Block (meteorology)

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An example of an omega block over western North America in May 2006 NAM 500 MB.PNG
An example of an omega block over western North America in May 2006

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. [1] These blocks can remain in place for several days or even weeks, causing 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). [2] In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans. [1] Whilst these events are linked to the occurrence of extreme weather events such as heat waves, [3] particularly the onset and decay of these events is still not well captured in numerical weather forecasts and remains an open area of research. [4] [5]

Contents

Impact of the polar vortex

Polar vortex and weather impacts due to stratospheric warming Polarvortexwinter.jpg
Polar vortex and weather impacts due to stratospheric warming

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 the polar vortex. [6] Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. [7] 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. [8]

Omega blocks

Omega blocks are so-named because the isobars or geopotential height contours with which they are associated in 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. [2]

Rex blocks

An example of a Rex block off the West coast of North America in January 2007 NGM 700 MB.PNG
An example of a Rex block off the West coast of North America in January 2007

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 the high–low close to form a circle. [9] Rex blocks are named after meteorologist Daniel F. Rex, who first identified them in 1950. [10]

Cut-off highs and lows

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. [11] Whether or not 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 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. [11]

Blocking high

If 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 [12] and 1967 [13] 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. [14] A blocking high in the southern Tasman Sea directs low pressure systems and troughs towards eastern Australia, whereby providing rainfall on the east coast of Australia. [15]

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. [16]

Cold air damming

When warm air ahead of an oncoming storm system overrides cool air trapped east of a mountain range, cloudiness and precipitation can occur for prolonged periods of time Cold2dam.png
When warm air ahead of an oncoming storm system overrides cool air trapped east of a mountain range, cloudiness and precipitation can occur for prolonged periods of time

Blocking of atmospheric systems near 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 the stable air mass of a cold air dam. [17]

Midlatitude cold waves

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. [18] These blocking patterns also have 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 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, [19] 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 the polar jet stream to Portugal and even Morocco. [18] Over Central Asia, unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and 1968/69 [20] 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 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

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 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 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. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Jet stream</span> Fast-flowing atmospheric air current

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of the Earth, Venus, Jupiter, Saturn, Uranus, and Neptune. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.

<span class="mw-page-title-main">Cyclone</span> Large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

<span class="mw-page-title-main">Anticyclone</span> Weather phenomenon of high pressure, as opposed to a cyclone

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

<span class="mw-page-title-main">Atmospheric circulation</span> Process which distributes thermal energy about the Earths surface

Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant. The smaller-scale weather systems – mid-latitude depressions, or tropical convective cells – occur chaotically, and long-range weather predictions of those cannot be made beyond ten days in practice, or a month in theory.

<span class="mw-page-title-main">High-pressure area</span> Region with higher atmospheric pressure

A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation.

<span class="mw-page-title-main">Low-pressure area</span> Area with air pressures lower than adjacent areas

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

<span class="mw-page-title-main">Synoptic scale meteorology</span> 1000-km-order method of measuring weather systems

In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".

<span class="mw-page-title-main">Prevailing winds</span> Strongest direction of wind on a region of Earths surface

In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface at any given time. A region's prevailing and dominant winds are the result of global patterns of movement in the Earth's atmosphere. In general, winds are predominantly easterly at low latitudes globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important cause of the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.

<span class="mw-page-title-main">Westerlies</span> Prevailing winds from the west

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 and trend towards the poles 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.

<span class="mw-page-title-main">Polar low</span>

A polar low is a mesoscale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres, as well as the Sea of Japan. The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s (38 mph).

<span class="mw-page-title-main">Arctic oscillation</span> Climatic cycle over Earths North Pole

The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called the Antarctic oscillation or Southern Annular Mode (SAM). The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N.

<span class="mw-page-title-main">Pressure system</span> Relative peak or lull in the sea level pressure distribution

A pressure system is a peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.

<span class="mw-page-title-main">Polar vortex</span> Persistent cold-core low-pressure area that circles one of the poles

A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air that encircles both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

<span class="mw-page-title-main">Trough (meteorology)</span> Elongated region of low atmospheric pressure

A trough 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.

<span class="mw-page-title-main">Ridge (meteorology)</span> Elongated region of high atmospheric pressure

In meteorology a ridge or barometric ridge is an elongated area of relatively high atmospheric pressure compared to the surrounding environment, without being a closed circulation. It is associated with an area of maximum anticyclonic curvature of wind flow. The ridge originates in the center of an anticyclone and sandwiched between two low-pressure areas, and the locus of the maximum curvature is called the ridge line. This phenomenon is the opposite of a trough.

<span class="mw-page-title-main">Extratropical cyclone</span> Type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to severe gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

<span class="mw-page-title-main">Cold-core low</span> Cyclone with an associated cold pool of air at high altitude

A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.

<span class="mw-page-title-main">Cut-off low</span> Meteorological phenomenon

A cut-off low, sometimes referred to as the weatherman's woe, is defined as "a closed upper-level low which has become completely displaced from basic westerly current, and moves independently of that current" by the National Weather Service. Cut-off lows form in mid-latitudes and would remain nearly stationary for days.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

  1. 1 2 Glossary of Meteorology, Second Edition; American Meteorological Society, 2000; ISBN   1-878220-34-9.
  2. 1 2 "The Omega Block". theweatherprediction.com. Archived from the original on 6 December 2017. Retrieved 27 July 2019.
  3. ↑ Stefanon, Marc; D’Andrea, Fabio; Drobinski, Philippe (2012-03-01). "Heatwave classification over Europe and the Mediterranean region". Environmental Research Letters. 7 (1): 014023. Bibcode:2012ERL.....7a4023S. doi: 10.1088/1748-9326/7/1/014023 . ISSN   1748-9326. Archived from the original on 2022-04-07. Retrieved 2022-04-07.
  4. ↑ Woollings, Tim; Barriopedro, David; Methven, John; Son, Seok-Woo; Martius, Olivia; Harvey, Ben; Sillmann, Jana; Lupo, Anthony R.; Seneviratne, Sonia (2018-09-01). "Blocking and its Response to Climate Change". Current Climate Change Reports. 4 (3): 287–300. doi:10.1007/s40641-018-0108-z. ISSN   2198-6061. PMC   6428232 . PMID   30956938. Archived from the original on 2023-09-16. Retrieved 2022-04-07.
  5. ↑ Lupo, Anthony R. (November 2021). "Atmospheric blocking events: a review". Annals of the New York Academy of Sciences. 1504 (1): 5–24. Bibcode:2021NYASA1504....5L. doi:10.1111/nyas.14557. ISSN   0077-8923. PMID   33382135. S2CID   229931718. Archived from the original on 2023-09-16. Retrieved 2022-04-07.
  6. ↑ Erik A. Rasmussen and John Turner (2003). Polar lows: mesoscale weather systems in the polar regions. Cambridge University Press. p. 174. ISBN   978-0-521-62430-5. Archived from the original on 2023-09-16. Retrieved 2016-09-27.
  7. ↑ Alan Robock (May 2000). "Volcanic Eruptions and Climate" (PDF). Reviews of Geophysics. 38 (2): 171. Bibcode:2000RvGeo..38..191R. doi:10.1029/1998rg000054. S2CID   1299888. Archived (PDF) from the original on 2016-03-04. Retrieved 2012-02-24.
  8. ↑ Todd Mitchell (2004). Arctic Oscillation (AO) time series, 1899 – June 2002. Archived 2003-12-12 at the Wayback Machine University of Washington. Retrieved on 2009-03-02.
  9. ↑ "Brief page about Rex blocks". Archived from the original on 2017-10-22. Retrieved 2006-01-31.
  10. ↑ Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus. 2 (4): 275–301. Bibcode:1950TellA...2..275R. doi:10.1111/j.2153-3490.1950.tb00339.x.
  11. 1 2 "Atmospheric Blocking". Archived from the original on 2017-10-22. Retrieved 2008-02-19.
  12. ↑ Archived 2015-09-23 at the Wayback Machine see p. 116
  13. ↑ Victoria. Department of Crown Lands and Survey (Victoria, Australia); The Victorian Drought, 1967/68; published 1968
  14. ↑ Widespread rain to fall in all states as two storms form in Australia's east and west Archived 24 March 2021 at the Wayback Machine by Ben Deacon from ABC News. 20 March 2021
  15. ↑ "What caused the 'rain bomb'? How the unprecedented Queensland and NSW 2022 floods unfolded". The Guardian. 28 February 2022. Archived from the original on 28 February 2022. Retrieved 3 March 2022.
  16. ↑ "Stalled weather: how stuck air pressure systems drive floods and heatwaves". The Conversation. 3 March 2022. Archived from the original on 3 March 2022. Retrieved 3 March 2022.
  17. ↑ C. David Whiteman (2000). Mountain Meteorology : Fundamentals and Applications. Oxford University Press. p. 166. ISBN   978-0-19-803044-7. Archived from the original on 2023-09-16. Retrieved 2016-09-27.
  18. 1 2 van Loon, Harry and Rogers, Jeffrey C.; ‘The Seesaw in Winter Temperatures Between Greenland and Northern Europe: Part I: General Description’; Monthly Weather Review , 106 (1978), pp. 296–310
  19. ↑ Carrera, M.L.; Higgins, R.W. and Kousky, V.E.; ‘Downstream Weather Impacts Associated with Atmospheric Blocking over the Northeast Pacific Archived 2019-08-01 at the Wayback Machine ’; Journal of Climate, 17 (2004), pp. 4823–4839
  20. ↑ Hirschi, Joël J.-M. and Sinha, Bablu; ‘Negative NAO and cold Eurasian winters: how exceptional was the winter of 1962/1963?’; Weather 62 (2007); pp. 43–48
  21. ↑ Mitchell, Dann; Kornhuber, Kai; Huntingford, Chris; Uhe, Peter (July 2019). "The day the 2003 European heatwave record was broken". The Lancet Planetary Health. 3 (7): e290–e292. doi: 10.1016/S2542-5196(19)30106-8 . PMID   31296455. Archived from the original on 2022-10-20. Retrieved 2022-10-17.
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