LEARNING OBJECTIVES
By the end of this topic, you should be able to:
Trade winds are the permanent east to west prevailing winds that flow in the equatorial region of the earth (between 30⁰N and 30⁰S latitudes). They are also referred to as easterlies. These winds blow predominantly from the southeast and the northeast in the Southern and Northern Hemisphere respectively. These winds strengthen during winter as well as when the Arctic oscillation is in its warm phase. These winds have been used for sailing ships by captains to cross the oceans of the world for centuries. This allowed for colonial expansion into the Americas as well as the establishment of trade routes across the Pacific and Atlantic oceans.
In meteorology, trade winds act as the steering flow for tropical storms forming over the Pacific, Atlantic and the southern Indian Oceans. Trade winds are also responsible for transporting African dust westward across the Atlantic Ocean into the Caribbean Sea. Shallow cumulus clouds can be seen within the regimes of trade winds but they are capped from becoming taller by an inversion of trade winds. This inversion comes as a result of descending air aloft from the subtropical ridge. The weaker the trade winds, the greater the rainfall that can be expected in the neighboring landmasses.
CAUSE
As a part of the Hadley cell, surface air flows toward the equator while the flow aloft occurs towards the poles. An area of low pressure of calm, light variable winds close to the equator is referred to as the doldrums, intertropical front, near-equatorial trough or the intertropical convergence zone. When it is located within the monsoon region, this zone of low pressure and wind convergence is also referred to as the monsoon trough. Around 30⁰ in both hemispheres, air starts to ascend towards the subtropical ridges (the surface in the subtropical high pressure belts). The sinking (subsident) air is relatively dry because as it descends, temperature increases, but the absolute humidity remains constant, this lowers the relative humidity of the air mass. The warm dry air is referred to as the superior air mass and usually resides above a maritime tropical (warm and moist) air mass. An increase of temperature with height is referred to as temperature inversion. When this occurs in a trade wind regime, it is referred to as a trade wind inversion.
The surface air flowing from these high-pressure subtropical belts toward the equator is deflected toward the west by the Coriolis effect in both hemispheres. These winds predominantly blow from the northeast and from the southeast in the Northern and Southern Hemispheres. Since the naming of winds is based on the directions from which they blow, these winds are referred to as the northeasterly trade winds in the Northern Hemisphere and southeasterly trade winds in the Southern Hemisphere. Doldrums is the place where the trade winds of both hemispheres meet.
As these winds blow across tropical regions, air masses heat up over lower latitudes as a result of more direct sunlight. Those winds that develop over land (continental) are hotter and drier than those that develop over oceans (maritime). Maritime tropical air masses are sometimes called trade air masses. The north Indian ocean is one region of the earth that does not have trade winds.
