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69-421: 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. Tropical cyclones which cross the subtropical ridge axis into

138-530: A rain shadow effect which limits further penetration of these systems and associated rainfall eastward. This trend reverses in the summer when strong heating of the land causes high pressure and tends to block moisture-rich air from the Pacific from reaching land. This explains why most of coastal Western North America in the highest latitude experiences dry summers, despite vast rainfall in the winter. The polar easterlies (also known as Polar Hadley cells) are

207-417: A greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the land rises, the land heats the air above it. The warm air is less dense and so it rises. This rising air over the land lowers the sea level pressure by about 0.2%. The cooler air above the sea, now with higher sea level pressure, flows towards

276-443: A particular location. Presented in a polar coordinate grid, the wind rose shows the frequency of winds blowing from particular directions. The length of each spoke around the circle is related to the proportion of the time that the wind blows from each direction. Each concentric circle represents a different proportion, increasing outwards from zero at the center. A wind rose plot may contain additional information, in that each spoke

345-678: A principal pathway of exchange among them. The current is strongly constrained by landform and bathymetric features. To trace it starting arbitrarily at South America, it flows through the Drake Passage between South America and the Antarctic Peninsula and then is split by the Scotia Arc to the east, with a shallow warm branch flowing to the north in the Falkland Current and a deeper branch passing through

414-486: A similar trajectory. Prevailing winds Wind roses are tools used to display the direction of the prevailing wind. Knowledge of the prevailing wind allows the development of prevention strategies for wind erosion of agricultural land, such as across the Great Plains . Sand dunes can orient themselves perpendicular to the prevailing wind direction in coastal and desert locations. Insects drift along with

483-503: A wealth of other species . The ACC has been known to sailors for centuries; it greatly speeds up any travel from west to east, but makes sailing extremely difficult from east to west, although this is mostly due to the prevailing westerly winds . Jack London 's story "Make Westing" and the circumstances preceding the mutiny on the Bounty poignantly illustrate the difficulty it caused for mariners seeking to round Cape Horn westbound on

552-482: Is allowed by salinity dominating density stratification, due to the lower temperatures. Farther south still is the SACC, which is determined as the southernmost extent of Circumpolar deep water (temperature of about 2 °C at 400 m). This water mass flows along the shelfbreak of the western Antarctic Peninsula and thus marks the most southerly water flowing through Drake Passage and therefore circumpolar. The bulk of

621-653: Is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica . An alternative name for the ACC is the West Wind Drift . The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups (Sv, million m /s), or possibly even higher, making it the largest ocean current. The current is circumpolar due to

690-499: Is broken down into color-coded bands that show wind speed ranges. Wind roses typically show 8 or 16 cardinal directions , such as north (N), NNE, NE, etc., although they may be subdivided into as many as 32 directions . The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator , equatorward of the subtropical ridge . These winds blow predominantly from

759-545: Is driven by the strong westerly winds in the latitudes of the Southern Ocean. In latitudes where there are continents, winds blowing on light surface water can simply pile up light water against these continents. But in the Southern Ocean, the momentum imparted to the surface waters cannot be offset in this way. There are different theories on how the Circumpolar Current balances the momentum imparted by

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828-527: Is removed by orographic lift, leaving drier air (see foehn wind ) on the descending and generally warming, leeward side where a rain shadow is observed. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming

897-590: Is reported to have opened to water circulation 33.5 million years ago (Ma). The timing of the opening of the Drake Passage, between South America and the Antarctic Peninsula, is more disputed; tectonic and sediment evidence show that it could have been open as early as pre-34 Ma, estimates of the opening of the Drake passage are between 20 and 40 Ma. The isolation of Antarctica by

966-521: 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. Throughout the year, the westerlies vary in strength with the polar cyclone . As

1035-564: The Great Basin and Mojave Deserts . Insects are swept along by the prevailing winds, while birds follow their own course. As such, fine line patterns within weather radar imagery, associated with converging winds, are dominated by insect returns. In the Great Plains , wind erosion of agricultural land is a significant problem, and is mainly driven by the prevailing wind. Because of this, wind barrier strips have been developed to minimize this type of erosion. The strips can be in

1104-685: The Macquarie Ridge south of New Zealand. The ACC varies with time. Evidence of this is the Antarctic Circumpolar Wave , a periodic oscillation that affects the climate of much of the southern hemisphere. There is also the Antarctic oscillation , which involves changes in the location and strength of Antarctic winds. Trends in the Antarctic Oscillation have been hypothesized to account for an increase in

1173-617: The Southern Ocean and, at approximately 125 Sverdrups , the largest ocean current. In the northern hemisphere, the Gulf Stream , part of the North Atlantic Subtropical Gyre , has led to the development of strong cyclones of all types at the base of the Westerlies, both within the atmosphere and within the ocean . The Kuroshio ( Japanese for "Black Tide") is a strong western boundary current in

1242-516: The clipper ship route from New York to California. The eastbound clipper route , which is the fastest sailing route around the world, follows the ACC around three continental capes – Cape Agulhas (Africa), South East Cape (Australia), and Cape Horn (South America). The current creates the Ross and Weddell Gyres . The ACC connects the Atlantic , Pacific , and Indian Oceans, and serves as

1311-519: The high-pressure area caused by a system traversing the Westerlies, 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, entering the Westerlies. A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to

1380-409: The windward side of mountains. It is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation . In mountainous parts of the world subjected to consistent winds (for example, the trade winds ), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture

1449-459: The ACC transport is carried in this front, which is defined as the latitude at which a subsurface salinity minimum or a thick layer of unstratified Subantarctic mode water first appears, allowed by temperature dominating density stratification. Still further south lies the PF, which is marked by a transition to very cold, relatively fresh, Antarctic Surface Water at the surface. Here a temperature minimum

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1518-782: The Arc more to the east before also turning to the north. Passing through the Indian Ocean, the current first retroflects the Agulhas Current to form the Agulhas Return Current before it is split by the Kerguelen Plateau , and then moving northward again. Deflection is also seen as it passes over the mid-ocean ridge in the Southeast Pacific. The current is accompanied by three fronts :

1587-415: The Circumpolar Current may directly transport momentum downward in the water column. This is because such flows can produce a net southward flow in the troughs and a net northward flow over the ridges without requiring any transformation of density. In practice both the thermohaline and the eddy/meander mechanisms are likely to be important. The current flows at a rate of about 4 km/h (2.5 mph) over

1656-514: 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

1725-470: 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. Due to persistent winds from west to east on the poleward sides of

1794-460: The Southern Hemisphere. 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. The westerlies explain why coastal Western North America tends to be wet, especially from Northern Washington to Alaska, during the winter. Differential heating from the Sun between

1863-477: The Southern Ocean, transformation of these waters into light surface waters, and a transformation of waters in the opposite direction to the north. Such theories link the magnitude of the Circumpolar Current with the global thermohaline circulation , particularly the properties of the North Atlantic. Alternatively, ocean eddies , the oceanic equivalent of atmospheric storms, or the large-scale meanders of

1932-636: The Subantarctic front (SAF), the Polar front (PF), and the Southern ACC front (SACC). Furthermore, the waters of the Southern Ocean are separated from the warmer and saltier subtropical waters by the subtropical front (STF). The northern boundary of the ACC is defined by the northern edge of the SAF, this being the most northerly water to pass through Drake Passage and therefore be circumpolar. Much of

2001-615: The airflow is more severe. Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors . Strong updrafts , downdrafts and eddies develop as the air flows over hills and down valleys. Wind direction changes due to the contour of the land. If there is a pass in the mountain range, winds will rush through the pass with considerable speed due to the Bernoulli principle that describes an inverse relationship between speed and pressure. The airflow can remain turbulent and erratic for some distance downwind into

2070-459: The amount of sea ice is lowest, and in August–September the sea ice is at its greatest extent. Ice levels have been monitored by satellite since 1973. Upwelling of deep water under the sea ice brings substantial amounts of nutrients. As the ice melts, the melt water provides stability and the critical depth is well below the mixing depth, which allows for a positive net primary production . As

2139-418: The continent. Diatom production continues through the summer, and populations of krill are sustained, bringing large numbers of cetaceans , cephalopods , seals, birds, and fish to the area. Phytoplankton blooms are believed to be limited by irradiance in the austral (southern hemisphere) spring, and by biologically available iron in the summer. Much of the biology in the area occurs along the major fronts of

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2208-572: The current is credited by many researchers with causing the glaciation of Antarctica and global cooling in the Eocene epoch. Oceanic models have shown that the opening of these two passages limited polar heat convergence and caused a cooling of sea surface temperatures by several degrees; other models have shown that CO 2 levels also played a significant role in the glaciation of Antarctica. Antarctic sea ice cycles seasonally, in February–March

2277-524: The current, the Subtropical, Subantarctic, and the Antarctic Polar fronts, these are areas associated with well defined temperature changes. Size and distribution of phytoplankton are also related to fronts. Microphytoplankton (>20 μm) are found at fronts and at sea ice boundaries, while nanophytoplankton (<20 μm) are found between fronts. Studies of phytoplankton stocks in

2346-484: 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. 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

2415-430: The daytime sea breeze to dissipate. If the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it. Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, creating an associated thermal low over

2484-455: The dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards the low-pressure areas within the westerlies at high latitudes. Like trade winds and unlike the westerlies, these prevailing winds blow from the east to the west , and are often weak and irregular. Due to the low sun angle, cold air builds up and subsides at

2553-472: The eastern boundary of an ocean. 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. 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

2622-430: 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

2691-408: The flatter countryside. These conditions are dangerous to ascending and descending airplanes. Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow, similar to the relationship between sea breeze and land breeze. At night, the sides of the hills cool through radiation of the heat. The air along the hills becomes cooler and denser, blowing down into

2760-490: The form of soil ridges, crop strips, crops rows, or trees which act as wind breaks. They are oriented perpendicular to the wind in order to be most effective. In regions with minimal vegetation, such as coastal and desert areas, transverse sand dunes orient themselves perpendicular to the prevailing wind direction, while longitudinal dunes orient themselves parallel to the prevailing winds. Antarctic Circumpolar Current Antarctic Circumpolar Current ( ACC )

2829-414: The front. Some research has been conducted on Antarctic phytoplankton as a carbon sink . Areas of open water left from ice melt are good areas for phytoplankton blooms. The phytoplankton takes carbon from the atmosphere during photosynthesis. As the blooms die and sink, the carbon can be stored in sediments for thousands of years. This natural carbon sink is estimated to remove 3.5 million tonnes from

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2898-419: The general public. These are the everyday phenomena which along with anticyclones , 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

2967-635: The global extra-tropical circulation at ≈ 10^4 kilometers". The current helps preserve wooden shipwrecks by preventing wood-boring " ship worms " from reaching targets such as Ernest Shackleton 's ship, the Endurance . The "State of the cryosphere" report found, that the Antarctic Circumpolar Current became weaker. By 2050 it expected to lose 20% of its strenght with "widespread impacts on ocean circulation and climate." The Weddell Sea Bottom Water has lost 30% of its volume in

3036-605: The lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet . Associated with the Circumpolar Current is the Antarctic Convergence , where the cold Antarctic waters meet the warmer waters of the subantarctic , creating a zone of upwelling nutrients. These nurture high levels of phytoplankton with associated copepods and krill , and resultant food chains supporting fish, whales, seals , penguins, albatrosses , and

3105-417: The land into the lower pressure, creating a cooler breeze near the coast. The strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. If an off-shore wind of 8 knots (15 km/h) exists, the sea breeze is not likely to develop. At night, the land cools off more quickly than the ocean due to differences in their specific heat values, which forces

3174-518: The land which is quite cool and the ocean which is relatively warm causes areas of low pressure to develop over land. This results in moisture-rich air flowing east from the Pacific Ocean, causing frequent rainstorms and wind on the coast. This moisture continues to flow eastward until orographic lift caused by the Coast Ranges, and the Cascade, Sierra Nevada, Columbia, and Rocky Mountains causes

3243-491: The latest 32 years, and the Antarctic Bottom Water is expected to shrunk. This will impact ocean circulation, nutrients, heat content and carbon sequestration. UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for

3312-436: 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

3381-690: The northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere . The trade winds act as the steering flow for tropical cyclones that form over world's oceans, guiding their path westward. Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean Sea , as well as portions of southeast North America. The westerlies or the prevailing westerlies are

3450-476: The northwest in the Southern Hemisphere. They are strongest in the winter when the pressure is lower over the poles, such as when the polar cyclone is strongest, and weakest during the summer when the polar cyclone is weakest 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

3519-464: The ocean each year. 3.5 million tonnes of carbon taken from the ocean and atmosphere is equivalent to 12.8 million tonnes of carbon dioxide. An expedition in May 2008 by 19 scientists studied the geology and biology of eight Macquarie Ridge sea mounts, as well as the Antarctic Circumpolar Current to investigate the effects of climate change of the Southern Ocean. The circumpolar current merges

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3588-461: The pole creating surface high-pressure areas, forcing an outflow of air toward the equator ; that outflow is deflected westward by the Coriolis effect . In areas where the wind flow is light, sea breezes and land breezes are important factors in a location's prevailing winds. The sea is warmed by the sun to a greater depth than the land due to its greater specific heat . The sea therefore has

3657-464: The prevailing wind, but the flight of birds is less dependent on it. Prevailing winds in mountain locations can lead to significant rainfall gradients, ranging from wet across windward-facing slopes to desert-like conditions along their lee slopes. Prevailing winds can vary due to the uneven heating of the Earth. A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at

3726-482: The prevailing winds in the middle latitudes (i.e. between 35 and 65 degrees latitude ), which blow in areas poleward of the high pressure area known as the subtropical ridge in the horse latitudes . These prevailing winds blow from the west to the east , and steer extra-tropical cyclones in this general direction. The winds are predominantly from the southwest in the Northern Hemisphere and from

3795-399: The radius from the center of low pressure. 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. When a tropical cyclone crosses the subtropical ridge axis, normally through a break in

3864-406: The right) in the Northern Hemisphere and eastward from south (to the left) in the Southern Hemisphere. This is why winds across the Northern Hemisphere tend to blow from the southwest, but they tend to be from the northwest in the Southern Hemisphere. 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

3933-400: The sea ice recedes epontic algae dominate the first phase of the bloom, and a strong bloom dominate by diatoms follows the ice melt south. Another phytoplankton bloom occurs more to the north near the Antarctic Convergence , here nutrients are present from thermohaline circulation . Phytoplankton blooms are dominated by diatoms and grazed by copepods in the open ocean, and by krill closer to

4002-595: The southern sea have shown that the Antarctic Circumpolar Current is dominated by diatoms, while the Weddell Sea has abundant coccolithophorids and silicoflagellates. Surveys of the SW Indian Ocean have shown phytoplankton group variation based on their location relative to the Polar Front, with diatoms dominating South of the front, and dinoflagellates and flagellates in higher populations North of

4071-430: The subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the westerlies of each hemisphere. The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on

4140-405: The terrain and enhancing any lows which would have otherwise existed, and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind can change direction and accelerate parallel to the wind obstruction. This barrier jet can increase the low level wind by 45%. In mountainous areas, local distortion of

4209-434: The transport is carried in the middle two fronts. The total transport of the ACC at Drake Passage is estimated to be around 135 Sv, or about 135 times the transport of all the world's rivers combined. There is a relatively small addition of flow in the Indian Ocean, with the transport south of Tasmania reaching around 147 Sv, at which point the current is probably the largest on the planet. The circumpolar current

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4278-494: The transport of the Circumpolar Current over the past two decades. Published estimates of the onset of the Antarctic Circumpolar Current vary, but it is commonly considered to have started at the Eocene / Oligocene boundary. The isolation of Antarctica and formation of the ACC occurred with the openings of the Tasmanian Passage and the Drake Passage . The Tasmanian Seaway separates East Antarctica and Australia, and

4347-484: The valley, drawn by gravity. This is known a mountain breeze. If the slopes are covered with ice and snow, the mountain breeze will blow during the day, carrying the cold dense air into the warmer, barren valleys. The slopes of hills not covered by snow will be warmed during the day. The air that comes in contact with the warmed slopes becomes warmer and less dense and flows uphill. This is known as an anabatic wind or valley breeze. Orographic precipitation occurs on

4416-493: The warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse. If the Earth were tidally locked to the Sun, solar heating would cause winds across the mid-latitudes to blow in a poleward direction, away from the subtropical ridge. However, the Coriolis effect caused by the rotation of Earth tends to deflect poleward winds eastward from north (to

4485-469: The waters of the Atlantic, Indian, and Pacific Oceans and carries up to 150 times the volume of water flowing in all of the world's rivers. The study found that any damage on the cold-water corals nourished by the current will have a long-lasting effect. After studying the circumpolar current it is clear that it strongly influences regional and global climate as well as underwater biodiversity. The subject has been characterized recently as "the spectral peak of

4554-485: 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 are called the Roaring Forties , between 40 and 50 degrees south latitude, within

4623-447: 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 . 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

4692-500: The western north Pacific Ocean , similar to the Gulf Stream, which has also contributed to the depth of ocean storms in that region. 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". The descriptor "extratropical" refers to

4761-450: The winds. The increasing eastward momentum imparted by the winds causes water parcels to drift outward from the axis of the Earth's rotation (in other words, northward) as a result of the Coriolis force . This northward Ekman transport is balanced by a southward, pressure-driven flow below the depths of the major ridge systems. Some theories connect these flows directly, implying that there is significant upwelling of dense deep waters within

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