Roaring Forties

The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S.[1] The strong west-to-east air currents are caused by the combination of air being displaced from the Equator towards the South Pole, the Earth's rotation, and the scarcity of landmasses to serve as windbreaks at those latitudes.

For the electric company, see Roaring 40s.
Not to be confused with Roaring Twenties.

The Roaring Forties in the Cook Strait of New Zealand produce high waves, and they erode the shore as shown in this image.

The Roaring Forties were a major aid to ships sailing the Brouwer Route from Europe to the East Indies or Australasia during the Age of Sail, and in modern times are favoured by yachtsmen on round-the-world voyages and competitions. The boundaries of the Roaring Forties are not consistent: The wind-stream shifts north or south depending on the season.

Similar but even stronger conditions that occur at more southerly latitudes are called the Furious Fifties and the Shrieking or Screaming Sixties.

Dynamics
See also: Prevailing winds and Atmospheric circulation

Hot air rises at the Equator and is pushed towards the poles by cooler air travelling towards the Equator (an atmospheric circulation feature known as the Hadley Cell).[1] At about 30°S, the outward-travelling air sinks to lower altitudes, and continues toward the poles closer to the ground (the Ferrel Cell), then rises up again from about 60°S as the air joins the Polar vortex.[1] This travel in the 30°–60°S zone combines with the rotation of the earth to move the air currents from west to east, creating westerly winds.[1]

Unlike in the northern hemisphere, the large tracts of open ocean south of 40°S are interrupted only by Tasmania, New Zealand, and the southern part of South America. These relatively small obstructions, which are themselves bordered by large tracts of open water along their southern shores, allow high wind speeds to develop – much higher than near 40°N, where two large continents (Eurasia and North America) impede the flow of circum-planetary westerly winds.[1] Similar but stronger wind conditions prevalent closer to the South Pole are referred to as the "Furious Fifties" (50°S to 60°S), and the "Shrieking" or "Screaming Sixties" (below 60°S).[2]

The latitude ranges for the Roaring Forties and similar winds are not consistent: All shift towards the South Pole in the southern summer, and towards the Equator in the southern winter.[1]

Use for sailing
The Clipper Route, taken by ships sailing from Europe to Australia in order to take advantage of the Roaring Forties

During the Age of Sail, ships travelling from Europe to the East Indies or Australasia would sail down the west coast of Africa and round the Cape of Good Hope to use the Roaring Forties to speed their passage across the Indian Ocean,[3] then on the return leg, continue eastwards across the Pacific Ocean and south of Cape Horn before sailing up the east coast of the Americas to home. It was first used by Dutch explorer Hendrik Brouwer in his Brouwer Route, discovered in 1611, which effectively halved the duration of the trip from Europe to Java. "To run the easting down" was the phrase used to describe the fast passages achieved in the Roaring Forties.[3] The story Easting Down by Shalimar describes the events that befall a steamship unwisely venturing into these latitudes to achieve a faster passage.[4]

Modern round-the-world sailors also take advantage of the Roaring Forties to speed travel times, in particular those involved in record attempts or races.[3]

Wind power

The strong and continuous winds in the Roaring Forties make this zone highly prospective for wind power such as in New Zealand and Tasmania.[5]

Impact of pollution

The peak band of winds has moved approximately 2.5 degrees south in the late 20th century, from a combination of human-induced ozone depletion and greenhouse gas emissions. This has caused faster warming in Antarctica and less rainfall in Australia, especially Western Australia.[6]

See also

References
  1. Catchpole, Heather (20 September 2007). "Roaring forties". ABC Science. In Depth. Australian Broadcasting Corporation. Retrieved 7 April 2011.
  2. "Exploring the Southern Ocean". Eco-Photo Explorers. 21 December 2009. Retrieved 7 April 2011.
  3. Dear, I.C.B.; Kemp, Peter, eds. (2007). "Roaring Forties". The Oxford Companion to Ships and the Sea. Oxford Reference Online. Oxford University Press. ISBN 978-0-19-860616-1. OCLC 60793921. Retrieved 14 April 2011.
  4. Tanner, Tony, ed. (2002) [1994]. The Oxford book of sea stories. Oxford University Press. ISBN 0192803700. OCLC 1200552340.
  5. Wind turbine
  6. Helen Davidson (11 May 2014). "Roaring Forties' shift south means more droughts for southern Australia". TheGuardian.com.
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