Hadley Cell - Mechanism

Mechanism

The major driving force of atmospheric circulation is solar heating, which on average is largest near the equator and smallest at the poles. The atmospheric circulation transports energy polewards, thus reducing the resulting equator-to-pole temperature gradient. The mechanisms by which this is accomplished differ in tropical and extratropical latitudes.

Between 30°N and 30°S latitude, this energy transport is accomplished by a relatively simple overturning circulation, with rising motion near the equator, poleward motion near the tropopause, sinking motion in the subtropics, and an equatorward return flow near the surface. In higher latitudes, the energy transport is instead accomplished by cyclones and anticyclones that cause relatively warm air to move polewards and cold air to move equator wards in the same horizontal plane. The tropical overturning cell is referred to as the Hadley cell. Why it extends only to 30 degrees latitude and what determines its strength are questions addressed by modern dynamical meteorology.

Near the tropopause, as the air moves polewards in the Hadley cell it is turned eastward by the Coriolis effect, which turns winds to the right in the Northern hemisphere and to the left in the Southern Hemisphere, creating the subtropical jet streams that flow from west to east. Analogously, near the surface, the equatorward return flow is turned to the west by the Coriolis effect. These resulting surface winds, with both an equatorward and a westward component, are referred to as the trade winds.