please note that this document covers: part1 air circulation (this page) and part2 ocean circulation
|Introduction to this chapter.|
|Solar energy reaches the planet on its outer atmosphere. Some is reflected and some reaches the surface through a number of natural filters that protect life on earth. What reaches the surface, warms it up and makes plants grow. Atmosphere, ionosphere, exosphere, Van Allen radiation belts, magnetosphere, satellites, orbits.|
|The solar radiation warms the surface of the planet and its atmosphere. Various gases filter incoming and outgoing radiation, resulting in a planet which is liveable. Solar radiation, blackbody, terrestrial radiation, atmospheric gas filters, absorption and shade, photosynthetic absorption, seasons.|
|The way air circulates in the atmosphere is surprisingly complicated. It governs the climate all over the world. During summer and winter, predictable areas of high and low pressure develop and the winds associated with them. General global circulation, Hadley Cell, jet stream, deserts,|
|water circulation||The water retained as moisture in the air, brings life to the continents where it circulates almost independently from that above the oceans. Humans have disrupted this water cycle by changing forests into agriculture and urban development, which in turn changed the climate.|
|Currents and winds are deflected by Coriolis forces. How do they work? How do winds and currents react? What is an Ekman Spiral? How do layers of air or water move?|
|The surface currents are driven by wind blowing over the ocean's surface. Where winds blow predictably, these currents are predictable also their associate climates. Map and names of currents.|
|Most of a water particle's life is spent in the deep sea and nutrients befall the same fate. Yet at places on Earth, the cool bottom water is able to resurface and sea life here is bountiful.|
|Wind-driven currents depend on the vagaries of climate and climate depends on ocean currents. An introduction to the El Niño climate variation that affects nearly the entire world's climates.|
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In this chapter we start with the atmosphere, since the sunlight arriving at the planet's surface is influenced by it, and thus climate and winds. The main current systems in the oceans arise from winds blowing over the water's surface. See also wind and waves. Where such winds blow for prolonged periods and with sufficient force, the ocean's water will be moved at speeds between 0.5 and 2 km/hr, contributing directly to the heat transport towards the poles. Much of the climate depends on these currents, which in turn, depend on the climate. Both the weather and the ocean currents are therefore in constant oscillation or instability. One of these is the El Nino climate/current cycle that influences rainfall and drought far afield and which will be treated in detail in its own chapter. Also ocean productivity depends largely on upwellings caused by ocean currents. More of that in the chapter on plankton and how to feed the world. Ultimately, the health and physics of the oceans, plays an important role in the way our planet will warm in response to the burning of fossil fuels. That too, will have its own chapter.
Because the circulation of water and air depend on the amount of heat arriving at the planet's surface, it is important to understand what happens to the sunlight from the moment it enters our atmosphere. This simple picture helps to relate the relative distances involved. If the planet were the size of a billiard ball, say 5 cm across, then the whole surface of the planet, from Mt. Everest to the deepest ocean trough (20 km) would be no thicker than a human hair. Here is where everything happens to life (the biosphere). Within this human hair, there are at least two layers in the atmosphere and two in the ocean that move almost independently of one another.
At a distance of 150-300 km, the atmosphere is so thin that spacecraft can orbit there in low orbit. The geostationary satellites orbit some 36000 km higher, a distance of about six times the radius of Earth.
is a blanket of protective gases providing insulation against otherwise
extreme alterations in temperature. Earth's gravity pulls the gases towards
its surface where the pressure is 1 bar, decreasing rapidly with height.
At 100 km height, the pressure is only one millionth. About 80% of the
atmosphere is found in the first 15 km; half in the first 5 km.
In the diagram, the various zones are shown. Note that their boundaries vary appreciably from pole to equator and from day to night. The outer zone is the exozone which gradually fades into space. At 600 km the pressure is a mere 1E-38 bar or less than a trillionth of a trillionth of a trillionth of surface pressure. Only the fastest atoms of the lightest gases, Hydrogen and Helium are found here.
Close to the surface, the troposphere (between 8 km at the poles and 15 km at the equator) is where the weather and climate reign. It contains 85% of the atmospheric mass. Air temperature falls steadily at about 4-8ºC for every km height, 8ºC at the tropics and almost nil at the North Pole in winter. Where the passenger airliners fly, 10 km, the temperature is -60 degrees, varying with day and night and other conditions. Near the surface of Earth, the temperature is on average 15 degrees and the air is composed of 78% nitrogen, 21% oxygen and 1% argon.
The tropopause is a region where the temperature starts to rise again, forming the boundary with the stratosphere, which extends upward to 50 km. Here is where protective ozone is found (20-50 km). The stratosphere absorbs ultraviolet light, which warms it. Just above the ozone layer, the temperature drops again gradually to -80 degrees at the mesopause.
In the ionosphere, air particles are electrically charged by the sun's radiation, being able to move at high speeds in the almost perfect vacuum. The temperatures shown in the diagram, therefore have little meaning. A space craft in this zone would not experience it. Four electrically charged layers are found here, the D, E, F1 and F2 layers. These are capable of bouncing radio waves of various frequencies, enabling radio transmitters to beam their programmes much further. Short-wave radio frequencies are able to reach the other side of the planet under favourable conditions. Medium wave (MW) transmissions benefit from the D layer, which becomes active in the night.
Polar auroras occur between 65 and 965 km height
in the polar regions. Meteors from outer space burn up in the lower ionosphere,
at around 100-150 km height.
In the outer exosphere, above 500 km, only protons and electrons and particles arriving by solar wind (400 km/s=1.4 million km/hr), are found. Spiralling along the sun's magnetic field lines, they arrive here in four days. They are trapped by the Earth's magnetic field lines, concentrating in two belts at 3000 and 16000 km height, named after their discoverer, James A Van Allen (1958). The inner belt contains mainly protons with energies exceeding 30MeV (million electronvolt), whereas the outer belt contains mainly fast moving electrons and slow moving helium ions, with energies exceeding 100MeV. Radiation in both belts is so high (20,000 hits per second per cm2) that it is effectively a no-go zone for spacecraft and astronauts. In the top left, the path of a charged particle arriving from the sun, is shown. As it encounters the magnetic field, it starts to corkscrew towards one of the poles, eventually being repelled by the intensity of field lines near the magnetic pole. At times it may interact with the atmosphere's molecules at some 100 km height, producing the flickering light curtains of the Aurora Borealis. Eventually these charged particles are swept into one of the Van Allen Belts. The sun's 'wind' of solar particles compresses the magnetosphere at the windward side, whereas expanding it at the leeward side, which gradually fades into space. The windward extent of the upper exosphere is about 57000 km. Here the magnetic field fluctuates erratically.
During intense solar activity (sunspots and flares), the solar wind
can play havoc with Earth's magnetic field, causing bright auroras and
magnetic storms that upset magnetic compasses, electronic equipment and
long power lines. The year 2001 is expected to be at the height of both
the 20 year and the 10 year sunspot cycle. It takes 4 days for the solar
wind to arrive here, enough to take preventative action in case of extreme
Since Russia took the world by surprise with its Sputnik I satellite (4 Oct 1957), there has been a race for space. The number of satellites launched, is astounding (and some are kept secret). At first, satellites provided an extension of the armament race between the USA and USSR but later, other types were launched:
The sunlight, as it arrives on Earth's atmosphere, almost perfectly matches what is expected of the theoretical radiation from a blackbody of 5780 degrees Kelvin. A blackbody is an idealised mass which is so black that it completely absorbs all radiation falling onto it and also that it radiates out as defined by various laws in physics (Stefan, Boltzmann, Planck, Wien). The diagram tries to simplify the confusing situation. The sun's light (yellow shape), delivers its energy mainly in the visible light wavelengths, from ultraviolet to near infrared (0.3-3 micron), peaking at 0.5 micron (blue-green colour). Although much of it is reflected back in space, the remaining light is converted to heat and radiated out by the Earth's surface (green shape). These wavelengths are in the far infrared range (5-25 micron), peaking at 11 micron. Note that absorption by water is a rather influential factor, causing daily, seasonal and latitudinal (N to S) differences in the measured and averaged radiation curves. The grey shapes show the way natural greenhouse gases filter both the incoming and the outgoing radiation (some curves are incomplete).
resulting incoming radiation on the surface and the outgoing radiation
in space, look rather confusing, as they are filtered both ways. In the
diagram on right, actual radiation is shown but with logarithmic scales.
From the shape of Earth's outgoing radiation, the planet behaves like a
blackbody at -18ºC rather than the average surface temperature of
It is interesting to note that the most abundant atmospheric gas, nitrogen (N2), plays an insignificant role, but the gases that are made by life on Earth, do the work. They filter out harmful ultraviolet radiation (less than 0.3 micron) and they maintain a warming blanket that raises the temperature of the surface from -60ºC, what it would have been without an atmosphere, to +15ºC. They keep the planet habitable.
|The diagrams shown before would be quite meaningless without understanding what solar radiation means to life. This diagram shows in the top half what we've seen before, the solar radiation reaching the atmosphere and what filters through it to sea level. The vertical scale is linear and the horizontal log scale not quite logarithmic, but it shows what the light looks like, just under the sea surface (light blue) (See also UW photography/water/colour). Notice that both the quantity and quality of the light have changed dramatically. The blue sky is caused by blue light bouncing off air molecules and its spectrum is shown here (dark blue), as well as the spectrum underneath a plant canopy, when plants have been filtering their needs out (green). These two qualities of light play havoc with a photographer's settings and are quite often misunderstood. Note that if an object absorbs light of one part of the spectrum, it reflects the remaining light. Thus a plant which absorbs blue light, looks red.|
The bottom graph projects the absorption curves of photosynthetic compounds
necessary for photosynthesis. Plants capture the energy from the sun by
means of a special pigment that turns a light photon into a chemical work
unit (electron). What is not directly obvious is that towards the left
of the graph, light photons become more energetic. Thus a purple photon
(wave length 0.4) is nearly twice as energetic as a red photon (wave length
0.7), and can do more work. A UV-B photon (wave length 0.25) is more energetic
still. During the course of evolution, plants evolved from photosynthesising
bacteria (blue-green algae). At the time there was no atmosphere and the
amount of UV-B radiation that must have existed, made terrestrial life
a death warrant. Notice how bacterio-chlorophyll has its absorption peak
towards the red. Modern plants use chlorophyll and beta-carotene, as well
as the other substances shown.
|This diagram shows how sunlight is distributed over the planet by latitude, going from the North Pole (left) to the South Pole (right), and in what proportions it is reflected back to space and absorbed by the planet. As can be expected, most sunlight falls in mid latitude, around the equator. The amount absorbed by the surface, depends mainly on what grows on the surface, green plants absorbing most. The amount absorbed by air (blue shape) follows a fixed proportion of the incoming radiation, but the part reflected by clouds (white) brings a surprise, mainly because of dense cloud cover in the temperate regions. Reflection by the surface is highest at the poles, but by Antarctica more than the arctic. Also deserts at around 30 degrees, reflect the light.|
The combined effects of radiation, reflection, latitude, season, vegetation and the sizes of the land masses, causes the continents and the oceans to warm and cool, producing winds and currents.
Also visit the CNES (Centre Nationale des Etudes
in earth observation web site for the theory of atmospheric principles,
satellite observing, etc. More details also in the chapters on Soil and
From the previous chapter it is clear that the equator warms up much more than the poles. The global wind circulation would simply have consisted of warm air rising at the tropics, travelling south and descending at the poles; cool wind travelling over the surface back to the tropics. The picture shows that the situation is more complicated, mainly because the Earth rotates and because warm air contains moisture. The amount of heat carried by moisture is unexpectedly large because water requires an enormous amount of heat to evaporate - heat which is released again when water condensates. Another important factor is that air is compressible, heating up when compressed while cooling when expanding. The picture on right tells the story of how air circulates around our planet.
Starting at the tropics, we find warm air, loaded with moisture, indeed rising (tropical Hadley Cells). Over the tropics, the effect of the rotating earth (the Coriolis force) is least. Warm air climbs, cools, loses its moisture (tropical rains) and travels south some distance. At about 30 degrees latitude the dry air descends (because it has cooled and because of the Coriolis force), compresses and warms up to form deserts on land and calm areas at sea (the doldrums). Curving slightly, winds return to the tropics as trade winds. Notice that the speed of up and down drafts (0.05-0.1 m/s) is 100 times less than that of horizontal winds (10-20 m/s). Imagine the Hadley Cells and surface winds as spiralling north and south of the equator.
Between latitude 30-60 degrees, the Coriolis force
is felt strong enough to cause winds to rotate horizontally as wandering
cyclones and anticyclones. In the northern hemisphere, winds are deflected
to the right, which makes them rotate clockwise around anticyclones (H)
and anti-clockwise around cyclones (L). On the southern hemisphere, the
situation is the reverse.
On average the barometric pressure above 30º latitude is high and low towards 60º. Between the high and low pressure bands, winds mainly move in an easterly direction (temperate westerlies, 'roaring forties'). Towards the poles, the winds move mainly in westerly direction (polar easterlies, 'screaming sixties'). At high altitude (9-15 km) in the down draft of the tropical Hadley cells, above the desert regions (at 30-40º latitude), runs a strong westerly wind (the jet stream) which is very focused (several hundred km wide) and which flows fast (200-300 km/hr). It is trapped in the frontal zone between polar and tropical air (see diagram below). The jet stream does not run around the world as a continuous ribbon but it interconnects weather systems, flowing fast in winter (100-200 km/hr) but much slower in summer (30-80 km/hr). (From R G Barry & R J Chorley: Atmosphere, weather & climate. 1982)
At the poles, air rises and descends by convection
in the polar Hadley Cells. Coriolis deflection turns this into spiralling
movements around the poles.
of continents and seasons, complicates the pattern further, as can be seen
from the maps below. As the earth tilts from summer to winter, the heat
equator moves from 23.5 degrees south to 23.5 degrees north and back, passing
over the true equator twice. It gives places around the equator four seasons
(monsoons) and everywhere else two (summer/winter). Note that monsoons
develop only where large continents border an ocean.
The top map gives the situation in the northern summer, whereas the bottom map pictures the southern summer.
Both the winds and the high/low pressure areas are of course averages
for each season, obscuring the enormous variations from day to day. Wherever
the wind blows over the sea, a surface current is induced. In places where
the wind blows from the same direction in all seasons, sea currents can
develop to great depths (to 200m deep) and high speeds (up to 2 km/hr).
See if you can predict from these maps, where the main ocean currents can
Heat is tranported from the equator to the poles by ocean gyres and wind. As a rule of thumb, about one third is done by the ocean currents and two thirds by the atmosphere. Compare the areas under the blue and red curves above the zero line. Total heat transport is maximal where the temperature gradient is maximal, at mid latitudes (30-50º). Ocean currents are more effective in the subtropics (10-40º), whereas the atmosphere is more effective at higher latitudes (30-70º). Note that heat transport by air is not done by the air itself, which has a very low heat capacity, but by successive evaporation and precipitation. As warm ocean water moves from the tropics northward, it enters a cool atmosphere, resulting in high evaporation and rain, until the ocean has cooled completely.
Although early seafarers gave account of parts of the oceans without any wind (the doldrums, dark blue areas) and other parts with never ending storms and towering waves (yellow-red areas), nobody really had any idea of the average height of the waves in all oceans. The noise level on early satellite maps of the height of the sea surface gave some indication, but it wasn't until 1992 that the TOPEX/Poseidon satellite gathered data by bouncing radio waves off the surface of the ocean and the reflected signals were analysed. The picture on right emerged. It shows that the ocean around the equator is generally calm (purple/blue), with very calm regions in between the archipelagos of Indonesia and the Caribbean, because there is not enough wind fetch to create high waves. Surprisingly, an area exists south of the Indian Ocean, where waves average up to 7m in height (yellow/red)! In this area, winds consistently blow eastward, which gives them an unequalled fetch on the ocean's water. This area may well be the power house behind the South Pacific Gyre, which by its El Niño and La Niña cycles, has a major influence on all climate of the planet.