Download WIND AND OCEAN CIRCULATION LECTURE 3 Low and High

WIND AND OCEAN CIRCULATION
LECTURE 3
Low and High Pressure
Air pressure gradients develop both horizontally and vertically within the atmosphere. A
horizontal pressure gradient refers to air pressure variation along a surface of constant altitude
(mean sea level, 500 mb surface, etc). A vertical pressure gradient is a permanent feature of the
atmosphere because air pressure is greatest at the earth's surface and decreases with altitude.
To appreciate the latter, ie the vertical variation of air pressure with height, let's examine the
following. Assume a column of air over a horizontal surface. Because air has weight, this
column exerts pressure on that surface. On a flat surface two identical columns exert the same
pressure at their respective surface areas (Figure 1). Variation in pressure does occur if one
surface is higher than the other by reason of topography. This means that the pressure on a
mountain peak is lower than at sea level because the column of air on top of a mountain is
reduced in thickness and hence its weight.
In a synoptic weather condition along the same horizontal surface on the ground, pressure
variation does occur. Why is this so? This happens because at the centre of low pressure there is
an up-ward movement of air and at the centre of high pressure downward movement of air. At
the point where the column of air experiences a vertical updraft, the pressure at the surface
would be less than what it should be. At another point, a column that experiences descent, the
downward movement would add additional weight to that of the column, resulting in higher
pressure than what it should be. Thus even at sea level, one may get low pressure in one location
and at high pressure in another.
Note this principle that all updraft of air creates low pressure and is always associated with
convection, cloud formation, and even rain. The reverse is true of all situations of downdraft of
air resulting in clear, dry weather.
Wind Circulation
Surplus heat at the tropics serves to drive circulations in the atmosphere and oceans.
a.
In the atmosphere, there is the primary circulation which comprises the persistent largescale features which cover large areas of the globe. Secondary circulation includes shortlived, rapidly moving cyclones and much slower anticyclones.
b.
In the oceans, the general circulation involves ocean currents.
Both circulations serve to redistribute energy and moisture, thus modifying their latitudinal
imbalances and establishing climates. Positive imbalance of energy in tropical and equatorial
regions is compensated by a net negative energy budget at higher latitudes. Superimposed on
these will be the variation with seasons - maximum absorbed solar energy in summer and the
reverse in winter.
Without the horizontal motions the temperatures at each latitude would be dictated by radiation
alone. Summers will be hotter for much of the globe and winter temperatures in the tropics will
be higher too.
The heat that is distributed by oceans and the atmosphere is accounted for by the following three
fluxes:
Oceanic flux
Atmospheric dynamic (sensible heat) flux
Atmospheric latent heat flux
25 %
60 %
15 %
Forces in wind movement
Heat transfer from the low latitudes to high latitudes is brought about by the general
atmospheric circulation as well as ocean currents. In this section, attention will be focused on
wind circulation.
Wind is generally considered the horizontal movement of air. No doubt there is also the updown or vertical movement of air but the speed of vertical movement of air is 1 % to 10 % of
the horizontal wind speed. Thus wind generally is measured in terms of its horizontal
components of velocity and direction.
Some basic principles govern wind movement. Pressure gradient is the basic activating force for
wind. The change in barometric pressure across the horizontal surface of the earth constitutes a
pressure gradient. This spatial variation in gradient develops between areas of contrasting
temperatures, or from differing water vapour concentrations, or both. Thus, pressure gradient
exists between cold, dry air and warm, humid air. Where this gradient exists, air molecules tend
to drift in the same direction as that gradient. This tendency for mass movement of the air is the
pressure gradient force. The magnitude of the force is directly proportional to the steepness of
the gradient.
The second important influence on wind motion is the earth's rotational reflective force or
coriolis force which deflects large-scale winds to the right in the Northern Hemisphere and left
in the Southern Hemisphere (Figure 2).
There are other forces that affect wind motion such as centripetal force and friction, the latter
affects winds at the earth's surface. Higher up, the effect on winds is absent.
The combined effects of the above forces result in winds in an anticyclone in the Northern
Hemisphere to blow clockwise and parallel to the isobars at high altitudes, while at the surface
the winds blow clockwise but outward from the centre (Figure 3). On the other hand, in a
Northern Hemisphere cyclone, above the friction layer, the gradient winds blow in
anticlockwise direction and parallel to the isobars. But in the friction layer the winds blow
inwards into the centre of low pressure.
General Circulation
Much of the energy that drives wind circulation comes from the tropical oceans. By right there
should be a simple direction in the transference of heat from the tropical to the polar areas
(Figure 4a). But the earth rotates and this effect diverts the wind into gigantic whirling systems,
or vortices, that are aligned more or less in latitudinal belts. The resulting zonal flow patterns
produce prevailing winds - easterly and westerly components which make up the general
circulation.
Because of the surplus energy in the tropical belt, the intesely heated equatorial air would rise.
Hence at the equator there is the large scale vertical movement of water vapour as a result of
intensive evaporation caused by surplus energy. At the surface, this area experiences calm
conditions where wind speeds are low, and where great thickness of clouds and heavy rainfall
of the convectional type occurs. It is this belt of low pressure that is known as the doldrums.
The rising air will at high altitude move polewards until at about 30 o N and S where it will
descend and at the surface they will move equatorward as easterlies or trade winds due to the
earth's rotation. A secondary upward movement of air is located at 60 oN and S which diverges
at high altitude into one component that moves poleward and the other towards the tropics. The
general scheme of primary circulation is shown below:
a.
Near the equator - Intertropical Cnvergence Zone (ITCZ) or doldrums. Its location
varies with seasons and at times it is a weak and discontinuous belt. Zone of low
pressure and ascending air. Plenty of cloud cover and rainfall all year round.
b.
On either side of the ITCZ are the trades - northeast and southeast. Due to the coriolis
force they are deflected and arrives at the equator at an acute angle instead of
perpendicular (easterlies).
c.
Horse latitudes - subtropical highs. Area of subsidence of upper air and divergence near
the surface. Place of relative calm. A part forms the trades and the northward part, the
prevailing westerlies. Generally dry and account for the location of hot deserts.
d.
Westerlies - in N Hemisphere they are from the southwest, and in the S Hemisphere,
they are from the northwest. These are zones of cyclonic storms which move generally
from west to east. The lack of landmasses acting as obstacles in the S Hemisphere
makes the westerlies known as the roaring forties.
e.
Polar easterlies and westerlies meet to form the subpolar lows or polar fronts. Region of
low pressure.
f.
Polar highs - source of the polar easterlies. Region of descending air from upper levels
(Figure 4b).
Mid-Latitude Rossby Waves
Due to differential surface heating, the presence of mountains and rotation of the earth, winds at
higher levels (several km above the surface) follow giant, undulating paths around the earth in
the latitude of the westerlies. These waves result from the tendency of winds in large-scale
motion systems to retain a constant spin, or angular momentum, about the earth's axis of
rotation. A stream of air moving towards the equator adopts a cyclonic curvature (counterclockwise in the N Hemisphere) relative to the surface at the lower latitude as the distance from
the axis of rotation increases. Eventually the curved path turns the wind back toward the pole.
Passing its original latitude, the wind takes an opposite (anticyclonic) curvature relative to the
earth as it comes closer to the polar axis, where the rotational velocity is less. The result Rossby waves which have lengths of 3000 to 6000 km (Figure 5)
In the upper level (8 - 15 km) tropospheric waves, there are the jet streams - they are narrow
bands of high velocity winds that follow the wave paths.
Seasonal variation in wind circulation
Due to variations in temperature and pressure patterns with seasons, the wind circulation also
migrates seasonally along the meridians - basically due to seasonal changes in the heat budget at
different latitudes.
In the Northern Hemisphere, wind and pressure belts shift northwards a few degrees in summer
and southwards in winter.
Monsoons
Monsoons are seasonal winds affecting parts of the world where the prevailing winds change
direction with season causing wet summers and relatively dry winters. Vigorous monsoon winds
are marked over Africa and Asia. Over much of India, for example, monsoon rains (between
June to September) account for 80% or more of total annual precipitation.
Monsoons are linked to seasonal shifts in the planetary-scale circulation, specifically northsouth shifts of the ITCZ. Seasonal contrasts in the heating of land and sea has been used to
explain the monsoon winds. The ocean has a greater thermal stability than does the land.
Beginning in spring, relatively cool air over the ocean and relatively warm air over the land give
rise to a horizontal air pressure gradient directed from the sea to the land, that produces a flow
of humid air inland. Over the land intense solar heating triggers convection. Hot, humid air
rises, and consequent expansional cooling leads to condensation, clouds and rain. Release of
latent heat intensifies the buoyant uplift, triggering even more rainfall. Aloft, the air spreads
seaward and subsides over the relatively cool ocean surface, thus completing the monsoon
cycle.
By early autumn, radiational cooling chills the land more than the adjacent sea, setting up a
horizontal air pressure gradient directed from land to sea. Air subsides over the land, and dry
surface winds sweep seaward. Air rises over the relatively warm sea surface; aloft, it drifts landward, completing the winter monsoon circulation. Over land, therefore, the summer monsoon is
wet, and the winter monsoon is dry (Figure 6).
Monsoon winds have sufficiently long trajectories and persist long enough to be influenced by
the Coriolis effect. In January surface monsoon winds are deflected to the right in the Northern
Hemisphere and in July to the left in the Southern Hemisphere
Topography complicates the monsoon circulation and the distribution of rainfall. For example,
the massive Tibetan plateau has a major influence on the Asian monsoon. The elevation of the
plateau tops 4000m over a large area. In winter, the westerly jet stream splits into two branches,
one to the south and the other to the north of the plateau. The southerly branch steers cyclones
that originate in the Mediterranean across northern India and brings significant precipitation to
the region. Meanwhile, the rest of India experiences the dry monsoon. In spring, the southern
branch weakens and by late May shifts northward over the plateau. It is not until this happens
that the moist monsoon flow begins.
Air masses
An air mass is an extensive portion of the atmosphere having characteristics of temperature and
moisture which are relatively homogeneous horizontally. For a large body of air to acquire
temperature and moisture properties that are approximately the same at a given level, that air
must rest for a time on a source region, which must itself have fairly homogeneous surface
conditions. A large land or water area which receives evenly distributed insolation affords a
good source region, but a second prerequisite is necessary if a distinctive air mass is to be
developed ie large scale subsidence or divergence of air over the source region. Air that
subsides over a homogeneous source region will become homogeneous itself and tends to retain
its characteristics when it moves away. The heat and moisture properties of the air mass
gradually change as it moves over other surface conditions.
Identification of air masses is done with the help of radiosonde. Three kinds of information are
necessary:
a.
b.
c.
History of change in the air since it left its source region
Horizontal characteristics at certain levels in the upper air
The vertical distribution of temperature, winds, and humidity.
In connection with b, maps can be produced. These maps are slices of the same pressure level or
through the atmosphere. Constant-level charts may be plotted either for a certain height or for a
constant-pressure level, the latter being most often used by meteorological services. Common
pressure levels are 850, 700, 500, 300 and 100 mb.
Classification of air masses is based mainly upon their source regions and secondarily upon
temperature and moisture properties. Two main categories are:
a.
b.
Tropical or subtropical - low latitudes
Polar or subpolar - high latitudes
Further subdivision is based on oceanic or continental, and further, according to what
modifications the masses experience as they travel from the source regions (Figure 7).
Extratropical cyclones/depressions
The air masses from polar and tropical sources converge in mid-latitudes and because of
distinctive temperature and moisture properties they do not mix readily but maintain a boundary
surface of discontinuity called a front. The warmer, lighter air mass is forced upwards over the
colder air mass.
In Northern Hemisphere cyclone, cold air moves along the ground southwards under the warm
air, which advances northwards. The resulting convergence and rising air along the front is
accompanied by low pressure. As the cyclonic wave develops, the pressure gradient is focused
toward the centre and the pattern of air flow takes on a counterclockwise, or cyclonic circulation
with winds making angles of 20o - 40o with the isobars. The coriolis effect deflects the winds to
the right of their intended path as they blow toward the low centre. The 'tongue' of warm air
advancing from the south is known as the warm sector. Where the advancing warm air mass is
replacing colder air at the surface the boundary is called the warm front. To the west of the
warm sector the leading edge of the of the cold air is the cold front. Because air is forced to rise
at these fronts, they are accompanied by appreciable cloudiness and precipitation. Such systems
are about 200 km to 1000 km in diameter. They may be weak but travel in groups whose
movements follow waves in the pressure pattern at upper levels. The general direction is from
west to east in the mid-latitude westerlies (Figure 8).
Life cycle of a wave cyclone (Lutgens, F.K. & E.J. Tarbuck, The Atmosphere, Prentice Hall,
1992, 5th Ed, pp. 216-218)
According to the wave cyclone model, cyclones form along fronts and proceed through a
somewhat predictable life cycle. This cycle can last for a few hours or for several days,
depending on whether conditions for development are favourable. Figure 9 is a schematic
representation of the stages in the development of a "typical" wave cyclone. As the figure
shows, cyclones originate along a front where air masses of different densities (temperatures)
are moving parallel to the front in opposite directions. In the classic model this would be
continental polar air associated with the polar easterlies north of the front and maritime tropical
air of the westerlies south of the front. The result of this opposing airflow is the development of
cyclonic shear, which produces a net counter-clockwise rotation. To better visualise this effect,
place a pencil between the palms of your hands. Now move your right hand ahead of your left
hand and notice that your pencil rotates in a counter-clockwise fashion. It is possible for
cyclonic flow to develop in other ways that may also initiate a wave cyclone. In any event, under
the correct conditions the frontal surface will take on a wave shape. The waves generated
between two contrasting air masses are usually several hundred kilometres long. Some waves
tend to dampen out whereas others become unstable and grow in amplitude.
Once a small wave forms, warm air invades this weak spot along the front and extends itself
pole-ward and the surrounding cold air moves equator-ward. This change causes a readjustment
in the pressure field that results in almost circular isobars, with the low pressure centred at the
apex of the wave. Once the cyclonic circulation develops, we would expect general convergence
to result in vertical lifting, especially where warm air is overrunning colder air. We can see from
Figure 10 that the air in the warm sector is flowing from the south west toward the colder air
flowing from the south east. Because the warm air is moving faster than the cold air in a
direction perpendicular to the front, we can conclude that warm air is invading a region formerly
occupied by cold air; therefore this must be a warm front. Similar reasoning indicates that in the
rear of the cyclonic disturbance cold air is underrunning the air of the warm sector, generating a
cold front there. Generally the position of the cold front advances faster than the warm front and
begins to close the warm sector. This process, called occlusion, results in the formation of an
occluded front with the displaced warm sector located aloft. The cyclone enters maturity when it
reaches this stage in its development. A steep pressure gradient and strong winds develop as
lifting continues. Eventually all the warm sector is forced aloft and cold air surrounds the
cyclone at low levels. One the sloping discontinuity (front) between the air masses no longer
exists, the pressure gradient weakens. At this point the cyclone has exhausted its source of
energy and the storm comes to an end.
Anticyclones
They are opposite to cyclones with pressure decreasing outwards from the centre. They are
composed of subsiding air which renders it stable. Because anticyclones occasionally become
stagnant and remain over a region for several days or weeks, they are becoming ever more
important in their effects on water supply, crop production and air pollution. Large stagnant
anticyclones can often block the eastward migration of cyclonic centres. This effect can keep
one part of a country dry for a week or longer while another area is continually under the
influence of a cyclonic storm.
Tropical storms
Tropical storms are like mid-latitude cyclones in some ways but unlike them in others. They do
not show sharp discontinuities of temperature. Many have weak pressure gradients but these are
associated with squally weather and precipitation.
The violent and destructive ones are less common, originating from tropical oceans. Known as
hurricanes in West Indies, typhoons in South China Sea or simply cyclones generally.
Hurricane or a typhoon is a giant heat engine that derives its energy mainly through the transfer
of sensible and latent heat from sea to air. The whirling motion is due to the coriolis effect and
is most likely to form when the ITCZ is at 5o to 10o from the equator in late summer or early
autumn. At the equator its deflective force is inadequate to generate the violent vortex.
Diameters vary from 150 to 1000 km. Some have small diameters of 30 km. Features:
a.
b.
c.
d.
e.
f.
Isobars are concentric and closely spaced
Very low pressure at the centre
No fronts but the centre is the calm "eye" - 10 - 50km in diameter where air is
descending
Temperature is evenly distributed in all directions from the centre
Rainfall is evenly distributed, but torrential
Wind force - velocities increase towards the centre reaching a maximum at the outer
edge of the eye (must reach 65 knots to be at hurricane force). Speeds can reach 130
knots or more
Typhoons are one of the most destructive types of cyclonic storms. Almost circular storm centre
of extremely low pressure into which winds spiral at high speeds (120-200 km/hr) and
accompanied by intense rainfall. At the centre is the eye which is an area of calm, cloud free and
air descends from high altitude and is adiabatically warmed (Figures 11 & 12).
Origin
Typhoons occur over warmer parts of the sea with sea-surface temperatures of over 27oc. They
require intense energy and moisture and hence occur over oceans in latitudes 8o to 10 oN & S.
Disturbance can initiate storm development where vertical movement and rotation can be
started but easterly wave can deepen and intensify the motions. Warming of the air over the sea
causes instability. Warm air rises rapidly creating a strong updraft of rising air currents in a
circular motion. A prominent low pressure centre results with a steep pressure gradient outward
from the centre. Together with the presence of the coriolis force which deflects wind motion,
this will result in strong winds spiralling inward. The winds do not converge to a central point
but reach their highest speed at the eye wall.
The strong energy comes from evaporation of water over oceans. Large amounts of latent
energy is released when the air ascends and the large mass of water vapour condenses. The
release of heat warms up the upper atmosphere causing further convective instability. This
forces the air upwards even higher and thus intensifies up-surgence of air. As long as heat is
available the process continues.
This up-surgence of air will eventually result in the formation of thick cumulonimbus clouds of
great depth. This is because the energy released from earlier condensations allows water vapour
to travel to higher altitudes for further condensation to take place. These dense clouds will then
form bands around the edge of the storm to the eye wall, producing heavy rains that generally
increases in intensity inward. This energy also allows the hygroscopic nuclei to travel up and
down the cloud accumulating enough small droplets to fall as large raindrops later.
Dissipation
1.
When it passes on land due to loss of energy
2.
Increased frictional force due to uneven terrain on land
3.
When it passes over oceans into the mid-latitude belt of cooler temperatures
Local winds
a. Foehn or chinook winds
A foehn or chinook wind originates when a steep pressure gradient develops with high pressure
on the windward side of a mountain and a low pressure trough on the leeward side. Air moves
down the pressure gradient, which means from the windward side to the leeward side (Figure
13). Because the air is warm it does not flow down-slope under the influence of gravity. As the
air descends it is compressed and warmed up adiabatically and significantly. For example, on 6
January 1966, at Pincher Creek, Alberta, a chinook sent the temperature soaring 21 oC in only 4
minutes. An even more dramatic temperature surge was recorded at Spearfish, South Dakota, on
22 January 1943: The air temperature rose from -20 oC at 7.30 am to 7 oC at 7.32 am, ie. 27 oC
in just 2 minutes.
Chinook is a native Indian word meaning "snow eater", because of its catastrophic effect on
snow cover. Air ascending on the windward slopes loses much of its water vapour to
condensation and precipitation. On the leeward slopes, as the air descends and is
compressionally warmed, the relative humidity drops dramatically. Because the chinook is both
warm and very dry, a snow cover melts and vapourises rapidly.
b. Land and sea breezes
When both land and water are exposed to the same intensity of solar radiation, the land surface
warms up more than the water surface. The relatively warm land heats the overlying air, thereby
lowering air density. Compared to the land, the water is relatively cool, as is the air overlying
the water. Consequently, a local horizontal air pressure gradient develops between land and
water, with high pressure over the water surface. In response to this gradient, cool air sweeps
inland as sea breeze. Aloft, continuity requires a return airflow from land to sea, with air rising
over the land and sinking over the water.
After sunset, the land surface cools faster by radiational cooling than the water and a reverse air
pressure gradient develops with higher pressure over land and low pressure over water. Air will
blow from land to sea giving rise to land breeze (Figure 14).
c. Anabatic and katabatic winds
Under the influence of gravity, a shallow mass of cold, dense air slides downhill. This katabatic
wind usually originates in winter over extensive snow-covered plateaus or highlands. Although
adiabatic compression warms the air to some extent, the air is so cold to start with that these
winds are still quite cold when they reach the lowlands.
Among the best know are the mistral originating in the Alps and moving down the Rhone River
Valley of France and along the Mediterranean coast, and the bora originating in the highlands of
the former Yugoslavia down the narrow Dalmatian coastal plain along the Adriatic Sea.
Most katabatic winds are weak, usually under 10 km per hour.
Anabatic winds on the other hand, blows upwards the slope of a mountain due to insolation of
the hill sides. Radiation received by the hillsides exposed to the sun heats up the overlying air
causing it to rise and in its place cooler and denser air from the valley bottom which is in the
shadow to move up-slope and fill its place (Figure 15).
Both anabatic and katabatic winds are mountain breezes.
OCEAN CIRCULATION
There is a relationship between atmospheric circulation and ocean circulation particularly in
function - transfer of energy from the tropics to higher latitudes. Also, energy is exchanged
between the oceans and atmosphere by transfer of sensible heat and latent heat and by
mechanical action along the ocean-air boundary surface. Movement of ocean currents are due
to:
a.
b.
c.
f.
Variations in water temperature
Density differences
Salinity differences
Wind
Direction is motivated by:
a.
b.
The coriolis effect
Configuration of ocean basin and shorelines
Large-scale slow movement of ocean water resulting primarily from prevailing winds, are called
drifts rather than currents. There is a close correspondence between general atmospheric
circulation and oceanic circulation in the middle and low latitudes. In the N Hemisphere
circulation is clockwise, in the S Hemisphere anticlockwise. The net effect is to carry cold water
to the equator along the east margins and warm water pole-wards along the west margins. At
high latitudes cold water subsides and moves towards the equator. Figure 16 shows the world's
ocean currents.
Read further on warm and cold currents and ocean circulation.
Summary
1.
Differential heating results in differential net energy distribution over the globe.
Redistribution of energy between the low and high latitudes is an ongoing process.
2.
Two ways of heat transfer are through atmospheric circulation and ocean currents. Both
ways are influenced by the coriolis effect due to the earth's rotation.
3.
Differential heating and cooling results in the formation of high and low pressure. All
wind circulations whether large scale or small scale are governed by this principle.
4.
The movement of air masses results in different weather phenomena - cyclonic and
anticyclonic features and weather conditions.
Differential heating on smaller scales results in localised wind movements and circulations.
Discussion
What instruments are used to measure wind speed and direction? Based on daily wind
recordings of a station (Changi - records are available in the NIE Library) for a period of
a year draw a wind-rose diagram. Briefly discuss what you have drawn.
Discuss the factors that influence wind movements near the ground surface. Draw some
diagrams to show the effects of surface roughness on wind movement.
With reference to specific locations show the ameliorating effects warm ocean currents on the
climate of adjacent lands.
Explain the importance of ocean currents in the distribution of fish resources of the world.
GohKC
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