Trade winds

Last updated
The westerlies (blue arrows) and trade winds (yellow and brown arrows) Map prevailing winds on earth.png
The westerlies (blue arrows) and trade winds (yellow and brown arrows)

The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly 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. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries. They enabled colonial expansion into the Americas, and trade routes to become established across the Atlantic Ocean and the Pacific Ocean.

Contents

In meteorology, they act as the steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian oceans and make landfall in North America, Southeast Asia, and Madagascar and East Africa. Shallow cumulus clouds are seen within trade wind regimes and 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 in the neighboring landmasses.

The trade winds also transport nitrate- and phosphate-rich Saharan dust to all Latin America, the Caribbean Sea, and to parts of southeastern and southwestern North America. Sahara dust is on occasion present in sunsets across Florida. When dust from the Sahara travels 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. [1]

History

A Spanish galleon Spanish Galleon.jpg
A Spanish galleon

The term originally derives from the early fourteenth century sense of trade (in late Middle English) still often meaning "path" or "track". [2] The Portuguese recognized the importance of the trade winds (then the Volta do mar , meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic ocean as early as the 15th century. [3] From West Africa, the Portuguese had to sail away from continental Africa, that is, to west and northwest. They could then turn northeast, to the area around the Azores islands, and finally east to mainland Europe. They also learned that to reach South Africa, they needed to go far out in the ocean, head for Brazil, and around 30°S go east again. (This is because following the African coast southbound means sailing upwind in the Southern hemisphere.) In the Pacific ocean, the full wind circulation, which included both the trade wind easterlies and higher-latitude Westerlies, was unknown to Europeans until Andres de Urdaneta's voyage in 1565. [4]

The captain of a sailing ship seeks a course along which the winds can be expected to blow in the direction of travel. [5] 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 effect on European empire-building and thus on modern political geography. For example, Manila galleons could not sail into the wind at all. [4]

Edmond Halley's map of the trade winds, 1686 Edmond Halley's map of the trade winds, 1686.jpg
Edmond Halley's map of the trade winds, 1686

By the 18th century, the importance of the trade winds to England's merchant fleet for crossing the Atlantic Ocean had led both the general public and etymologists to identify the name with a later meaning of "trade": "(foreign) commerce". [6] Between 1847 and 1849, Matthew Fontaine Maury collected enough information to create wind and current charts for the world's oceans. [7]

Cause

3D map showing Hadley cells in relationship to trade winds on the surface Earth Global Circulation.svg
3D map showing Hadley cells in relationship to trade winds on the surface

As part of the Hadley cell, surface air flows toward the equator while the flow aloft is towards the poles. A low-pressure area of calm, light variable winds near the equator is known as the doldrums, [8] near-equatorial trough, [9] intertropical front, or the Intertropical Convergence Zone. [10] When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. [11] Around 30° in both hemispheres, air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. The subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the moisture content remains constant, which lowers the relative humidity of the air mass. This warm, dry air is known as a superior air mass and normally resides above a maritime tropical (warm and moist) air mass. An increase of temperature with height is known as a temperature inversion. When it occurs within a trade wind regime, it is known as a trade wind inversion. [12]

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. [13] These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. [14] Because winds are named for the direction from which the wind is blowing, [15] these winds are called the northeasterly trade winds in the Northern Hemisphere and the southeasterly trade winds in the Southern Hemisphere. The trade winds of both hemispheres meet at the Doldrums. [8]

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. [16] Maritime tropical air masses are sometimes referred to as trade air masses. [17] All tropical oceans except the northern Indian Ocean have extensive areas of trade winds. [18]

Weather and biodiversity effects

Clouds which form above regions within trade wind regimes are typically composed of cumulus which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by the trade wind inversion. [19] Trade winds originate more from the direction of the poles (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer. [20] As an example, the windy season in the Guianas, which lie at low latitudes in South America, occurs between January and April. [21] When the phase of the Arctic oscillation (AO) is warm, trade winds are stronger within the tropics. The cold phase of the AO leads to weaker trade winds. [22] When the trade winds are weaker, more extensive areas of rain fall upon landmasses within the tropics, such as Central America. [23]

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 (Florida and Gulf Coast). When dust from the Sahara moving around the southern periphery of the ridge travels 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. [1] Although the Southeast USA has some of the cleanest air in North America, much 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]

Every year, millions of tons of nutrient-rich Saharan dust cross the Atlantic Ocean, bringing vital phosphorus and other fertilizers to depleted Amazon soils. [27]

See also

Related Research Articles

Cyclone Large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

Monsoon Seasonal changes in atmospheric circulation and precipitation associated with the Intertropical Convergence Zone moving

A monsoon is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains.

Horse latitudes Latitudes 30–35 degrees north and south of the Equator

The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure area at the divergence of trade winds and the westerlies.

Anticyclone Weather phenomenon

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

Air mass Volume of air defined by its temperature and water vapor content

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass' characteristics, as well as modification.

Subtropics Geographic and climate zone

The subtropical zones or subtropics are geographical and climate zones located to the north and south of the Tropics. Geographically part of the North and South temperate zones, they cover the latitudes from 23°26′11.0″ (or 23.4364°) to approximately 35° in the Northern and Southern hemispheres. The horse latitudes lie within this range.

Intertropical Convergence Zone Meteorological phenomenon

The Intertropical Convergence Zone, known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal equator though its specific position varies seasonally. When it lies near the geographic Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonial circulation, it is sometimes referred to as a monsoon trough, a usage that is more common in Australia and parts of Asia.

High-pressure area Region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment

A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment.

Low-pressure area Atmospheric condition

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Generally, A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

Prevailing winds Strongest direction of wind on a region of Earths surface

In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface. A region's prevailing and dominant winds are the result of global patterns of movement in the Earth's atmosphere. In general, winds are predominantly easterly at low latitudes globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important cause of the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.

Westerlies Prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude

The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner. Tropical cyclones which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

Tropical wave Type of atmospheric trough

A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equator-ward side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

Pressure system Relative peak or lull in the sea level pressure distribution

A pressure system is a relative peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.

Index of meteorology articles Wikipedia index

This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting.

Mesoscale convective system Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Convergence zone

A convergence zone in meteorology is a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions. This causes a mass accumulation that eventually leads to a vertical movement and to the formation of clouds and precipitation. Large-scale convergence, called synoptic-scale convergence, is associated with weather systems such as baroclinic troughs, low-pressure areas, and cyclones. The large-scale convergence zone formed over the equator, the Intertropical Convergence Zone, has condensed and intensified as a result of the global increase in temperature. Small-scale convergence will give phenomena from isolated cumulus clouds to large areas of thunderstorms.

Monsoon trough Weather phenomenon

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

South Atlantic High

South Atlantic High is a semipermanent pressure high centered at about 25°S, 15°W, in the Atlantic Ocean. It is also called the St. Helena High, Saint Helena island being the only land in the area. It can stretch thousands of miles across the South Atlantic. This does not mean that the position and the intensity of this anticyclone are permanent, but just that we find an anticyclone on the maps describing the average monthly pressure. This area of high pressure is part of the great subtropical belt of anticyclones called the subtropical ridge. The centre of the high pressure region tends to follow the seasonal variation in position of the sun, moving south in the southern summer and north in the southern winter. This affects the climate of the adjacent continental areas, bringing seasonal changes to the climate and weather as the position of the high oscillates.

Wind Natural movement of air or other gases relative to a planets surface

Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet. Within the tropics and subtropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can prevail.

Upper tropospheric cyclonic vortex

An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season. Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low. A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level. In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex. Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.

References

  1. 1 2 Science Daily (1999-07-14). African Dust Called A Major Factor Affecting Southeast U.S. Air Quality. Retrieved on 2007-06-10.
  2. Carol G. Braham; Enid Pearsons; Deborah M. Posner; Georgia S. Maas & Richard Goodman (2001). Random House Webster's College Dictionary (second ed.). Random House. p.  1385. ISBN   978-0-375-42560-8.
  3. Hermann R. Muelder (2007). Years of This Land - A Geographical History of the United States. Read Books. p. 38. ISBN   978-1-4067-7740-6.
  4. 1 2 Derek Hayes (2001). Historical atlas of the North Pacific Ocean: maps of discovery and scientific exploration, 1500–2000. Douglas & McIntyre. p. 18. ISBN   978-1-55054-865-5.
  5. Cyrus Cornelius Adams (1904). A text-book of commercial geography. D. Appleton and company. p.  19.
  6. Oxford English Dictionary (2 ed.). p. 225.
  7. Derek Hayes (2001). Historical atlas of the North Pacific Ocean: maps of discovery and scientific exploration, 1500–2000. Douglas & McIntyre. p. 152. ISBN   978-1-55054-865-5.
  8. 1 2 Sverre Petterssen (1941). Introduction to Meteorology. Mcgraw-Hill Book Company, Inc. p. 110. ISBN   978-1-4437-2300-8.
  9. Glossary of Meteorology (June 2000). "Doldrums". American Meteorological Society. Archived from the original on 2009-09-25. Retrieved 2009-11-09.
  10. Glossary of Meteorology (June 2000). "Intertropical Convergence Zone". American Meteorological Society. Archived from the original on 2009-06-02. Retrieved 2009-11-09.
  11. Glossary of Meteorology (June 2000). "Monsoon Trough". American Meteorological Society. Archived from the original on 2009-06-17. Retrieved 2009-11-09.
  12. Glossary of Meteorology (June 2000). "Superior air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  13. Glossary of Meteorology (2009). "trade winds". Glossary of Meteorology. American Meteorological Society. Archived from the original on 2008-12-11. Retrieved 2008-09-08.
  14. Ralph Stockman Tarr; Frank Morton McMurry; Almon Ernest Parkins (1909). Advanced geography. State Printing. p.  246.
  15. JetStream (2008). "How to read weather maps". National Weather Service. Archived from the original on 2012-07-05. Retrieved 2007-05-16.
  16. Glossary of Meteorology (June 2000). "Tropical air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  17. Glossary of Meteorology (June 2000). "Trade air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  18. John E. Oliver (2005). Encyclopedia of world climatology. Springer. p. 128. ISBN   978-1-4020-3264-6.
  19. Bob Rauber (2009-05-22). "Research-The Rain in Cumulus over the Ocean Campaign" . Retrieved 2009-11-08.
  20. James P. Terry (2007). Tropical cyclones: climatology and impacts in the South Pacific. Springer. p. 8. ISBN   978-0-387-71542-1.
  21. G. E. Pieter & F. Augustinus (2004). "The influence of the trade winds on the coastal development of the Guianas at various scale levels: a synthesis". Marine Geology. 208 (2–4): 145–151. Bibcode:2004MGeol.208..145A. doi:10.1016/j.margeo.2004.04.007. hdl: 1874/12170 .
  22. Robert R. Steward (2005). "The Ocean's Influence on North American Drought". Texas A&M University.
  23. John E. Oliver (2005). Encyclopedia of world climatology. Springer. p. 185. ISBN   978-1-4020-3264-6.
  24. Science Daily (2001-06-15). Microbes And The Dust They Ride In On Pose Potential Health Risks. Retrieved on 2007-06-10.
  25. Usinfo.state.gov (2003). Study Says African Dust Affects Climate in U.S., Caribbean. Archived 2007-06-20 at the Wayback Machine Retrieved on 2007-06-10.
  26. U. S. Geological Survey (2006). Coral Mortality and African Dust. Retrieved on 2007-06-10.
  27. Yu, Hongbin; Chin, Mian; Yuan, Tianle; Bian, Huisheng; Remer, Lorraine A.; Prospero, Joseph M.; Omar, Ali; Winker, David; Yang, Yuekui; Zhang, Yan; Zhang, Zhibo; Zhao, Chun (2015). "The fertilizing role of African dust in the Amazon rainforest: A first multiyear assessment based on data from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations". Geophysical Research Letters. 42 (6): 1984–1991. doi: 10.1002/2015GL063040 .