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    Why Do We Have a Hurricane Season?

    By By Jose Maliekal
    May 14, 2013, 5:26:04 AM EDT

    Jose Maliekal, an Associate Professor of Earth Sciences at State University of New York at Brockport, explains:

    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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    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.

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