trade wind

The westerlies and trade winds

The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator^[1]. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Historically, the trade winds have been used by captains of sailing ships to cross the world's oceans for centuries, and enabled European empire expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.

In meteorology, the trade winds act as the steering flow for tropical storms that form over the Atlantic, Pacific, and south Indian Oceans that make landfall in North America, Southeast Asia, and India, respectively. Trade winds also steer African dust westward across the Atlantic ocean into the Caribbean sea, as well as portions of southeast North America. Shallow cumulus clouds are seen within trade wind regimes, which are capped from becoming taller by a trade wind inversion, which is caused by descending air aloft from within the subtropical ridge. The weaker the trade winds become, the more rainfall can be expected within neighboring landmasses.

History

A Spanish galleon

Their term trade winds originally derives from the late Middle English word 'trade' (borrowed from Middle Low German, and cognate with English 'tread'), meaning "path" or "track," and thence the obsolete nautical phrase "the wind blows trade," that is to say, on a consistent track. The Portuguese recognized the importance of the trade winds in navigation in the Atlantic ocean as early as the 15th century.^[2] The full wind circulation, which included both the trade wind easterlies and higher-latitude Westerlies, was not known across the Pacific ocean until 1565.^[3]

The captain of a ship seeks a course along which the winds can be expected to blow in the direction of travel.^[4] During the Age of Sail the pattern of prevailing winds made various points of the globe easy or difficult to access, and therefore had a direct impact on European empire-building and thus on modern political geography. For example, Manila galleons could not sail into the wind at all.^[3]

By the 18th century, the importance of the trade winds to England's merchant fleet crossing the Atlantic Ocean led both etymologists and the general public had come to identify them with a later meaning of 'trade', "(foreign) commerce".^[5] Between 1847 and 1849, Matthew Fontaine Maury collected enough information to create wind and current charts for the world's oceans.^[6]

Cause

General distribution of air masses near North America

As part of the Hadley cell circulation, surface air flows toward the equator while the flow aloft is poleward. A low-pressure area of calm, light variable winds near the equator is known as the doldrums,^[7] equatorial trough,^[8], intertropical front, or the Intertropical Convergence Zone.^[9] When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough.^[10] Around 30° in both hemispheres air begins to descend toward the surface in subtropical high-pressure belts. The sinking air is relatively dry because its moisture has already been released near the equator. This superior air mass is dry and subsident, or sinking through the troposphere, originating within the subtropical ridge, and rarely reaches the ground. The superior air normally resides over the top of maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below. The superior air mass normally forms a trade wind inversion over the maritime tropical air mass due to its warmer character.^[11]

The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis effect.^[12] south of the subtropical ridge. These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.^[13] Because winds are named for the direction from which the wind is blowing,^[14] these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The trade winds meet at the doldrums.^[7]

As they blow across tropical regions, air masses heat up over lower latitudes due to more direct sunlight. Those that develop over land (continental) are drier and hotter than those that develop over oceans (maritime), and travel northward on the western periphery of the subtropical ridge.^[15] Maritime tropical air masses are sometimes referred to as trade air masses.^[16] The one region of the Earth which has an absence of trade winds is the north Indian ocean.^[17]

Weather effects

Nā Pali coast, Kauaʻi, showing trade wind cumuli

Clouds which form within trade wind regimes are typically composed of shallow cumulus, which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by the trade wind inversion.^[18] Trade winds become more poleward in origin (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer.^[19] As an example, the windy season in the Guianas, which lie at low latitudes in South America, occurs between January and April.^[20] When the phase of the Arctic oscillation (AO) is warm, trade winds are stronger within the deep tropics. The cold phase of the AO leads to weaker trade winds.^[21] When the trade winds are weaker, more extensive areas of rainfall fall upon landmasses within the tropics, such as Central America.^[22]

During mid-summer in the Northern Hemisphere (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America. When dust from the Sahara moving around the southern periphery of the ridge moves over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates.^[23] Over 50% of the African dust that reaches the United States affects Florida.^[24] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year.^[25] Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.^[26]

See also

• Intertropical Convergence Zone

References