The energy that drives surficial processes including oceanic and atmospheric circulation comes from the sun. This energy arrives at the Earth as radiative energy. As an atom gains energy its electrons jump to a higher energy state, as the atom losses energy that energy is emitted as radiation. There is abundant evidence that the mean temperature of the Earth has been nearly constant at 255° K for a very long time. In order for the temperature to remain constant, there must be a long term balance of energy between the Earth and space. Although this balance does exist for the Earth as a whole, orbital parameters of the Earth cause solar radiation to fluctuate over latitudinal bands. The redistribution of this heat from the equator where radiation is maximum to the poles, coupled with the Coriolis force which arises due to Earth's rotation gives rise to the atmospheric circulation pattern we call the General Circulation Pattern. Let's look at this balance that has resulted in a stable temperature that supports life and allows plate tectonics to operate.
Radiation is a wave phenomena. An important concept governing
radiative heat transport is that bodies of different temperature
radiate energy with different wavelengths. Recall that wavelength is
the length of a single cycle of the waveform. Wien's Law tells us
that hotter bodies radiate more short wavelength energy than cool
bodies. This means that the energy radiated by the sun which is a
relatively hot body (temperature about 5780° K) will have a lot
of its energy in the visible range but dominated by a certain
wavelength which we call ultraviolet. As a comparison, the Earth at a
temperature of about 255° K radiates most of its energy in the
infrared band. We will shortly see why this is important, but for now
let's look at what happens to the radiation from the sun as it
arrives at the Earth. When this radiation encounters the Earth's
atmosphere some is absorbed, some is scattered back to space, some is
reflected back to space, and some is transmitted. The total amount
that is sent back to space (reflected and scattered by the atmosphere
and surface of the Earth) is about 30%. This fraction of the total
solar energy that is not transmitted to the Earth is called the
Earth's albedo. If this fraction were higher than it is, less
radiation would reach the Earth and the equilibrium temperature would
be colder. Of the 70% of the solar radiation that penetrates the
atmosphere, 20% is absorbed by the atmosphere and clouds and 50% is
absorbed by the Earth's surface. Particular gasses in the atmosphere
absorb this energy. Ozone (O3) is the most important of these gases.
Ozone is very efficient at absorbing ultraviolet radiation which the
sun dominantly radiates. This is extremely fortunate since
ultraviolet radiation is very damaging to life on Earth. Since 70% of
solar radiation reaches Earth, 70% must return to space or the Earth
would keep heating up. This energy is radiated back to space,
however, since the Earth is at a colder temperature than the sun, it
radiates this energy in the longer wavelength infrared region of the
electromagnetic radiation spectrum. The significance of this is that
gases such as 
and 
, which are very abundant,
preferentially absorb infrared radiation and therefore heat the
Earth's surface. These gases are known as "greenhouse" gases because
of they absorb the long wavelength energy radiated by the Earth and
warm it up. The Earth would be colder if it was not for the presence
of these greenhouse gases. Of course if the Earth's atmosphere had
much greater concentrations of these greenhouse gases, like Venus,
the surface temperature would be too great to support life. In fact,
the presence of abundant greenhouse gases in the atmosphere could
even inhibit plate tectonics by making the surface temperature of the
planet too hot to have lithospheric plates. This may indeed by the
case for Venus with a surface temperature of about 427°.
Although over the entire planet the incoming radiation from the sun is balanced by outgoing radiation (although the wavelength of the energy has been modified), the intensity of the solar radiation varies as a function of latitude due to the tilt of the rotation axis of the Earth. The intensity of the radiation is greatest near the equator where the angle between the radiation path and the Earth's surface is the largest (the cross-sectional area of the sun's rays is smallest and thus intensity is the largest). In addition to latitudinal variations in solar radiation the elliptical orbit of the Earth about the sun results in seasonal variations of solar radiation as well.
Atmospheric circulation results from differential heating of the planet and planetary rotation.
A. Driving mechanisms
Let's look at the forces acting on a volume of air. These are gravity acting in a downward direction (towards the center of the Earth) and the force due to the decrease in pressure as one moves from the surface of the Earth into the atmosphere. This force is directed from high pressure to low pressure or away from the center of the Earth. The force due to gravity is (mass) x (acceleration due to gravity) and the force due to the pressure change is the difference in the pressure at z =z1 and z2 times the area or: (P1 -P2) x (Area). If we set these two forces equal and divide both sides by area we get:
This equation is called the hydrostatic equation and it says that in warm regions where density is small, the change in pressure over some vertical distance z is small compared to cold regions where density is greater. This results in contours of equal pressure (isobars) being far apart at the equator and close together at the poles. This produces horizontal flow that goes from pole to equator near the surface of the Earth and from equator to pole at high altitude. This is a pressure dominated horizontal flow of air. Vertical motion of the atmosphere is dominated by density so it flows up at the equator where the air is warm and not dense and down at the pole where the air is cold and dense. This circulation pattern that is perpendicular to isobars is not what we observe and the reason is that it is disturbed by rotation of the Earth. The disturbing agent is known as the Coriolis Force.
B. Coriolis Force
Freely moving objects on the surface of the Earth appear to curve to the right in the northern hemisphere and to the left in the southern hemisphere due to the Earth's rotation. This effect is known as the Coriolis effect and it has a large influence on atmospheric and ocean circulation. The Coriolis effect arises from observing these motions from a moving reference frame: the spinning Earth. The object is actually moving in a straight line but the Earth where we are observing the motion is moving counterclockwise so the object appears to be veering away from us in a clockwise direction or to the right in the northern hemisphere and to the left in the southern hemisphere. The magnitude of the Coriolis effect increases from zero at the equator where the surface of the Earth is parallel to the spin axis to a maximum at the poles where the surface of the Earth is perpendicular to the spin axis.
The effect of the Coriolis Force is to produce the northeast and southeast trade winds in the latitudinal band between the equator and 30° north and south respectively. The surface circulation traveling from high pressure in high latitudes to low pressure at the equator is turned to the right (west) and left (west) by the Coriolis force in the north and south hemispheres respectively. The result is a wind that blows from the northeast to the southwest in the northern hemisphere (called the northeast trade wind) and from southeast to northwest in the southern hemisphere (called the southeast trade winds). This pattern is distrubed in the higher latitudes, primarily due to the increase in the Coriolis force which produces vortices or circular motions of the atmosphere called cyclones when rotation is towards a low pressure center or anticyclone when motion is away from a high pressure center. Also important is the distribution of land masses which heat up and cool down at different rates from the ocean and therefore complicated simple circulation patterns for a uniform surface.
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