Global Atmospheric Circulation
The circulation of the atmosphere is responsible for about 50% of the transport of energy from the tropics to the poles. The basic mechanism is very simple: hot air rises in the tropics, reducing the pressure at the surface and increasing it higher up. This forces the air to spread away polewards at high levels, and to be drawn in at low levels. As the warm, polewards moving air comes into regions with less incoming solar radiation, it cools and sinks, thus completing the circulation.
Global Circulation, from the Met Office
If the Earth were not rotating, we would see this very simple pattern: hot air would rise in the tropics, move away from the equator, gradually cool, sink at high latitudes near the poles, and finally re-circulate near the surface towards the equator.
However, the Earth’s rotation complicates things. For a given point on the Earth’s surface to do a full rotation, it has to travel a lot further at the equator (2 π times the radius of the Earth; i.e. 6,371 km), than at mid-latitudes, and at the poles it doesn’t have to travel at all, just rotate. Speed is defined as a distance divided by time, so for a full rotation in 24 hours, this means that the speed of the surface of the Earth is greatest at the equator, and falls with increasing latitude.
Now, imagine a cannonball fired towards North from the Equator. In addition to its northward speed, the cannonball also has the same easterly speed as the Earth from which it was launched. But, as it travels further North, the Earth underneath it is moving slower than the Earth at the equator was. So the cannonball appears to drift to the east in flight. This is called the Coriolis effect or force. The Coriolis force is the reason why the upper level, polewards travelling air (wind!) is westerly (west to east); whereas the equatorwards travelling, surface winds are easterly (east to west). In the tropics, these easterlies are known as the trade winds.
In the case of the atmosphere, this means that winds travelling polewards get a bigger and bigger westerly speed. This peaks in the sub-tropical jet streams where air speeds are typically 40m/s in the upper troposphere. With such large vertical velocity gradients, the air becomes unstable, and waves develop in the westerly flow. We experience these as the low pressure systems which regularly pass over the North of Europe. These systems mix the air, which results in the transport of cold air equatorwards and of warm air polewards. Their net effect is the transport of heat polewards and they set up the so-called Ferrell cell, which rotates in the opposite sense to the Hadley Cell.
In the Polar regions, the circulation pattern is very similar to the Hadley Cell and is called the Polar Cell. It is driven by the ascent of warmer air and the descent of colder air. The mid-latitude jet stream is found in the upper troposphere where the Ferrell and Polar cells meet.