General circulation of the atmosphere - (n.)

The science of meteorology may well be said to have begun when George Hadley, in 1735, attempted to explain from physical principles why the trade winds exist. The trades are the easterly winds north and south of the equatorial doldrum belt. The trades got their name from English sailors, for their steadiness (as in "the wind blows trade," i.e. on track). Doldrums refers to the becalmed ships with flapping sails (as in the term "in the doldrums"). Hadley pointed out that equatorial solar heating would make tropical air lighter than air at high latitudes. Tropical air would thus rise, high latitude air would sink, winds at the surface would blow towards the equator, and winds aloft would blow towards the poles. Hadley realized that the rotation of the earth would cause the surface wind blowing toward the equator to veer towards the west, thus producing the system of trades familiar to the mariners of his day. This picture is quite realistic as concerns the trades and the doldrums. It had to be modified considerably, however, to account for the other global wind belts, notably the westerlies. The physicist Carl-Gustaf Rossby did this in the 1930's and 1940's, thus founding modern meteorology. Not one, but several circulation cells are involved, with air rising not only at the tropics, but also at the polar front, and with air descending in the zone separating the westerlies and the trades, the great high pressure belt which contains the centers of the ocean gyres and the deserts on land.

A general scheme of circulation has the following elements: Moist air rises in a low pressure zone along an equatorial belt. This is the Intertropical Convergence Zone, or ITCZ. In the Northern Hemisphere, the ITCZ tends to move north in summer and south in winter. Also, it is deflected away from the equator by monsoonal winds, especially in the Indian Ocean region, and over Africa. Away from the ITCZ the de-watered air sinks, heating up while doing so, and produces an inversion on top of the trades. The term "inversion" refers to a vertical temperature profile that discourages convection (that is, it describes slower-than-normal cooling upward). North and south of the ITCZ, air is being sucked in close to the sea surface, just as Hadley envisioned. This air, moving toward the equator, picks up moisture (and heat) from the tropical ocean. The air's motion is deflected to the right on the northern hemisphere, and to the left on the southern hemisphere. The reason for the deflection is the rotation of the Earth, which subjects moving objects and fluid particles to the Coriolis Force. The Coriolis Force deflects every moving particle toward the right on the northern hemisphere and toward the left on the southern hemisphere. It is responsible for converting the Hadley convection into zonal winds. In the mid and high latitudes, the actual flow is much more complicated than a zonal scheme would suggest. The belt around the polar front region (roughly at 35 to 50°N), where the zone of excess heat meets that of heat deficiency, is dominated by cyclones. These are the eddies of counter-clockwise winds. The reason for the formation of eddies is that temperature gradients, heat exchange, and Coriolis forces are at a maximum here in mid-latitudes. This makes zonal air flow highly unstable. The distribution of land and sea, in mid-latitudes, greatly influences where high pressure centers and low pressure centers will develop. On the whole, the ocean gives off heat to the atmosphere. Thus, air tends to rise over the sea which establishes low pressure centers especially where warm water penetrates into high latitudes. The prime examples for more or less fixed low pressure centers resulting from this mechanism are the Iceland Low and the Aleutian Low.