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African easterly jet

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The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is at a maximum and the wind is easterly. The temperature contrast between the Sahara Desert and the Gulf of Guinea causes the jet to form to the north of the monsoon trough . The jet's maximum wind speeds are at a height of 3 kilometres (1.9 mi). The jet moves northward from its south-most location in January, reaching its most northerly latitude in August. Its strongest winds are in September while it begins shifting back towards the equator. Within the easterly jet, tropical waves form. Convective complexes associated with these waves can form tropical cyclones . If the jet is south of its normal location during August and September, tropical cyclogenesis is suppressed. If desertification continues across Sub-Saharan Africa , the strength of the jet could increase, although tropical wave generation probably would decrease, which would decrease the number of tropical cyclones in the Atlantic basin.

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47-630: During January, the African easterly jet lies at 3,000 metres (9,800 ft) above sea level at five degrees north latitude. Winds within it increase in speed from 30 km/h (19 mph) in January to 40 km/h (25 mph) in March. Shifting northward in April to the seventh parallel, winds within the jet increase to 45 km/h (28 mph). By June, it shifts northward into northwest Africa. From June into October,

94-409: A defined barometric pressure . Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an automatic terminal information service (ATIS). Since the terrain elevation is also referenced to MSL,

141-432: A few metres, in timeframes ranging from minutes to months: Between 1901 and 2018, the average sea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s. This was faster than the sea level had ever risen over at least the past 3,000 years. The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022. Climate change due to human activities

188-419: A higher temperature than the air over the ocean. The hot air over the land tends to rise, creating an area of low pressure . That creates a steady wind blowing toward the land, bringing the moist near-surface ocean air with it. Similar rainfall is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at

235-666: A pronounced thermal trough aligned with the Central Valley, and typically linked to the broader thermal low across the North American deserts. As a consequence, a strong pressure gradient is created which draws cool marine air landward. As temperatures plummet, fog and stratus stream in and through the gaps of the Coast Ranges, and especially through the Golden Gate at San Francisco ( see San Francisco fog ). The same thermal trough

282-438: A reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level ), and in the atmospheric sciences , and in land surveying . An alternative is to base height measurements on a reference ellipsoid approximating the entire Earth, which is what systems such as GPS do. In aviation, the reference ellipsoid known as WGS84

329-463: A sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century. Local factors like tidal range or land subsidence will greatly affect the severity of impacts. For instance, sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century. Yet, of the 20 countries with

376-403: A term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, much stronger, and seasonal. The sea is warmed by the sun to a greater depth than the land due to its greater specific heat . The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than

423-483: A weak cyclonic circulation. As they are strongest at the surface and warm near their center, and weaker aloft where the air is more stable, the thermal low is considered warm core. The strongest versions of these features globally are over Arabia, the northern portion of the Indian subcontinent , Arizona , Mexican Plateau , northwest Argentina , southwestern Spain , Australia, and northern Africa . The formation of

470-417: Is an average surface level of one or more among Earth 's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum  – a standardised geodetic datum  – that is used, for example, as a chart datum in cartography and marine navigation , or, in aviation, as the standard sea level at which atmospheric pressure

517-488: Is because the sea is in constant motion, affected by the tides, wind , atmospheric pressure, local gravitational differences, temperature, salinity , and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as a datum . For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge . Still-water level or still-water sea level (SWL)

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564-427: Is due to change in either the volume of water in the world's oceans or the volume of the oceanic basins . Two major mechanisms are currently causing eustatic sea level rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands. Many factors can produce short-term changes in sea level, typically within

611-483: Is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and local mean sea level. Another alternative is to use a geoid -based vertical datum such as NAVD88 and the global EGM96 (part of WGS84). Details vary in different countries. When referring to geographic features such as mountains, on a topographic map variations in elevation are shown by contour lines . A mountain's highest point or summit

658-512: Is measured to calibrate altitude and, consequently, aircraft flight levels . A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location. Sea levels can be affected by many factors and are known to have varied greatly over geological time scales . Current sea level rise is mainly caused by human-induced climate change . When temperatures rise, mountain glaciers and polar ice sheets melt, increasing

705-401: Is sometimes pushed toward the coast, especially in late fall, when higher pressure develops to the east due to cooling even further east. That situation often brings the warmest temperatures of the year to the normally cool coastline, because the sea breeze stops or is even replaced by a dangerously dry land breeze (see also Diablo wind and Santa Ana wind ). In hilly or mountainous areas near

752-447: Is the level of the sea with motions such as wind waves averaged out. Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides , also have zero mean. Global MSL refers to a spatial average over the entire ocean area, typically using large sets of tide gauges and/or satellite measurements. One often measures the values of MSL with respect to

799-723: Is the main cause. Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise , with another 42% resulting from thermal expansion of water . Sea level rise lags behind changes in the Earth 's temperature by many decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened. What happens after that depends on human greenhouse gas emissions . If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. It could then reach by 2100 slightly over 30 cm (1 ft) from now and approximately 60 cm (2 ft) from

846-527: Is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such as Death Valley, California , the elevation AMSL is negative. It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid . In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of

893-971: The Sonoran Desert , on the Mexican Plateau , in California's Great Central Valley , in the Sahara , in the Kalahari , over north-west Argentina , in South America , over the Kimberley region of north-west Australia , over the Iberian Peninsula , and over the Tibetan Plateau . On land, intense, rapid solar heating of the Earth's surface causes the heating of the lowest layers of the atmosphere, via re-radiated energy in

940-564: The geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean , whose surface dips as much as 106 m (348 ft) below the global mean sea level (excluding minor effects such as tides and currents). Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level. Instantaneous sea level varies substantially on several scales of time and space. This

987-418: The height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today. Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at the poles and 6,371.001 km (3,958.756 mi) on average. This flattened spheroid , combined with local gravity anomalies , defines

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1034-777: The thermal low over northern Africa leads to a low-level westerly jet stream to the south of the Intertropical Convergence Zone . The mid-level African easterly jet develops because heating of the West African land mass during this time of year creates a surface temperature and moisture gradient between the Gulf of Guinea and the Sahara Desert. The atmosphere responds by generating vertical wind shear to maintain thermal wind balance. The jet reaches its zenith in August, lying between

1081-544: The 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES , TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level ( AMSL ) is the elevation (on the ground) or altitude (in the air) of an object, relative to

1128-412: The 16th and 17th parallels. In September, winds maximize near 50 km/h (31 mph) between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November. The mid-level African easterly jet is considered to play a crucial role in the West African monsoon , and helps form the tropical waves that move across the tropical Atlantic and eastern Pacific oceans during

1175-973: The 19th century. With high emissions it would instead accelerate further, and could rise by 1.0 m ( 3 + 1 ⁄ 3  ft) or even 1.6 m ( 5 + 1 ⁄ 3  ft) by 2100. In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F). Rising seas affect every coastal and island population on Earth. This can be through flooding, higher storm surges , king tides , and tsunamis . There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves . Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade. The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without

1222-448: The Earth's gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits gravity anomalies such as those measured by NASA's GRACE satellites . In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and

1269-672: The Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and the main part of Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante , while the European Vertical Reference System is calibrated to the Amsterdam Peil elevation, which dates back to

1316-456: The amount of water in the oceans, while the existing seawater also expands with heat. Because most of human settlement and infrastructure was built in response to a more-normalized sea level with limited expected change, populations affected by sea level rise will need to invest in climate adaptation to mitigate the worst effects or, when populations are at extreme risk, a process of managed retreat . The term above sea level generally refers to

1363-469: The coastline, thermally-forced sea breezes, combined with wind circulations up the sides of the mountains, can encourage the production of chemicals which can lead to the development of smog . Pollution has been tracked into the mid-levels of the troposphere in the form of ozone , which is concentrated over the circulation of the thermal low as well as adjacent oceanic areas. Sea level Mean sea level ( MSL , often shortened to sea level )

1410-482: The geoid is referred to as (mean) ocean surface topography . It varies globally in a typical range of ±1 m (3 ft). Several terms are used to describe the changing relationships between sea level and dry land. The melting of glaciers at the end of ice ages results in isostatic post-glacial rebound , when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic subsidence from

1457-624: The greatest exposure to sea level rise, twelve are in Asia , including Indonesia , Bangladesh and the Philippines. The resilience and adaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts. The greatest impact on human populations in the near term will occur in the low-lying Caribbean and Pacific islands . Sea level rise will make many of them uninhabitable later this century. Pilots can estimate height above sea level with an altimeter set to

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1504-430: The heat low over northern Africa leads to a low-level westerly jet stream from June into October. Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. That differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty meters deep, due to the action of wind and buoyancy-generated turbulence , whereas

1551-551: The height of planetary features. Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time long enough that fluctuations caused by waves and tides are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to sea level changes (millimetres per year). Some land movements occur because of isostatic adjustment to

1598-539: The infrared spectrum. The hotter air is less dense than surrounding cooler air and rises, leading to the formation of a low-pressure area. Elevated areas can enhance the strength of the thermal low because they warm more quickly than the atmosphere which surrounds them at the same altitude. Over water, instability lows form during the winter when the air overlying the land is colder than the warmer water body. Thermal lows can extend to 3,100 metres (10,200 ft) in height and tend to have weak circulations. Thermal lows over

1645-405: The lack of ground and plant moisture, that would normally provide evaporative cooling , can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. That, combined with the rise of the hot air, results in a low pressure area called a thermal low. Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at

1692-431: The land surface conducts heat slowly, with the seasonal signal penetrating perhaps a meter or so. Additionally, the specific heat capacity of liquid water is significantly higher than that of most materials that make up land. Together, those factors mean that the heat capacity of the layer involved in the seasonal cycle is much larger over the oceans than over land, meaning that the air over the land warms faster and reaches

1739-411: The land's surface. As the temperature of the surface of the land rises, the land heats the air above it. The less dense warm air rises, which lowers the sea level pressure by about 0.2%. The cooler air above the sea, with higher sea level pressure, flows towards the land into the area of lower pressure, creating a cooler breeze near the coast. The strength of the sea breeze is directly proportional to

1786-576: The land; hence a change in relative MSL or ( relative sea level ) can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates, or both. In the UK, the ordnance datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool . Since

1833-481: The melting of ice sheets at the end of the last ice age . The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds . Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the geoid and true polar wander . Atmospheric pressure , ocean currents and local ocean temperature changes can affect LMSL as well. Eustatic sea level change (global as opposed to local change)

1880-584: The same altitude above sea level , which creates an associated heat low over the terrain, and enhances any thermal lows which would have otherwise existed. During the cold season, ( winter ), warm water bodies such as the Great Lakes can induce an instability low. Thermal lows which develop near sea level can build in height during the warm season, or summer , to the elevation of the 700 hPa pressure surface, which lies near 3,100 metres (10,200 ft) above sea level. Heat lows normally are stationary and have

1927-485: The surface. However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation . In winter, the land cools off quickly, but the ocean retains its heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons are similar to sea and land breezes ,

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1974-412: The temperature difference between the land and the sea. If the environmental wind field is greater than 8 knots (15 km/h) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop. Along the California coast, the cooler water creates a surface marine layer that is much cooler than inland areas during the summer. At the same time, the intense heating inland generates

2021-507: The times of the Russian Empire , in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) above chart datum and 1.304 m (4 ft 3.3 in) below the average sea level. In France,

2068-518: The tropical Atlantic, mainly during August and September. When the jet is south of normal during the peak months of the Atlantic hurricane season , tropical cyclone formation is suppressed. Thermal low Thermal lows , or heat lows , are non- frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near

2115-496: The warm season. The jet exhibits both barotropic and baroclinic instability , which produces synoptic scale , westward-propagating disturbances in the jet known as African easterly waves or tropical waves. These instabilities, particularly in the presence of moist-convection, cause intense lower-layer cyclonic vortices at the northern flank of the jet. A small number of mesoscale storm systems embedded in these waves develop into tropical cyclones after they move from west Africa into

2162-411: The weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise. On planets that lack a liquid ocean, planetologists can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level" or zero-level elevation , serves equivalently as a reference for

2209-448: The western and southern portions of North America, northern Africa, and Southeast Asia are strong enough to lead to summer monsoon conditions. Thermal lows inland of the coastline lead to the development of sea breezes which, combined with rugged topography near the coast, can lead to poor air quality. Owing to the very high temperatures in the centre of heat lows, there are relatively few direct observations of thermal lows. In deserts,

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