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The General Circulation of the Atmosphere

Introduction

The circulation of the atmosphere across the globe is chaotic; as a result, long-term weather prediction is theoretically impossible. However, large-scale structures in atmospheric circulation are stable over time and can be described by basic ideas about energy balance in the Earth system. 

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R_O = \frac{|V_\theta|}{2 \Omega r}

General Circulation in the Atmosphere

The earth receives heat energy from incoming solar radiation. Because the earth is round and its rotating axis is slightly tilted, the amount of income radiation is different depending on the latitude and time of the year. The mean incoming solar radiation is at a maximum in the tropics, and decreases as the latitude increases to the pole. The difference in radiative budget results in atmosphere’s meridional instability. To counteract the instability, the atmosphere transports heat from the equator poleward. 

The meridional heat flux consists of two parts: the mean circulation and eddies. The mean circulation is most prominent in the tropics and called Hadley cell circulation, transporting heat from the equator to the sub-tropics. Eddies are the transient heat flux prominent in the mid-latitudes, transferring heat poleward. Together, they transport the heat from the equator to the pole. 

Hadley Cell

We can plot climatological fields showing the meridional mean features of the atmosphere to observe the Hadley cell circulation. There are two phenomena that can be observed in the Hadley cell: the overturning circulation and thermal wind.

Overturning Circulation

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Figure 1: Zonally averaged temperature in January and July from 1948 to 2016. The warmest air is always at the tropics, but the temperature gradient is strongest over the mid-latitude in the northern hemisphere in January and in the southern hemisphere is July.

The overturning circulation functions by the convection of heat in the tropics. In Figure 1, the meridional temperature plot has its maximum in the tropics, where the incoming solar radiation is more perpendicular to the surface than the pole, hence the larger effective solar radiation. 

In January when it is summer in the southern hemisphere, the solar radiation comes slightly more from the south, so the temperature peak is slightly to the south. As it is winter in the northern hemisphere, the temperature gradient in the northern hemisphere is larger than in the southern hemisphere. While in July where it is summer in the northern hemisphere and winter in the southern hemisphere, the temperature peak is slightly to the north, and the temperature gradient is larger in the northern hemisphere.

Because the incoming solar radiation in January and July is not equal in southern and northern hemisphere, the Hadley cell will not be two symmetrical cells over the equator.  One cell will be weaker and one stronger, where it is easier to observe the stronger cell.

Thermal Wind

Tank Experiment

Two rotating tank experiments were set up. Tank rotation speed was the important parameter varied: one tank rotated at a speed of 1 

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