The adage "follow the money" is often invoked to explain the actions of politicians. The behavior of the atmosphere too can be explained by a balance sheet--not one of money, but of energy.
Solar radiation is the main energy source driving the earth's atmospheric system. Through a cycle of radiation absorption and re-emission, the atmosphere and the underlying surface exchange energy. Calculations of annually and globally averaged conditions show that the earth's surface has a net radiation surplus, while the atmosphere comes up short. A balance is established through non-radiative processes, namely conduction and convection, that heat the atmosphere. So-called latent heating is the dominant mechanism. Examining the radiation balance as a function of latitude, we see that tropical regions have a radiation surplus; the deficit over the higher latitudes peaks at the poles. Once again, atmospheric and oceanic processes act to alleviate such imbalances. And, in fact, the general circulation--the global system of ocean and air currents that we observe--results from this north-south imbalance.
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Image: UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN HURRICANE ALLEYS. Every day, somewhere on the planet, it is hurricane season. Such storms arise to adjust imbalances that occur as the atmosphere transfers latent heat from the equator to the poles. |
In the tropics, the general circulation consists of a north-south cell, known as the Hadley cell. Near the equator, large-scale convection causes air to rise.On reaching the upper troposphere, this air flows poleward. Beneath the diverging air, surface pressure drops and an equatorial trough develops. Over the maritime tropics, tall convective clouds form in the vicinity of the equatorial trough, where the sea surface temperature is quite high. This deep convection, the most conspicuous feature of the tropical circulation, in the company of precipitation transports latent heat from the earth's surface to the upper atmosphere.
Elsewhere in the tropics, however, non-precipitating cumulus clouds dominate the landscape. Dynamical constraints force poleward-flowing air to sink, which--on reaching the surface of the earth--returns back to the equator, completing the Hadley cell. The upper-tropospheric sinking motion stabilizes the underlying atmosphere, which accounts for the omnipresence of shallow non-precipitating clouds. Notwithstanding the name, the equatorial trough gets displaced well into the southern hemisphere during the solstitial seasons. In short, assisted by tropical convection, the Hadley circulation exports energy from the upper-tropospheric tropics to higher latitudes, and circulation features of the middle latitudes link the tropics with the polar regions.
In general, the observed temperature is the highest in the vicinity of the equatorial trough; it is the lowest in the upper atmosphere above poles. Therefore it is logical to treat the atmosphere as a heat engine. It extracts energy from a warm source, performs work and deposits the unused energy at a cold sink. The work performed by this atmospheric engine drives the atmospheric general circulation. As long as the Sun warms the surface of the earth non-uniformly, the atmospheric heat engine will continue to drive the general circulation.
That said, the efficiency of the atmospheric heat engine is rather low; from time to time, inefficiency causes the disparity between the warm source and the cold sink to increase. And this gap calls for a more efficient energy transfer mechanism, namely hurricanes. The inward spiraling cyclonic circulation associated with hurricanes (also known as typhoons and tropical cyclones) extracts latent heat very efficiently. Also, the movement of hurricanes helps export energy to the higher latitudes. Thus the hurricane season falls during that period when the tropical meridional circulation cell is unable to facilitate the requisite equator-to-pole transfer of energy. In response, the potential for the development of hurricanes builds up in the atmosphere. Although every day of the year, somewhere on this planet, it is hurricane season, only when a set of unique conditions come together do hurricanes actually form.
