Westerlies

The Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east,^[1] 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. They are strongest in the winter hemisphere and when the pressure is lower over the poles, and weakest in the summer hemisphere and when pressures are higher over the poles.

Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific oceans, as the Westerlies lead to the development of strong ocean currents in both hemispheres. The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes can come in the Roaring Forties, between 40 and 50 degrees 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.

Behavior

Winds across the northern hemisphere tend to blow from the southwest, while they tend to be from the northwest in the Southern Hemisphere.^[2] When pressures are lower over the poles, the strength of the Westerlies increases, which warms the mid-latitudes. This occurs when the Arctic oscillation is positive. When it is negative and pressures are higher over the poles, the flow is more meridional which brings cold air into the mid-latitudes.^[3] 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.^[4] Dust plumes originating in the Gobi desert combine with pollutants and spread large distances downwind, or eastward, into North America due to the prevailing westerly winds in the mid-latitudes.^[5]

The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds 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 and Shrieking Sixties according to the varying degrees of latitude.^[6]

Impact on ocean currents

Due to persistent winds from west to east on the poleward sides of the subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres, with the currents in the Northern Hemisphere weaker than its southern counterpart due to the strength differences between the winds in the Westerlies of both hemispheres.^[7] The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary of an ocean.^[8] These western ocean currents transport warm, tropical water polewards toward the polar regions. Ships crossing both oceans have taken advantage of the ocean currents for centuries.

Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) is an ocean current that flows from west to east around Antarctica. An alternate name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current.^[9] It keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet. The ACC has been known to sailors for many years; circumstances preceding the Mutiny on the Bounty and Jack London's story "Make Westing" poignantly illustrated the difficulty it caused for mariners seeking to round Cape Horn on the clipper ship route between New York and California.

Gulf Stream

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. At about 30°W, 40°N, it splits in two, with the northern stream crossing to northern Europe and the southern stream recirculating off West Africa.^[10] The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe.^[11]^[12]^[13] ^[14] Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be; and that this is due to the North Atlantic drift, one of the branches from the tail of the Gulf Stream.^[15] ^[16]^[17] It is part of the North Atlantic Subtropical Gyre. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean.^[18]^[19]^[20]^[21] The Gulf Stream is also a significant potential source of renewable power generation.^[22]

Kuroshio

The Kuroshio (Japanese for "Black Tide") is a strong western boundary current in the western north Pacific Ocean. It begins off the east coast of Taiwan and flows northeastward past Japan, where it merges with the easterly drift of the North Pacific Current. The current is also sometimes known as the Black Stream—the English translation of kuroshio, and an allusion to the deep blue of its water. The current is also known as the Japan Current. The path of Kuroshio south of Japan is reported every day.^[23] The warm waters of the Kuroshio Current sustain the coral reefs of Japan, the northernmost coral reefs in the world. The branch into the Sea of Japan is called Tsushima Current. The Japan Current is also responsible for the mild weather experienced around Alaska's southern coast. It has given its name to a poem written by sailor and poet Nikos Kavvadias (Kuro Siwo).

Extratropical cyclones

A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries.

An extratropical cyclone is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[24]

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,^[24]^[25] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with anti-cyclones, 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 with radius.^[26] 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.^[27]

Interaction with tropical cyclones

Storm track of Typhoon Ioke, showing recurvature off the Japanese coast in 2006

When a tropical cyclone crosses the subtropical ridge axis, its general track around the high-pressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun recurvature.^[28] A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon encounters southwesterly winds (blowing northeastward) around a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by extratropical cyclones in this manner, which move from west to east to the north of the subtropical ridge. An example of a tropical cyclone in recurvature was Typhoon Ioke in 2006, which took a similar trajectory.^[29]

References

^ Glossary of Meteorology (2009). Westerlies. American Meteorological Society. Retrieved on 2009-04-15.
^ Ralph Stockman Tarr and Frank Morton McMurry (1909).Advanced geography. W.W. Shannon, State Printing, pp. 246. Retrieved on 2009-04-15.
^ National Snow and Ice Data Center (2009). The Arctic Oscillation. Arctic Climatology and Meteorology. Retrieved on 2009-04-11.
^ Halldór Björnsson (2005). Global circulation. Veðurstofu Íslands. Retrieved on 2008-06-15.
^ News Staff (2007-04-18).Scientists track migration of Asian dust and pollution. Scientific Blogging.com. Retrieved on 2009-03-19.
^ Walker, Stuart (1998). The sailor's wind. W. W. Norton & Company. p. 91. ISBN 0393045552, 9780393045550. {{cite book}}: Check |isbn= value: invalid character (help)
^ Wunsch, Carl (November 8, 2002). "What Is the Thermohaline Circulation?". Science. 298 (5596): 1179–1181. doi:10.1126/science.1079329. PMID 12424356. (see also Rahmstorf.)
^ National Environmental Satellite, Data, and Information Service (2009). Investigating the Gulf Stream. North Carolina State University. Retrieved on 2009-05-06.
^ 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.
^ Joanna Gyory, Arthur J. Mariano, Edward H. Ryan (2005). "The Gulf Stream". Cooperative Institute for Marine and Atmospheric Studies. Retrieved 2009-01-06.{{cite web}}: CS1 maint: multiple names: authors list (link)
^ Geoff Samuels (2008). "Caribbean Mean SSTs and Winds". Cooperative Institute For Marine and Atmospheric Studies. Retrieved 2009-01-16.
^ National Climatic Data Center (1998). Climatic Wind Data for the United States via the Internet Wayback Machine. NOAA. Retrieved on 2007-06-02.
^ Dr. Sarah Oktay (2009). "Description of Nantucket Island". University of Massachusetts. Retrieved 2009-01-06.
^ Professor Hennessy (1858). "Report of the Annual Meeting: On the Influence of the Gulf-stream on the Climate of Ireland". Richard Taylor and William Francis. Retrieved 2009-01-06.
^ NASA (2004). "Satellites Record Weakening North Atlantic Current Impact". NASA. Retrieved 2008-09-10.
^ Barbie Bischof, Arthur J. Mariano, Edward H. Ryan (2003). "The North Atlantic Drift Current". The National Oceanographic Partnership Program. Retrieved 2008-09-10.{{cite web}}: CS1 maint: multiple names: authors list (link)
^ Erik A. Rasmussen, John Turner (2003). Polar Lows. Cambridge University Press. p. 68. {{cite book}}: |access-date= requires |url= (help)
^ National Hurricane Center (2009).Atlantic Hurricane Database. Retrieved on 2009-04-14.
^ S. Businger, T. M. Graziano, M. L. Kaplan, and R. A. Rozumalski (2004). Cold-air cyclogenesis along the Gulf-Stream front: investigation of diabatic impacts on cyclone development, frontal structure, and track. Meteorology and Atmospheric Physics, pp. 65-90. Retrieved on 2008-09-21.
^ David M. Roth (2000). P 1.43 A FIFTY YEAR HISTORY OF SUBTROPICAL CYCLONES. American Meteorological Society. Retrieved on 2008-09-21.
^ D. K. Savidge and J. M. Bane (1999). Cyclogenesis in the deep ocean beneath the Gulf Stream. 1. Description. Journal of geophysical research, pp. 18111-18126. Retrieved on 2008-09-21.
^ Jeremy Elton Jacquot (2007). Gulf Stream's Tidal Energy Could Provide Up to a Third of Florida's Power. Treehugger.com. Retrieved on 2008-09-21.
^ Japan Coast Guard (2006). "Quick Bulletin of Ocean Conditions". Hydrographic and Oceanographic Department. Retrieved 2009-04-14.
^ ^a ^b Dr. DeCaria (2005-12-07). "ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones". Department of Earth Sciences, Millersville University, Millersville, Pennsylvania. Retrieved 2006-10-21. {{cite web}}: External link in |publisher= (help) Cite error: The named reference "ExtraLessonMillUni" was defined multiple times with different content (see the help page).
^ Robert Hart and Jenni Evans (2003). "Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space" (PDF). American Meteorological Society. Retrieved 2006-10-03. {{cite web}}: External link in |publisher= (help)
^ Ryan N. Maue (2009). CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS. Florida State University. Retrieved on 2008-06-15.
^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2004). "Frequently Asked Questions: What is an extra-tropical cyclone?". NOAA. Retrieved 2006-07-25.
^ Joint Typhoon Warning Center (2009). Section 2: Tropical Cyclone Motion Terminology. United States Navy. Retrieved on 2007-04-10.
^ Powell, Jeff; et al. (2007). "Hurricane Ioke: 20-27 August 2006". 2006 Tropical Cyclones Central North Pacific. Central Pacific Hurricane Center. Retrieved 2007-06-09. {{cite web}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)