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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 (about 30 degrees) and trend towards the poles and steer extratropical cyclones in this general manner. which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow. The are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
The westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles. The westerlies are particularly strong, especially in the Southern Hemisphere (called also 'Brave West winds' at striking Chile, Argentina, Tasmania and New Zealand), in areas where land is absent, because land amplifies the flow pattern, making the current more north–south oriented, slowing the westerlies. The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40 and 50 degrees south latitude. The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.
(1909). [https://books.google.com/books?id=OLMXAAAAIAAJ&dq=direction+of+the+prevailing+westerlies+in+northern+hemisphere+southwest&pg=PA246 Advanced geography.] W. W. Shannon, State Printing, pp. 246. Retrieved on 2009-04-15.When pressures are lower over the poles, the strength of the westerlies increases, which has the effect of warming the mid-latitudes. This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer. When it is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the [[Equator]], which brings cold air into the mid-latitudes.National Snow and Ice Data Center (2009). [http://nsidc.org/arcticmet/patterns/arctic_oscillation.html The Arctic Oscillation.] Arctic Climatology and Meteorology. Retrieved on 2009-04-11.
Throughout the year, the westerlies vary in strength with the polar cyclone. As the cyclone reaches its maximum intensity in winter, the westerlies increase in strength. As the cyclone reaches its weakest intensity in summer, the Westerlies weaken.Halldór Björnsson (2005). Global circulation. Veðurstofu Íslands. Retrieved on 2008-06-15. An example of the impact of the westerlies is when dust plumes, originating in the Gobi Desert combine with pollutants and spread large distances downwind, or eastward, into North America. The westerlies can be particularly strong, especially in the Southern Hemisphere, where there is less land in the middle to cause the progression of west to east winds to slow down. In the Southern hemisphere, because of the stormy and cloudy conditions, it is usual to refer to the westerlies as the roaring forties, furious fifties, or shrieking sixties according to the varying degrees of latitude.
The Antarctic Circumpolar Current (ACC), or the West Wind Drift, is an ocean current that flows from west to east around Antarctica. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 , the largest ocean current.Ryan Smith, Melicie Desflots, Sean White, Arthur J. Mariano, Edward H. Ryan (2005). The Antarctic CP Current. The Cooperative Institute for Marine and Atmospheric Studies. Retrieved on 2009-04-11. In the northern hemisphere, the Gulf Stream, part of the North Atlantic Subtropical Gyre, has led to the development of strong of all types at the base of the Westerlies, both within the atmosphere and within the ocean.David M. Roth (2000). A FIFTY YEAR HISTORY OF SUBTROPICAL CYCLONES. American Meteorological Society. Retrieved on 2008-09-21. The Kuroshio (Japanese for "Black Tide") is a strong western boundary current in the western north Pacific Ocean, similar to the Gulf Stream, which has also contributed to the depth of ocean storms in that region.
The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet, where the Westerlies steer the system generally from west to east. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where extratropical transition has occurred, and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with , drive the weather over much of the Earth.
Although extratropical cyclones are almost always classified as baroclinic since they form along zones of temperature and dewpoint gradient, they can sometimes become barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform along the radius from the center of low pressure.Ryan N. Maue (2009). CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS. Florida State University. Retrieved on 2008-06-15. An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core and causes temperature and dewpoint gradients near their centers to fade.
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