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Encyclopedia > Tropical cyclogenesis
Global Tropical Cyclone Tracks
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Global Tropical Cyclone Tracks

Cyclogenesis is the technical term describing the development or strengthening of a surface low pressure system, or cyclone, in the atmosphere.[1] The mechanisms through which cyclogenesis occurs are distinctly different in mid-latitude regions, when compared to the tropics. In the tropics, cyclogenesis involves the development of a warm-core cyclone, usually known as a tropical cyclone, due to significant convection in a favorable atmospheric environment. Image File history File links Download high-resolution version (8000x4000, 5864 KB) Summary This map shows the tracks of all Tropical cyclones which formed worldwide from 1985 to 2005. ... Image File history File links Download high-resolution version (8000x4000, 5864 KB) Summary This map shows the tracks of all Tropical cyclones which formed worldwide from 1985 to 2005. ... diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure above any area in the Earths atmosphere caused by the weight of air. ... The CYCLONE, an early computer built in 1959 by Iowa State University, was based on the Institute for Advanced Study (IAS) architecture developed by John von Neumann. ... Saturns atmosphere is made up of hydorgen, helium and methane ... Latitude, usually denoted symbolically by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ...

Contents

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Requirements for tropical cyclogenesis

Depth of 26°C isotherm on October 1, 2006
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Depth of 26°C isotherm on October 1, 2006

There are six main requirements for tropical cyclogenesis: warm enough sea surface temperatures, atmospheric instability, mid-level moisture, enough Coriolis force to develop a low pressure center, a low level focus or disturbance, and low vertical wind shear. An isotherm is a line of equal or constant temperature on a graph, plot, or map; an isopleth of temperature. ... Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ...

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Warm waters, instability, and mid-level moisture

Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.
Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.

Normally, ocean temperatures of 26.5°C (80°F) through at least a 50-metre depth are considered the minimum to maintain the special mesocyclone which is the tropical cyclone. These warm waters are required to maintain the warm core that fuel tropical systems. This value is well above the global average surface temperature of the oceans, which is 16.1°C (60.9°F).[2] However, this sea surface temperature requirement is a general baseline, and assumes that the ambient atmospheric environment has average conditions; tropical cyclones are known to form even when these conditions are not met. For example, cooler air temperatures at a higher altitude (i.e. at the 500 hPa level) can lead to tropical cyclogenesis at lower water temperatures. At the 500 hPa level, the temperature averages -7°C (18°F) within the tropics. At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing between 1961 and 1990) was -77°C (-132°F).[3] Recent examples of tropical cyclones which maintained themselves over cooler waters include Delta, Epsilon, and Zeta of the 2005 Atlantic hurricane season. Moisture in the mid-levels of the troposphere, roughly at the 500 hPa level, is also a requirement for development. Dry air at that level does not promote large areas of thunderstorms.[4] Image File history File links http://earthobservatory. ... Image File history File links http://earthobservatory. ... The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed it in 1742. ... This article is about the temperature scale; see also Fahrenheit graphics API. Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... The metre, or meter (US), is a measure of length. ... This article or section may be confusing for some readers, and should be edited to be clearer or more simplified. ... Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ... Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ... HPA means Physiology Hypothalamic-Pituitary-Adrenal axis: The hypothalamus, pituitary and adrenal glands work together to regulate hormone levels and maintain homeostasis. ... The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. ... This article is about weather phenomena. ... Tropical Storm Delta was the 25th named storm of the 2005 Atlantic hurricane season. ... Hurricane Epsilon was the twenty-sixth named storm and the fifteenth hurricane of the record-breaking 2005 Atlantic hurricane season. ... Tropical Storm Zeta was the late-developing twenty-seventh named storm of the 2005 Atlantic hurricane season. ... The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, shattering previous records on repeated occasions. ... The Troposphere is the lowermost portion of Earths atmosphere. ...

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The role of maximum potential intensity (MPI)

Dr. Kerry Emanuel created a mathematical model around 1988 to compute the upper limit of tropical cyclone intensity based on the sea surface temperature and atmospheric profiles from the latest global model runs. Emanuel's model is called the maximum potential intensity, or MPI. Maps created from this equation show regions where tropical storm and hurricane formation is possible, based upon the thermodynamics of the atmosphere at the time of the last model run (either 0000 or 1200 UTC). This does not take into account vertical wind shear.[5] A mathematical model is an abstract model that uses mathematical language to describe the behaviour of a system. ... Hurricane Epsilon defied and frustrated forecasters by persisting in a hostile environment for a remarkably long time A tropical cyclone prediction model is a computer program that uses meteorological data to predict the motion and intensity of tropical cyclones. ... ‹ The template below has been proposed for deletion. ... ... Wind shear is a difference in wind speed or direction between two points in the atmosphere. ...

Schematic representation of flow around a low-pressure area (in this case, Hurricane Isabel) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows
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Schematic representation of flow around a low-pressure area (in this case, Hurricane Isabel) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows
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Hurricane Isabel was the ninth named storm, the fifth hurricane, the second major hurricane, and the only Category 5 hurricane of the 2003 Atlantic hurricane season. ...

Coriolis force

A minimum distance of 500 km (300 miles) from the equator is normally needed for tropical cyclogenesis. The role of the coriolis force is to provide for gradient wind balance. Bays and gulfs can enhance local rotation of a storm, and cause formation close to the equator, similar to that witnessed during Typhoon Vamei's life cycle. km redirects here. ... A mile is the name of a unit of length, usually used to measure distance, in a number of different systems, including English units, Imperial units, and United States customary units. ... The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ... In physics, the Coriolis effect is an inertial force first described by Gaspard-Gustave Coriolis, a French scientist, in 1835. ...

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A low level disturbance

Whether it be the monsoon trough, a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required for tropical cyclogenesis. Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Tropical waves, also known as easterly waves, are elongated areas of relatively low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms. ... A guide to the symbols for weather fronts that may be found on a weather map: 1. ... An outflow boundary is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. ... Vorticity is a mathematical concept used in fluid dynamics. ...

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Weak vertical wind shear

Less than 10 m/s (22 mph) of vertical wind shear between the surface and the tropopause is required for tropical cyclone development. Strong wind shear can displace the mid-level warm core from the surface circulation, dry out the mid-levels of the troposphere, and halt development. In smaller systems, the development of a significant mesoscale convective complex in a sheared environment can send out a large enough outflow boundary to destroy the surface cyclone. Moderate wind shear can lead to the initial development of the convective complex and surface low similar to the mid-latitudes, but it must relax to allow tropical cyclogenesis to continue. Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. ... Miles per hour is a unit of speed, expressing the number of international miles covered per hour. ... Wind shear is a difference in wind speed or direction between two points in the atmosphere. ... The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. ... The Troposphere is the lowermost portion of Earths atmosphere. ...

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Favorable trough interactions during cyclogenesis

Limited vertical wind shear can be positive for tropical cyclone formation. When an upper level trough or upper level low is roughly the same scale as the tropical disturbance, the system can be steered by the upper level system into an area of better diffluence aloft, which can cause further development. Weaker upper cyclones are better candidates for a favorable interaction. There is evidence that weakly sheared tropical cyclones initially develop more rapidly than non-sheared tropical cyclones, though they plateau at a much weaker intensity.[6] This process is also known as baroclinic initiation of a tropical cyclone. Trailing upper cyclones and upper troughs can cause additional outflow channels and aid in the intensification process. It should be noted that developing tropical disturbances can help create/deepen upper troughs or upper lows in their wake due to the outflow jet eminating from the developing tropical disturbance/cyclone.[7][8]


There are cases where large, mid-latitude troughs can help with tropical cyclogenesis when an upper level jet streak passes to the northwest of the developing system, which will aid divergence aloft and inflow at the surface, spinning up the cyclone. This type of interaction is more often associated with disturbances already in the process of recurvature.[9]

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Large scale climate cycles and tropical cyclogenesis

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Loop of SST anomalies in the Tropical Pacific
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Influence of ENSO

Warm waters during the El Niño-Southern Oscillation lower the potential of tropical cyclone formation primarily in the Atlantic Basin, but tends to cause an increase in activity in the northeastern Pacific Ocean. Since tropical cyclones in the eastern north Pacific and Atlantic basins are both generated in large part by tropical waves from the same wave train, decreased tropical cyclone activity from the north Atlantic translates to increased tropical cyclone activity in the northeast Pacific. Although El Niño climate shifts do not impact numbers of tropical cyclones in the northwest Pacific, it does shift their formation, as typhoons form more to the east than normal. Near the International Date Line, on both sides of the equator, there is a net increase in tropical cyclone development during El Niño events.[10] Chart of ocean surface temperature anomaly (outlined) in the Pacific [°C] during a La Niña cycle in January 2006. ... The Atlantic Ocean is the second-largest ocean, covering approximately one-fifth of the Earths surface. ... Tropical waves, also known as easterly waves, are elongated areas of relatively low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms. ... The International Date Line around 180° This article is about the line dividing time zones; see Dateline for other meanings, including the television program. ...

5-day running mean of MJO. Note how it moves eastward with time.
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5-day running mean of MJO. Note how it moves eastward with time.
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Influence of the MJO

In general, westerly wind increases associated with the Madden-Julian Oscillation lead to increased tropical cyclogenesis in all basins. As the oscillation propagates from west to east, it leads to an eastward march in tropical cyclogenesis with time during that hemisphere's summer season.[11] There is an inverse relationship between tropical cyclone activity in the western Pacific basin and the north Atlantic basin, however. When one basin is active, the other is normally quiet, and vice versa. The main reason for this appears to be the phase of the Madden-Julian oscillation, or MJO, which is normally in opposite modes between the two basins at any given time. The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall. ... The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall. ...

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Influence of equatorial Rossby waves on tropical cyclogenesis

Research has shown that trapped equatorial Rossby wave packets can increase the likelihood of tropical cyclogenesis in the Pacific Ocean, as they increase the low-level westerly winds within that region, which then leads to greater low-level vorticity. The individual waves can move at approximately 1.8 m/s (4 mph) each, though the group tends to remain stationary.[12] Rossby (or planetary) waves are large-scale motions in the ocean or atmosphere whose restoring force is the variation in Coriolis effect with latitude. ... Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. ... Miles per hour is a unit of speed, expressing the number of international miles covered per hour. ...

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Seasonal forecasts of tropical cyclogenesis

Since 1984, Colorado State University has been issuing seasonal tropical cyclone forecasts for the north Atlantic basin, with results that are better than climatology. They have found several statistical relationships for this basin that appear to allow long range prediction of the number of tropical cyclones. Since then, numerous others have followed in their steps, with some organizations issuing seasonal forecasts for the northwest Pacific and the Austrailian region.[13] The predictors are related to regional oscillations in the global climate system: the Walker circulation which is related to ENSO (El Niño and La Niña) and the Southern Oscillation Index, the North Atlantic oscillation or NAO, the Arctic oscillation or AO, and the PNA.[14] Colorado State University Colorado State University is a public land grant institution of higher learning located in Fort Collins, Colorado in the United States. ... The Walker circulation is an atmospheric circulation of air at the equatorial Pacific Ocean, responsible for creating ocean upwelling off the coasts of Peru and Ecuador. ... Chart of ocean surface temperature anomaly (outlined) in the Pacific [°C] during a La Niña cycle in January 2006. ... The North Atlantic oscillation (NAO) is a complex climatic phenomenon in the North Atlantic Ocean (especially associated with fluctuations of climate between Iceland and the Azores). ... The Arctic oscillation (AO) is the dominant pattern of non-seasonal sea-level pressure (SLP) variations north of 20N, and it is characterized by SLP anomalies of one sign in the Arctic and anomalies of opposite sign centered about 37-45N. The North Atlantic oscillation (NAO) is a close relative... PNA is peptide nucleic acid, a chemical similar to DNA or RNA but differing in the composition of its backbone. ...

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References

  1. ^ Arctic Climatology and Meteorology Definition for Cyclogenesis
  2. ^ National Climatic Data Center Mean Global Sea Surface temperature
  3. ^ Dian J. Gaffen-Seidel, Rebecca J. Ross, James K. Angell Climatological characteristics of the tropical tropopause as revealed by radiosondes
  4. ^ Chris Landsea Tropical Cyclogenesis Parameters
  5. ^ Kerry A. Emanuel Maximum Intensity Estimation
  6. ^ M. E. Nicholls and R. A. Pielke A Numerical Investigation of the Effect of Vertical Wind Shear on Tropical Cyclone Intensification
  7. ^ Clark Evans Favorable trough interactions on tropical cyclones
  8. ^ Deborah Hanley, John Molinari, and Daniel Keyser A Composite Study of the Interactions between Tropical Cyclones and Upper-Tropospheric Troughs
  9. ^ Eric Rappin and Michael C. Morgan Jet streak interactions on tropical cyclones
  10. ^ Australian Bureau of Meteorology ENSO impacts on tropical cyclone cyclogenesis location
  11. ^ John Molinari and David Vollaro Planetary- and Synoptic-Scale Influences on Eastern Pacific Tropical Cyclogenesis
  12. ^ Kelly Lombardo Equatorial Rossby waves and Western Pacific Tropical Cyclogenesis
  13. ^ Mark Saunders and Peter Yuen Tropical Storm Risk Group Seasonal Predictions
  14. ^ Philip J. Klotzbach, Willam Gray, and Bill Thornson Colorado State University's seasonal forecast predictors
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