Anticyclone

The Hadley cell carries heat and moisture from the tropics towards the northern and southern mid-latitudes.

An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the National Weather Service's glossary as "A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere".^[1] 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. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows, such as tropical cyclones, due to descending cool air from the backside of upper troughs, such as polar highs, or from large scale sinking, such as the subtropical ridge. Anticyclonic flow spirals in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Anticyclones were first described by Francis Galton in the 1860s.

History

Sir Francis Galton first discovered anticyclones in the 1860's while studying meteorology.

Formation

Surface based high-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. The subtropical ridge forms due to the Hadley cell circulation between the equator and the subtropics of the Northern Hemisphere and Southern Hemisphere. Upper-level high pressure areas lie over tropical cyclones due to their warm core nature.

Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere.^[2][3] Because they weaken in intensity with height, these high pressure systems are cold.

Subtropical ridge

The subtropical ridge shows up as a large area of black (dryness) on this water vapor satellite image from September 2000.

Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or Intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As it moves towards the Mid-Latitudes, the air cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres. This circulation is known as the Hadley cell and leads to the formation of the subtropical ridge.^[4] Many of the world's deserts are caused by these climatological high-pressure areas.^[5] Because these anticyclones strengthen with height, they are known as warm core ridges.

Formation aloft

The development of anticyclones aloft occurs in warm core cyclones, such as tropical cyclones, when latent heat caused by the formation of clouds is released aloft, which increases air temperatures and the resultant atmospheric thickness of the layer, which increases high pressure aloft which acts to evacuate their outflow.

Structure

Wind flows from areas of high pressure to areas of low pressure.^[6] This is due to density differences between the two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in the vicinity of low pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low pressure system, the stronger the wind. The coriolis force caused by the Earth's rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward, or flowing more ageostrophically, from their centers.^[7]

Effects

Surface-based systems

Golden Gate Bridge in fog

High pressure systems are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. Subsidence will generally dry out an air mass by adiabatic, or compressional, heating.^[8] Thus, high pressure typically brings clear skies.^[9] During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a build up of particulates in urban areas under the ridge, leading to widespread haze.^[10] If the low level relative humidity rises towards 100 percent overnight, fog can form.^[11]

Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses.^[12] The low, sharp inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming.

Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system.^[13] When extremely cold air moves over relatively warm oceans, polar lows can develop.^[14] However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses.^[15]

Mid-tropospheric systems

Mean July subtropical ridge position in North America

The circulation around mid-level ridges, and the subsidence at their center, act to steer tropical cyclones around their periphery. Due to the subsidence within this type of system, a cap can be set up which inhibits the development of free convection. This limits thunderstorm activity near their center, and traps low-level pollutants such as ozone as haze under their base, which is a significant problem in large urban centers during summer months such as Los Angeles, California and Mexico City, Mexico.

Upper tropospheric systems

The existence of an upper level ridge allows upper level divergence which leads to surface convergence. If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Since tropical cyclones strengthen these ridges, a positive feedback loop develops between the convective tropical cyclone and the upper level high, where the strength of both systems intensifies. This loop stops once ocean temperatures under the system cool sufficiently, under 26.5 °C (79.7 °F),^[16] which forces the thunderstorm activity to wane, which then weakens the upper level ridge.

Importance to global monsoon regimes

When the subtropical ridge in the northwest Pacific is stronger than normal, it leads to a wet monsoon season for Asia.^[17] The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States. Typically, the subtropical ridge across North America migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September.^[18] When the subtropical ridge is farther north than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona. When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime.^[19]

Depiction on weather maps

A surface weather analysis for the United States on October 21, 2006

On weather maps, high-pressure centers are associated with the letter H in English,^[20] or A in Spanish,^[21] because alta is the Spanish word for high, within the isobar with the highest pressure value. On constant pressure upper level charts, anticyclones are located within the highest height line contour.^[22]

Extraterrestrial versions

On Jupiter, there are two examples of an extraterrestrial anticyclonic storm; the Great Red Spot and the recently formed Oval BA. Unlike any typical anticyclonic storm that happens on Earth when there is water, there's no water powering them. Instead, it is powered by smaller storms merging together. Another theory is that warmer gases rise in a column of cold air, creating a vortex. It is the case of other storms that include Anne's Spot on Saturn, and the Great Dark Spot on Neptune. In addtion, anticyclones have been detected near the poles of Venus.

See also

• Atmospheric circulation
• Atmospheric pressure
• Block (meteorology)
• Earth's atmosphere
• Gyre
• North American High
• Pressure system
• Anticyclonic tornado

References

External links

• Intertropical Convergence Zone photo - NASA Goddard Space Flight Center