Atmospheric Circulation

Macroscale Circulation

Planetary scale features are the largest, occurring on spatial scales ranging from 10,000 to 40,000 km (kilometers) (6,000–24,000 mi. [miles]) and with life cycles on the order of weeks to months. These features include three major latitudinal wind and pressure bands encircling the globe in each hemisphere: the convective Hadley cells of the tropics; the Ferrel cells of the midlatitudes, and the polar cells of the high latitudes. The three circulation cells represent an idealized model of the dominant planetary wind motions by means of two major simplifications: (1) a uniform surface (to eliminate complications with differential surface heating) and (2) no seasonal or diurnal insolation changes (i.e., the model assumes solar noon on the equinox so that all latitudes are receiving 12 hrs. [hours] of daylight and the sun is always directly over the equator).

The low latitudes, between approximately 0° and 30° latitude, are governed by the heat-driven Hadley cells, named after the English physicist and meteorologist George Hadley (1685–1768), who first proposed their existence in 1753. Around the equatorial region, warm, humid air ascends from daytime convective heating, often condensing into towering cumulonimbus clouds. These convective thunderstorms release large amounts of latent heat energy that increases the buoyancy of the air and provides the energy to direct the Hadley cell. The ascension of air is further enhanced by surface convergence of the northeast and southeast trade winds that produce a low-pressure zone known as the equatorial trough or intertropical convergence zone (ITCZ). Consequently, locations around the ITCZ are associated with high annual precipitation, a marked daily regularity in rainfall, and a tropical wet climate with thick, broadleaf vegetation forests. Tropical rain forests are common around the Congo River basin in Africa, the Amazon River basin in South America, and the South China Sea region of southeast Asia.

Aloft, the upper-level flow of the Hadley cells is characterized by poleward-moving air that cools along the tropopause (the boundary between the weather-producing troposphere and the stable stratosphere). As the air propagates farther away from the ITCZ, it loses buoyancy and latent heat energy, and radiational cooling commences. The density of the air increases and begins to sink between 20° and 35° latitude. Since the Coriolis effect increases with increasing latitude, the poleward-moving air is also deflected, forcing the air into a west-east flow. The result is a convergence of air aloft that strengthens the subsidence and increases the surface pressure to produce an elongated subtropical high-pressure belt. Subtropical air is comparatively dry with a relatively low humidity, a by-product of (a) most of the moisture content being liberated near the equator and (b) the further warming of the air by adiabatic compression. Accordingly, the subtropics are often associated with dry conditions and clearer skies and are the site of the largest deserts in the world. The Sahara Desert of Northern Africa, the Atacama Desert of Western South America, the Arabian Desert of the Middle East, and the Great Sandy Desert of Australia are all positioned along this subtropical high-pressure zone.

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