Centers of action
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Centers of action are extensive and almost stationary
low Low or LOW or lows, may refer to: People * Low (surname), listing people surnamed Low Places * Low, Quebec, Canada * Low, Utah, United States * Lo Wu station (MTR code LOW), Hong Kong; a rail station * Salzburg Airport (ICAO airport code: LO ...
or high pressure areas which control the movement of atmospheric disturbances over a large area. This does not mean that the position of the center is constant over a specific area but that the monthly atmospheric pressure corresponds to a high or a low pressure. The French meteorologist
Léon Teisserenc de Bort Léon Philippe Teisserenc de Bort (5 November 1855 in Paris, France – 2 January 1913 in Cannes, France) was a French meteorologist and a pioneer in the field of aerology. Together with Richard Assmann (1845-1918), he is credited as co-discovere ...
was the first in 1881 to apply this term to maxima and minima of pressure on daily charts. The main centers of action in the Northern Hemisphere are the
Icelandic Low The Icelandic Low is a semi-permanent centre of low atmospheric pressure found between Iceland and southern Greenland and extending in the Northern Hemisphere winter into the Barents Sea. In the summer, it weakens and splits into two centres, one ...
, the Aleutian Low, the Azores/Bermuda High, the Pacific High, the Siberian High (in winter), and the Asiatic Low (in summer). Sir
Gilbert Walker Gilbert Walker may refer to: * Gilbert Walker (cricketer) (1888–1952), English cricketer *Gilbert Walker (physicist) (1868–1958), English physicist *Gilbert Carlton Walker Gilbert Carlton Walker (August 1, 1833 – May 11, 1885) was a Unit ...
used the same term to relate meteorological elements in a region to weather in the following season in other regions for the Southern Oscillation.


Causes

In the region called the
Horse latitudes The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure ...
, between 30 and 35 degrees of latitude North or South, there is a series of semi-permanent anticyclones on the downward side of the Hadley cell of the general atmospheric circulation. The thermal contrasts over oceanic waters leads to the formation of these centers of action as a direct convective circulation is established between the equator and higher latitudes in a very weak
Coriolis force In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
zone.. The Azores/Bermuda High is found in this zone. Similarly, low pressure areas are formed in the oceanic circulation near the Polar regions were sea water is much warmer than the land. Such centers are the
Icelandic Low The Icelandic Low is a semi-permanent centre of low atmospheric pressure found between Iceland and southern Greenland and extending in the Northern Hemisphere winter into the Barents Sea. In the summer, it weakens and splits into two centres, one ...
, the Aleutian Low and numerous lows near the coast of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
.. Finally, in both hemispheres, there are centers of action inland in large continental zones where cold or warm air can be trapped by surrounding mountainous zones or high contrast or temperature by surrounding seas. This is the case for
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
where the temperature is very cold in winter forming the Siberian High, or very hot in summer to give a
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.Glossary of Meteorology (200 ...
in summer. Antarctica is so cold in all seasons that it has a permanent anticyclone at its center.


Climatology

The intensity of these systems vary during the year depending on the temperature of water and air. The action centers falter when the temperature contrasts decrease and conversely, they reinforce when these increases. Their position also varies, always following the maximum contrast. For example, the Azores anticyclone moves between the
Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...
and
Bermuda ) , anthem = "God Save the King" , song_type = National song , song = " Hail to Bermuda" , image_map = , map_caption = , image_map2 = , mapsize2 = , map_caption2 = , subdivision_type = Sovereign state , subdivision_name = , e ...
, according to the position of the
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...
and the difference in warming between the Poles and the equator. The distribution of the action centers in the Southern Hemisphere changes far less than in the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
during the year because the landmass in that hemisphere is relatively small. Apart from a thermal anticyclone over Australia during the southern winter, the centers of action are fairly stable over the oceans. The southern parts of the Atlantic, Indian, and Pacific oceans have three anticyclones over the ''Horse latitudes''. Further South, a fairly continuous series of low pressure areas cover the ocean near Antarctica during the whole year and does the Antarctica High over the continent.


References

{{Cyclones Atmospheric circulation