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The three experiments develop the framework for a model of atmospheric circulation on Earth. The equator-to-pole temperature gradient creates an energy imbalance which is equalized when air moves along pressure gradients and is affected by conservation of angular momentum as it travels meridionally. These constraints produce two structural regimes visible in the atmosphere: the Hadley Cell and mid-latitude eddies.
Hadley Cell
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The Hadley cell is a region of convective overturning from about 0° to 30° N and S: the northern and southern extent shifts seasonally. Warm air rises at the equator, moves towards the poles, sinks in the subtropics, and returns along the surface towards the equator. Zonal winds are generated due to conservation of angular momentum. In the upper atmosphere, the westerly subtropical jet forms at the poleward boundary of the cell. Towards the equator, the easterly trade winds can be observed, although their speed is reduced by surface friction.
Poleward of 30° N and S, the Hadley cell breaks down. In a non-rotating Earth, the cell might extend all the way to the pole, but we observe that Coriolis deflection turns wind parallel to the equator at 30° where it sinks and returns to the equator. Temperature gradients poleward that cannot be equalized by convection lead to baroclinic instability.
Mid-latitude Eddies
Baroclinic instability leads to eddy formation. While there is a mid-latitude convective cell, its importance to heat transport is dwarfed by these eddies, which stir heat from the equator towards the pole. The mid-latitude region dominated by weather systems extends from 30° to 60° N and S, where it gives way to the polar convective cell.
General Circulation in the Atmosphere
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