Tropical Cyclogenesis
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Tropical cyclogenesis is the development and strengthening of a
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. The mechanisms through which
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
cyclogenesis occurs are distinctly different from those through which
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
cyclogenesis Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low-pressure area). Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyc ...
occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
in a favorable atmospheric environment. Tropical cyclogenesis requires six main factors: sufficiently warm
sea surface temperatures Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...
(at least ), atmospheric instability, high
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
in the lower to middle levels of the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance, and low vertical
wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
. Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins.
Climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
cycles such as ENSO and the
Madden–Julian oscillation The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal (30- to 90-day) variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric R ...
modulate the timing and frequency of tropical cyclone development. There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach strength higher than , and 20 become intense tropical cyclones (at least Category 3 intensity on the
Saffir–Simpson scale The Saffir–Simpson hurricane wind scale (SSHWS) classifies hurricanes—which in the Western Hemisphere are tropical cyclones that exceed the intensities of tropical depressions and tropical storms—into five categories distinguished b ...
).


Requirements for tropical cyclone formation

There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
in the lower to middle levels of the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, enough Coriolis force to sustain a low pressure center, a preexisting low level focus or disturbance, and low vertical
wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
. While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.


Warm waters, instability, and mid-level moisture

Normally, an ocean temperature of 26.5 °C (79.7 °F) spanning through at least a 50-metre depth is considered the minimum to maintain a
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
. These warm waters are needed to maintain the warm core that fuels tropical systems. This value is well above 16.1 °C (60.9 °F), the global average surface temperature of the oceans. Tropical cyclones are known to form even when normal conditions are not met. For example, cooler air temperatures at a higher altitude (e.g., at the 500 
hPa HPA may refer to: Organizations * Harry Potter Alliance, a charity * Halifax Port Authority, Canada * Hamburg Port Authority, Germany * Hawaii Preparatory Academy, a school in Hawaii, US * Health Protection Agency, UK * Heerespersonalamt, the Ger ...
level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certain
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
is required to force the atmosphere to be
unstable In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...
enough for convection. In a moist atmosphere, this lapse rate is 6.5 °C/km, while in an atmosphere with less than 100%
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
, the required lapse rate is 9.8 °C/km. At the 500 hPa level, the air temperature averages −7 °C (18 °F) within the tropics, but air in the tropics is normally dry at this level, giving the air room to wet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection. A wetbulb temperature at 500 hPa in a tropical atmosphere of −13.2 °C is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by 1 °C in the sea surface temperature for each 1 °C change at 500 hpa. Under a cold cyclone, 500 hPa temperatures can fall as low as −30 °C, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, roughly at the 500 hPa level, is normally a requirement for development. However, when dry air is found at the same height, temperatures at 500 hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres. At heights near the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77 °C (−105 °F). A recent example of a
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
that maintained itself over cooler waters was
Epsilon Epsilon (, ; uppercase , lowercase or lunate ; el, έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel or . In the system of Greek numerals it also has the value five. It was der ...
of the
2005 Atlantic hurricane season The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in history, until the record was broken 15 years later in 2020. The season broke numerous records at the time, with 28 tropical or subtropical storms recorded. ...
.


Role of Maximum Potential Intensity (MPI)

Kerry Emanuel Kerry Andrew Emanuel (born April 21, 1955) is an American professor of meteorology currently working at the Massachusetts Institute of Technology in Cambridge. In particular he has specialized in atmospheric convection and the mechanisms acting t ...
created a
mathematical model A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used in the natural sciences (such as physics, ...
around 1988 to compute the upper limit of tropical cyclone intensity based on sea surface temperature and atmospheric profiles from the latest global model runs. Emanuel's model is called the ''
maximum potential intensity The maximum potential intensity of a tropical cyclone is the theoretical limit of the strength of a tropical cyclone. Maximum potential intensity Due to surface friction, the inflow only partially conserves angular momentum. Thus, the sea surface ...
'', or MPI. Maps created from this equation show regions where tropical storm and hurricane formation is possible, based upon the
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
of the atmosphere at the time of the last model run. This does not take into account vertical
wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
.


Coriolis force

A minimum distance of from the
equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
(about 4.5 degrees from the equator) is normally needed for tropical cyclogenesis. The Coriolis force imparts rotation on the flow and arises as winds begin to flow in toward the lower pressure created by the pre-existing disturbance. In areas with a very small or non-existent Coriolis force (e.g. near the Equator), the only significant atmospheric forces in play are the ''pressure gradient force'' (the pressure difference that causes winds to blow from high to low pressure) and a smaller
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
force; these two alone would not cause the large-scale rotation required for tropical cyclogenesis. The existence of a significant Coriolis force allows the developing vortex to achieve gradient wind balance. This is a balance condition found in mature tropical cyclones that allows
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understo ...
to concentrate near the storm core; this results in the maintenance or intensification of the vortex if other development factors are neutral.


Low level disturbance

Whether it be a depression in the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
(ITCZ), a
tropical wave A tropical wave (also called easterly wave, tropical easterly wave, and African easterly wave), in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which ...
, a broad surface front, or an
outflow boundary An outflow boundary, also known as a gust front, 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 ...
, a low level feature with sufficient
vorticity In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wi ...
and convergence is required to begin 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 cyclones can form when smaller circulations within the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
come together and merge.


Weak vertical wind shear

Vertical wind shear of less than 10 m/s (20  kt, 22 mph) between the surface and the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
is favored for tropical cyclone development. A weaker vertical shear makes the storm grow faster vertically into the air, which helps the storm develop and become stronger. If the vertical shear is too strong, the storm cannot rise to its full potential and its energy becomes spread out over too large of an area for the storm to strengthen. Strong wind shear can "blow" the tropical cyclone apart, as it displaces the mid-level warm core from the surface circulation and dries out the mid-levels of the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, halting development. In smaller systems, the development of a significant
mesoscale convective complex A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. They are long-lived, often form nocturnally, and commonly contain heavy rainfall, wi ...
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.


Favorable trough interactions

Limited vertical wind shear can be positive for tropical cyclone formation. When an upper-level
trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...
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 with 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, although this comes at the cost of a peak in intensity with much weaker wind speeds and higher minimum pressure. 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. Developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet emanating from the developing tropical disturbance/cyclone. There are cases where large, mid-latitude troughs can help with tropical cyclogenesis when an upper-level
jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...
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.


Times of formation

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. Each basin, however, has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active. In the North Atlantic, a distinct hurricane season occurs from June 1 through November 30, sharply peaking from late August through October. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar time frame to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, tropical cyclone activity generally begins in early November and generally ends on April 30. Southern Hemisphere activity peaks in mid-February to early March. Virtually all the Southern Hemisphere activity is seen from the southern African coast eastward, toward South America. Tropical cyclones are rare events across the south Atlantic Ocean and the far southeastern Pacific Ocean.


Unusual areas of formation


Middle latitudes

Areas farther than 30 degrees from the equator (except in the vicinity of a warm current) are not normally conducive to tropical cyclone formation or strengthening, and areas more than 40 degrees from the equator are often very hostile to such development. The primary limiting factor is water temperatures, although higher shear at increasing latitudes is also a factor. These areas are sometimes frequented by cyclones moving poleward from tropical latitudes. On rare occasions, such as Pablo in 2019, Alex in 2004, Alberto in 1988, and the 1975 Pacific Northwest hurricane, storms may form or strengthen in this region. Typically, tropical cyclones will undergo
extratropical transition 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 ...
after recurving polewards, and typically become fully extratropical after reaching 45–50˚ of latitude. The majority of
extratropical cyclones 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 p ...
tend to restrengthen after completing the transition period.


Near the Equator

Areas within approximately ten degrees latitude of the equator do not experience a significant Coriolis Force, a vital ingredient in tropical cyclone formation. However, a few tropical cyclones have been observed forming within five degrees of the equator.


South Atlantic

A combination of
wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
and a lack of tropical disturbances from the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
(ITCZ) makes it very difficult for the South Atlantic to support tropical activity. At least 5 tropical cyclones have been observed here such as— a weak tropical storm in 1991 off the coast of Africa near
Angola , national_anthem = " Angola Avante"() , image_map = , map_caption = , capital = Luanda , religion = , religion_year = 2020 , religion_ref = , coordina ...
,
Hurricane Catarina Hurricane Catarina, or Cyclone Catarina () was an extraordinarily rare South Atlantic tropical cyclone, the only recorded hurricane strength storm on record in the South Atlantic Ocean. Catarina made landfall on South Brazil at peak intensity, ...
, which made landfall in Brazil in 2004 at Category 2 strength, a smaller storm in January 2004 located east of Salvador, Brazil, and Tropical Storm Iba in March 2019. The January storm is thought to have reached tropical storm intensity based on
scatterometer A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves scattered by diffusion in a medium such as air. Diffusionmeters using visible light are found in airports or along roads to measur ...
wind measurements.


Mediterranean and Black Seas

Storms that appear similar to tropical cyclones in structure sometimes occur in the
Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...
. Examples of these "
Mediterranean tropical cyclone Mediterranean tropical-like cyclones, often referred to as medicanes (a portmanteau of Mediterranean hurricanes) but sometimes also as Mediterranean cyclones or as Mediterranean hurricanes, are meteorological phenomena occasionally observed over ...
s" formed in September 1947, September 1969, September 1973, August 1976, January 1982, September 1983, December 1984, December 1985, October 1994, January 1995, October 1996, September 1997, December 2005, September 2006, November 2011, November 2014, November 2017, September 2018, September 2020, November and December 2020. However, there is debate on whether these storms were tropical in nature. The
Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Roma ...
has, on occasion, produced or fueled storms that begin
cyclonic rotation Cyclonic rotation, or cyclonic circulation, is atmospheric motion in the same direction as a planet's rotation, as opposed to anticyclonic rotation. For Earth, the Coriolis effect causes cyclonic rotation to be in a counterclockwise direction in the ...
, and that appear to be similar to tropical-like cyclones observed in the Mediterranean. Two of these storms reached tropical storm and subtropical storm intensity in August 2002 and September 2005 respectively.


Elsewhere

Tropical cyclogenesis is extremely rare in the far southeastern Pacific Ocean, due to the cold sea-surface temperatures generated by the
Humboldt Current The Humboldt Current, also called the Peru Current, is a cold, low- salinity ocean current that flows north along the western coast of South America.Montecino, Vivian, and Carina B. Lange. "The Humboldt Current System: Ecosystem components and pr ...
, and also due to unfavorable
wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
; as such, there are no records of a tropical cyclone impacting western South America. However, several systems have been observed developing in the region east of 120°W, which is the official eastern boundary of the South Pacific basin. On May 11, 1983, a tropical depression developed near 110°W, which was thought to be the easternmost forming South Pacific tropical cyclone ever observed in the satellite era. In mid-2015, a rare subtropical cyclone was identified in early May, slightly near
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
, even further east than the 1983 tropical depression. This system was unofficially dubbed ''Katie'' by researchers.Katie Katie is an English feminine name. It is a form Katherine, Kate, Caitlin, Kathleen, Katey and their related forms. It is frequently used on its own. People Sports * Katie Boulter (born 1996), British tennis player * Katie Clark (born 1994), Bri ...
"> Another subtropical cyclone was identified at 77.8 degrees longitude west in May 2018, just off the coast of Chile. This system was unofficially named ''Lexi'' by researchers. A subtropical cyclone was spotted just off the Chilean coast in January 2022, named ''Humberto'' by researchers.
Vortices In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
have been reported off the coast of
Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...
in the past. However, it is debatable if they are truly tropical in character. Tropical activity is also extremely rare in the
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
. However, a storm system that appeared similar to a subtropical or tropical cyclone formed in September 1996 over
Lake Huron Lake Huron ( ) is one of the five Great Lakes of North America. Hydrology, Hydrologically, it comprises the easterly portion of Lake Michigan–Huron, having the same surface elevation as Lake Michigan, to which it is connected by the , Strait ...
. The system developed an eye-like structure in its center, and it may have briefly been a subtropical or tropical cyclone.


Inland intensification

Tropical cyclones typically began to weaken immediately following and sometimes even prior to landfall as they lose the sea fueled heat engine and friction slows the winds. However, under some circumstances, tropical or subtropical cyclones may maintain or even increase their intensity for several hours in what is known as the
brown ocean effect The brown ocean effect is an observed weather phenomenon involving some tropical cyclones after landfall. Normally, hurricanes and tropical storms lose strength when they make landfall, but when the brown ocean effect is in play, tropical cyclon ...
. This is most likely to occur with warm moist soils or marshy areas, with warm ground temperatures and flat terrain, and when upper level support remains conducive.


Influence of large-scale climate cycles


Influence of ENSO

El Niño (ENSO) shifts the region (warmer water, up and down welling at different locations, due to winds) in the Pacific and Atlantic where more storms form, resulting in nearly constant
accumulated cyclone energy Accumulated cyclone energy (ACE) is a metric used by various agencies to express the energy released by a tropical cyclone during its lifetime. It is calculating by summing the square of a tropical cyclone's maximum sustained winds, measured ever ...
(ACE) values in any one basin. The El Niño event typically decreases hurricane formation in the Atlantic, and far western Pacific and Australian regions, but instead increases the odds in the central North and South Pacific and particular in the western North Pacific typhoon region. Tropical cyclones in the northeastern Pacific and north Atlantic basins are both generated in large part by
tropical wave A tropical wave (also called easterly wave, tropical easterly wave, and African easterly wave), in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which ...
s from the same wave train. In the Northwestern Pacific, El Niño shifts the formation of tropical cyclones eastward. During El Niño episodes, tropical cyclones tend to form in the eastern part of the basin, between 150°E and the
International Date Line The International Date Line (IDL) is an internationally accepted demarcation on the surface of Earth, running between the South and North Poles and serving as the boundary between one calendar day and the next. It passes through the Pacific O ...
(IDL). Coupled with an increase in activity in the North-Central Pacific (IDL to 140°W) and the South-Central Pacific (east of 160°E), there is a net increase in tropical cyclone development near the International Date Line on both sides of the equator. While there is no linear relationship between the strength of an El Niño and tropical cyclone formation in the Northwestern Pacific, typhoons forming during El Niño years tend to have a longer duration and higher intensities. Tropical cyclogenesis in the Northwestern Pacific is suppressed west of 150°E in the year following an El Niño event.


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. 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 cause 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.


Influence of equatorial Rossby waves

Research has shown that trapped equatorial
Rossby wave Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby. They are observed in the atmospheres and ...
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.


Seasonal forecasts

Since 1984,
Colorado State University Colorado State University (Colorado State or CSU) is a public land-grant research university in Fort Collins, Colorado. It is the flagship university of the Colorado State University System. Colorado State University is classified among "R1: ...
has been issuing seasonal tropical cyclone forecasts for the north Atlantic basin, with results that they claim are better than climatology. The university claims to 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 issued seasonal forecasts for worldwide basins. The predictors are related to regional oscillations in the global climate system: the
Walker circulation The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical dir ...
which is related to the
El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea te ...
; the
North Atlantic oscillation The North Atlantic Oscillation (NAO) is a weather phenomenon over the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. Through fluctuations in the ...
(NAO); the
Arctic oscillation 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 s ...
(AO); and the Pacific North American pattern (PNA).


See also

*
Invest (meteorology) An invest in meteorology (short for ''investigative area'', alternatively written INVEST) is a designated area of disturbed weather that is being monitored for potential tropical cyclone development. Invests are designated by three separate United ...
*
Monsoon trough The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific,Bin WangThe Asian Monsoon.Retrieved 2008-05-03. as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, ...
*
Tropical cyclone forecasting Tropical cyclone forecasting is the science of forecasting where a tropical cyclone's center, and its effects, are expected to be at some point in the future. There are several elements to tropical cyclone forecasting: track forecasting, intensi ...


References


External links


Current AO conditionsCurrent ENSO conditionsCurrent MJO conditionsCurrent NAO conditionsCurrent PNA conditionsTropical Cyclone Heat Potential
{{DEFAULTSORT:Tropical Cyclogenesis Tropical cyclone meteorology