pressure region is situated at about 50 to 60 latitude in a
position corresponding to the polar front. Here the polar easterlies and westerlies
meet to form a convergent zone known as the subpolar low. Finally, near Earth’s
poleward extremes are the polar highs, from which the polar easterlies originate.
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The real Earth, LT 7-6b:
• The only true zonal distribution of pressure exists along the subpolar low in the Southern
Hemisphere, where the ocean is continuous.
• To a lesser extent, the equatorial low is also continuous.
• At other latitudes this zonal pattern is replaced by semipermanent cells of high and low
pressure. A better approximation of global pressure patterns and resulting winds is
shown in LT 7-7.
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Monsoons
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The greatest seasonal change in the global
circulation is the development of monsoons.
The word monsoon is derived from an Arab
word meaning ‘winds that change
seasonally’.
• What regions of the ocean are affected by
monsoons? The most obvious region is the
Indian Ocean, north of about 15° S. Also
affected is the westernmost part of the
Pacific Ocean, including the regions of the
Malaysian and Indonesian archipelagos.
• In general, the winter season is associated
with an overall flow off the continents,
which is accompanied by dry conditions.
By contrast, the wind pattern in summer is
from the sea toward the land, and is often
associated with heavy rainfall. In more
detail...
• Continued...
Monsoons - continued...
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In the northern winter, the air over southern Asia is cooler and denser than air over the
ocean, and so the surface atmospheric pressure is greater over the continent than over the
ocean. The resulting pressure gradient leads to a northerly (from the north) or northeasterly (from the north-east) flow of air from Asia to south of the Equator. This flow of
air is the North-East Monsoon. After crossing the Equator, the flow is turned to the left
by the Coriolis force and converges with the South-East Trades at about 10-20° S.
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During the North-East Monsoon the winds bring dry cool air to India from the Asian land
mass.
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As the year progresses, increased heating weakens the high pressure over southern Asia.
By the northern summer, a low has developed so that from May/June to September a
southerly or south-westerly wind blows across the region. This is the South-West
Monsoon, the stronger of the two monsoons.
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During the South-West Monsoon the winds cross the Arabian Sea and bring humid
maritime air to India. The moisture that provides the heavy monsoon rains is partly a
direct result of evaporation from the warmed surface of the Arabian Sea, and partly the
result of upward convection of warm moist air above the Arabian Sea which leads to the
formation of cyclonic vortices which draw in more moisture-laden air from adjacent
regions.
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The monsoonal nature of the winds may also be thought of as a manifestation of the
seasonal change in the position of the ITCZ, from about 20° S in January to about 25° N,
over Asia, in July.
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Summary - 1
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The subtropical gyres are characterized by intense western boundary currents and
diffuse eastern boundary currents. In the North Atlantic, the western boundary current is
the Gulf Stream, and the eastern boundary current is the Canary Current.
The Gulf Stream consists of water that has come from equatorial regions (largely via the
Gulf of Mexico) and water that has recirculated within the subtropical gyre. The lowlatitude origin of much of the water means that the Gulf Stream has warm surface
waters, although the warm ‘core’ becomes progressively eroded by mixing with adjacent
waters as the Stream flows north-east.
The prevailing Trade Winds cause sea-levels to be higher in the western part of the
Atlantic basin than in the eastern part, and the resulting ‘head’ of water in the Gulf of
Mexico provides a horizontal pressure gradient acting downstream. The flow leaving
the Straits of Florida therefore has some of the characteristics of a jet.
The Gulf Stream follows the continental slope as far as Cape Hatteras where it moves
into deeper water and has an increasing tendency to form eddies and meanders; the flow
also becomes more filamentous, with cold counter-currents. Beyond the Grand Banks,
the current becomes even more diffuse and is generally known as the North Atlantic
Drift.
Flow in the Gulf Stream is in approximate geostrophic equilibrium, and the strong
lateral gradients in temperature and salinity mean that the flow is baroclinic.
Continued….
Summary - 2
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The fast, deep currents in the Gulf Stream are associated with the steep downward slope
of the isotherms and isopycnals towards the Sargasso Sea. The Gulf Stream may be
regarded as a ribbon of high velocity water forming a front between the warm Sargasso
Sea water and the cool waters over the continental margin. This frontal region with
strong velocity shear is subject to baroclinic instabilities and, especially downstream of
Cape Hatteras, to mesoscale eddy production. Eddies that are formed from meanders
with an anticyclonic tendency are known as warm-core eddies, and those formed from
meanders with a cyclonic tendency are called cold-core eddies. These eddies extend to
significant depths.
One of the tools for studying fluid flow is the concept of vorticity, or the tendency to
rotate. All objects on the surface of the Earth of necessity share the component of the
Earth’s rotation appropriate to the latitude; this is known as planetary vorticity and,
because vorticity is defined as 2 x angular velocity, is equal to 2 sin  and given the
same symbol as the Coriolis parameter, f. Rotary motion relative to the Earth is known
as relative vorticity, . The vorticity of a fluid parcel relative to fixed space -- its
absolute vorticity -- is given by f + . In the absence of external influences, potential
vorticity (f + )/D (where D is the depth of the water parcel) remains constant. Away
from coastal waters and other regions of strong velocity shear, f is much greater than .
By convention, an anticlockwise rotatory tendency is described as positive, and a
clockwise one as negative (regardless of hemisphere).
Continued….
Summary - 3
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Early theories about oceanic circulation were restricted because the effects of the Earth’s
rotation -- the Coriolis force and hence geostrophic currents -- were not appreciated.
Stommel demonstrated that the intensification of the western boundary currents of
subtropical gyres is a consequence of the increase in the Coriolis parameter with
latitude, and that western intensification can be explained in terms of vorticity balance.
Sverdrup showed that when horizontal pressure gradient forces, caused by sea-surface
slopes, are taken into account, the total wind driven, mreidional (north-south) flow is
proportional to the torque, or curl, of the wind stress. Sverdrup’s ideas were extended
by Munk who used real wind data and allowed for frictional forces resulting from
turbulent mixing in both vertical and horizontal directions. The circulation pattern he
derived bears a close resemblance to that of the real oceans.
The eastern boundary currents of the subtropical gyres are associated with coastal
upwelling which occurs in response to equatorward longshore winds. Areas of
divergence and upwelling are characterized by cooler than normal surface waters, a
raised thermocline and, because nutrients are continually being supplied to the photic
zone, high productivity of phytoplankton, the basis of marine life. By contrast, areas of
convergence and sinking, such as the regions of the subtropical gyres, are the oceanic
equivalent of barren deserts.