The Azores High also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda-Azores High , is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean , at the Horse latitudes . It forms one pole of the North Atlantic oscillation , the other being the Icelandic Low . The system influences the weather and climatic patterns of vast areas of North Africa , Western Asia , Southern Europe , and to a lesser extent, eastern North America . The aridity of the Sahara Desert and the summer drought of the Mediterranean Basin is due to the large-scale subsidence and sinking motion of air in the system. In its summer position, the high is centered near Bermuda , and creates a southwest flow of hot tropical air toward the East Coast of the United States . In summer, the Azores-Bermuda High is strongest. The central pressure hovers around 1024 mbar ( hPa ).
34-625: This high-pressure block exhibits anticyclonic behaviour, circulating the air clockwise . Due to this direction of movement, African eastern waves are impelled along the southern periphery of the Azores High away from coastal West Africa towards the Caribbean , Central America , or the Bahamas , favouring tropical cyclogenesis , especially during the hurricane season . Research into global warming suggests that it may be intensifying
68-416: A geographic saddle between two mountain peaks. A trough is the result of the movements of the air in the atmosphere. In regions where there is upward movement near the ground and divergence at altitude, there is a loss of mass. The pressure becomes lower at this point. At upper levels of the atmosphere, this occurs when there is a meeting of a mass of cold air and another hot one along a thin ribbon called
102-407: A low pressure center . A trough is an elongated area of lower air pressure. Since pressure is closely linked to wind, there are often changes in wind direction across a trough. If a trough forms in the mid-latitudes, a temperature difference at some distance between the two sides of the trough usually exists and the trough might become a weather front at some point. However, such a weather front
136-407: A weather front associated with clouds, showers, and a wind direction shift. Upper-level troughs in the jet stream (as shown in diagram) reflect cyclonic filaments of vorticity . Their motion induces upper-level wind divergence, lifting and cooling the air ahead (downstream) of the trough and helping to produce cloudy and rain conditions there. Unlike fronts, there is not a universal symbol for
170-420: A frontal baroclinic zone. We then have the creation of a jet stream that plunges the cold air towards the equator and hot air towards the poles, creating a ripple in the circulation that is called a Rossby wave . These undulations give the hollows and peaks of altitude. In general, absolute vorticity advection is positive between these two features, but closer to the ridge, whereas it is negative just behind
204-598: A qualifying term indicating a specific or a set of characteristics. An inverted trough is an atmospheric trough which is oriented opposite to most troughs of the mid-latitudes. Most (but not all) inverted troughs are tropical waves (also commonly called easterly waves). Most troughs of low pressure in the mid-latitudes of the Northern Hemisphere are characterized by decreasing atmospheric pressure from south to north while inverted troughs are characterized by decreasing pressure from north to south. This situation
238-519: A tendency to produce anomalously mild conditions at very high latitudes, at least in those regions exposed to anomalous flow from the ocean as in Greenland and Beringia , or from chinook winds as in Interior Alaska . Such cold winters over the contiguous United States and southern Canada as 1911/12 , 1935/36 , 1949/50, 1977/78 and 1978/79, 1993/94, and 2017/18 resulted from blocks in
272-640: A trough on a surface weather analysis chart. The weather charts in some countries or regions mark troughs by a line. In the United States, a trough may be marked as a dashed line or bold line. In the UK, Hong Kong and Fiji, it is represented by a bold line extended from a low pressure center or between two low pressure centers; in Macau and Australia, it is a dashed line. If they are not marked, troughs may still be identified as an extension of isobars away from
306-453: A trough. At the surface, lifting air under positive vorticity advection is reflected by the formation of depressions and troughs. There will therefore be a slope between the barometric high altitude and that on the ground, this slope going towards the mass of cold air at high altitude (generally towards the Pole). Troughs have an orientation relative to the poles which is rarely North-South. In
340-609: A year that North America experienced markedly below normal temperatures and higher than normal precipitation, especially during the spring months. The high amount of rain in North America increased the energy levels in the polar jet, driving it far to the north in Europe and resulting in a prolonged, static high pressure ridge that drove up hot air from the Sahara Desert into Europe. Trough (meteorology) A trough
374-400: Is an elongated region of relatively low atmospheric pressure without a closed isobaric contour that would define it as a low pressure area . Since low pressure implies a low height on a pressure surface, troughs and ridges refer to features in an identical sense as those on a topographic map . Troughs may be at the surface, or aloft, at altitude. Near-surface troughs sometimes mark
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#1732764734524408-522: Is the opposite in the Southern Hemisphere. Inverted troughs in both hemispheres move to the west from the east, while mid-latitude troughs generally move with the westerlies toward the east. A lee trough, also known as a dynamic trough, is "A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of
442-408: Is usually less convective than a trough in the tropics or subtropics (such as a tropical wave ). Inversely, sometimes collapsed frontal systems will degenerate into troughs. Sometimes the region between two high pressure centers may also assume the character of a trough when there is a detectable wind shift noted at the surface. In the absence of a wind shift, the region is designated a col , akin to
476-692: The Gulf of Alaska or to the east of the Mackenzie Mountains directing very cold Arctic air with a long trajectory as far as the American South , as did the Western cold waves of 1889/90 and January 1950. In Northern and Western Europe , cold winters such as 1683/84, 1739/40, 1794/95, 1829/30, 1894/95 , 1916/17, 1941/42, February 1947 and 1962/63 are almost always associated with high latitude Atlantic blocking and an equatorward shift of
510-608: The Bermuda High in some years, independently of oscillations such as ENSO , leading to more precipitation extremes across the Southeastern United States . Latitudinal displacement of the ridge is also occurring, and computer models depict more westward expansion of the anticyclone in the future. However, during the winter of 2009–2010 , the Azores High was smaller, displaced to the northeast and weaker than usual, allowing sea surface temperatures in
544-540: The Central Atlantic to increase quickly. 34°N 30°W / 34°N 30°W / 34; -30 Block (meteorology) Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones . They are also known as blocking highs or blocking anticyclones . These blocks can remain in place for several days or even weeks, causing
578-501: The Northern Hemisphere resemble an Ω, the uppercase Greek letter omega . They typically have a low-high-low pattern, arranged in the west–east direction. Rex blocks (or dipole blocks) consist of a high situated poleward (north in the Northern Hemisphere; south in the Southern Hemisphere) of a low. Very often both the high and the low are closed, meaning that the isobars (or constant geopotential height lines) defining
612-422: The Northern Hemisphere, however, cold winters in Europe (e.g. 1916/17, 1962/63) are often very mild over Central Asia, which can gain warm air advection from subtropical cyclones pushed to the south under negative NAO conditions. Heat waves in summer are the result of similar blocking patterns, typically involving the placement of the semi-permanent subtropical ridge . Some unusually intense summers such as 1936 in
646-528: The Northern Hemisphere, positively tilted troughs will extend from the lowest pressure northeast to southwest while negatively tilted troughs have a northwest to southeast orientation. In the Southern Hemisphere, the positive tilt will be southeast to northwest and the negative one southwest to northeast. A trough will usually begin with a positive tilt as cold air moves toward the Equator. The trough will become neutral (North-South) and then negatively tilted as
680-548: The Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced." It can be formed either as a result of the adiabatic compression of sinking air on the lee side of a mountain range, or through cyclogenesis resulting from "the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope." Convective cells may develop in
714-482: The United States, 1999, 2002, and 2011, and in Europe summers such as 1976, 2003 European heat wave , and 2019, were the result of entrenched highs that became detached from the jet stream for a prolonged period of time and allowed warm, dry air to build in place. In many cases such as the 1999 US drought, the heat wave was preceded by prior months of below normal precipitation that prevented temperatures from cooling. The 2003 heat wave in Europe occurred, conversely, during
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#1732764734524748-469: The areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others). In the Northern Hemisphere , extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans. Whilst these events are linked to the occurrence of extreme weather events such as heat waves, particularly
782-573: The block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed, as happened in southeastern Australia in 2006 and 1967 with resultant extreme droughts . However, when a blocking high is situated in the Tasman Sea it can cause torrential rains in eastern Australia, as in the cases of the 2021 and 2022 flood events . A blocking high in the southern Tasman Sea directs low pressure systems and troughs towards eastern Australia, whereby providing rainfall on
816-609: The coast of Georgia that caused a drought in the Southeast that same year. Rainy, cooler weather results if the block is a low in the US. Hurricane Ian in the last week of September 2022 drifted northward and its remnants became detached from the jet stream, resulting in a stationary low pressure system spinning off the Northeastern US and bringing several days of precipitation until a front finally moved through on October 6. If
850-504: The east coast of Australia. In Australia, blocking highs generally occur in the Great Australian Bight and the Tasman Sea , which are powerful high-pressure systems that usually develop further south than normal. They stay virtually unmoving for a lengthy period (i.e. several days to weeks) and thus block the regular easterly motion of weather systems across southern Australia. Blocking of atmospheric systems near
884-434: The energy carried by the cold air races east through the atmospheric circulation and distorts its shape. The positive tilt is thus the building phase of the trough and the negative tilt is the dissipation of its energy. Therefore, the clouds and precipitation will develop in the positive phase and the most severe weather will be in the negative phase. In addition to standard troughs, some troughs may be described further with
918-412: The high–low close to form a circle. Rex blocks are named after meteorologist Daniel F. Rex, who first identified them in 1950. When an upper-level high - or low -pressure system becomes stuck in place due to a lack of steering currents , it is known as being "cut off". The usual pattern which leads to this is the jet stream retreating poleward, leaving the then cut-off system behind. Whether or not
952-637: The onset and decay of these events is still not well captured in numerical weather forecasts and remains an open area of research. Polar cyclones are climatological features which hover near the poles year-round. They are weaker during summer and strongest during winter. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Extratropical cyclones which occlude and migrate into higher latitudes create cold-core lows within
986-635: The polar jet stream to Portugal and even Morocco . Over Central Asia , unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and 1968/69 are associated with blocking near the Ural Mountains extending the Siberian High westwards to push the very cold air from the Siberian " cold pole " outward towards the Aral and Caspian Seas . Unlike other midlatitude regions of
1020-527: The polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gauge its magnitude is the Arctic oscillation . Omega blocks are so-named because the isobars or geopotential height contours with which they are associated in
1054-489: The stable air mass of a cold air dam. In the middle latitudes of the Northern Hemisphere, areas on the eastern side of blocking anticyclones or under the influence of anomalous flows from colder continental interiors related to blocks experience severe winters, a phenomenon which has been known since the discovery of the North Atlantic Oscillation (NAO) in the 1840s. These blocking patterns also have
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1088-604: The surface of the Earth occurs when a well-established poleward high pressure system lies near or within the path of the advancing storm system. The thicker the cold air mass is, the more effectively it can block an invading milder air mass. The depth of the cold air mass is normally shallower than the mountain barrier which created the cold air damming, or CAD. Some events across the Intermountain West can last for ten days. Pollutants and smoke can remain suspended within
1122-410: The system is of high- or low-pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southern United States during late spring and early summer of 2007, when a cut-off-low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to Texas and Oklahoma (see June 2007 Texas flooding ), and a cut-off-high near
1156-540: The vicinity of troughs and give birth to a tropical cyclone . Some tropical or subtropical regions such as the Philippines or South China are greatly affected by convection cells along a trough. In the mid-latitude westerlies , upper level troughs and ridges often alternate in a high-amplitude pattern. For a trough in the westerlies, the region just west of the trough axis is typically an area of convergent winds and descending air – and hence high pressure –, while
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