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Tropical cyclogenesis is the development and strengthening of atropical cyclone in theatmosphere.[1] The mechanisms through whichtropical cyclogenesis occur are distinctly different from those through whichtemperatecyclogenesis occurs. Tropical cyclogenesis involves the development of awarm-core cyclone, due to significantconvection in a favorable atmospheric environment.[2]
Tropical cyclogenesis requires six main factors: sufficiently warmsea surface temperatures (at least 26.5 °C (79.7 °F)), atmospheric instability, highhumidity in the lower to middle levels of thetroposphere, enoughCoriolis force to develop alow-pressure center, a pre-existing low-level focus or disturbance, and low verticalwind shear.[3]
Tropical cyclones tend to develop during the summer, but have been noted in nearly every month inmost basins.Climate cycles such asENSO and theMadden–Julian oscillation modulate the timing and frequency of tropical cyclone development.[4][5] Themaximum potential intensity is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path.[6]
An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach strengths higher than 119 km/h (74 mph), and 20 become intense tropical cyclones (at least Category 3 intensity on theSaffir–Simpson scale).[7]
There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, highhumidity in the lower to middle levels of thetroposphere, enoughCoriolis force to sustain a low-pressure center, a preexisting low-level focus or disturbance, and low verticalwind shear.[3] While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.[3]
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 atropical cyclone.[3] These warm waters are needed to maintain thewarm core that fuels tropical systems. This value is well above 16.1 °C (60.9 °F), the global average surface temperature of the oceans.[8]
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 level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certainlapse rate is required to force the atmosphere to beunstable 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, the required lapse rate is 9.8 °C/km.[9]
At the 500 hPa level, the air temperature averages −7 °C (19 °F) within the tropics, but air in the tropics is normally dry at this level, giving the air room towet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection. A wet-bulb temperature at 500 hPa in a tropical atmosphere of −13.2 °C (8.2 °F) is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by 1 °C (1.8 °F) 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 (−22 °F), which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of thetroposphere, 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.[10][11] At heights near thetropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77 °C (−107 °F).[12] A recent example of atropical cyclone that maintained itself over cooler waters wasEpsilon of the2005 Atlantic hurricane season.[13]
Kerry Emanuel created amathematical model around 1988 to compute the upper limit of tropical cyclone intensity based on sea surface temperature and atmospheric profiles from the latestglobal model runs. Emanuel's model is called themaximum potential intensity, or MPI. Maps created from this equation show regions where tropical storm and hurricane formation is possible, based upon thethermodynamics of the atmosphere at the time of the last model run. This does not take into account verticalwind shear.[14]
A minimum distance of 500 km (310 mi) from theequator (about 4.5 degrees from the equator) is normally needed for tropical cyclogenesis.[3] TheCoriolis 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 thepressure gradient force (the pressure difference that causes winds to blow from high to low pressure[15]) and a smallerfriction 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.[16] This is a balance condition found in mature tropical cyclones that allowslatent heat to concentrate near the storm core; this results in the maintenance or intensification of the vortex if other development factors are neutral.[17]
Whether it be a depression in theIntertropical Convergence Zone (ITCZ), atropical wave, a broadsurface front, or anoutflow boundary, a low-level feature with sufficientvorticity and convergence is required to begin tropical cyclogenesis.[3] 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.[3] Tropical cyclones can form when smaller circulations within theIntertropical Convergence Zone come together and merge.[18]
Vertical wind shear of less than 10m/s (20 kt, 22 mph) between the surface and thetropopause is favored for tropical cyclone development.[3] 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.[19] Strong wind shear can "blow" the tropical cyclone apart,[19] as it displaces the mid-level warm core from the surface circulation and dries out the mid-levels of thetroposphere, halting development. In smaller systems, the development of a significantmesoscale 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 diminish to allow tropical cyclogenesis to continue.[19]
Limited vertical wind shear can be positive for tropical cyclone formation. When an upper-leveltrough 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 betterdiffluence 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 higherminimum pressure.[20] This process is also known asbaroclinic 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.[21][22]
There are cases where large, mid-latitude troughs can help with tropical cyclogenesis when an upper-leveljet stream 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.[23]
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.[24]
In the North Atlantic, a distinct hurricane season occurs from June 1 through November 30, sharply peaking from late August through October.[24] The statistical peak of the North Atlantic hurricane season is September 10.[25] 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 Indianbasin, storms are most common from April to December, with peaks in May and November.[24]
In the Southern Hemisphere, tropical cyclone activity generally occurs between early November and April 30. Southern Hemisphere activity peaks in mid-February to early March.[24] 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.[26]
| Basin | Season start | Season end | Tropical cyclones | Refs |
|---|---|---|---|---|
| North Atlantic | June 1 | November 30 | 14.4 | [27] |
| Eastern Pacific | May 15 | November 30 | 16.6 | [27] |
| Western Pacific | January 1 | December 31 | 26.0 | [27] |
| North Indian | January 1 | December 31 | 12 | [28] |
| South-West Indian | July 1 | June 30 | 9.3 | [27][29] |
| Australian region | November 1 | April 30 | 11.0 | [30] |
| Southern Pacific | November 1 | April 30 | 7.1 | [31] |
| Total: | 96.4 | |||
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 asPablo in 2019,Alex in 2004,[32]Alberto in 1988,[33] and the1975 Pacific Northwest hurricane,[34] storms may form or strengthen in this region. Typically, tropical cyclones will undergoextratropical transition afterrecurving polewards, and typically become fully extratropical after reaching 45–50° of latitude. The majority ofextratropical cyclones tend to restrengthen after completing the transition period.[35]
Areas within approximately ten degrees latitude of the equator do not experience a significantCoriolis force, a vital ingredient in tropical cyclone formation.[36] However, a few tropical cyclones have been observed forming within five degrees of the equator.[37]
A combination ofwind shear and a lack of tropical disturbances from theIntertropical Convergence Zone (ITCZ) makes it very difficult for the South Atlantic to support tropical activity.[38][39] At least six tropical cyclones have been observed here, includinga weak tropical storm in 1991 off the coast of Africa nearAngola,Hurricane Catarina in March 2004, which made landfall in Brazil atCategory 2 strength,Tropical Storm Anita in March 2010,Tropical Storm Iba in March 2019,Tropical Storm 01Q in February 2021, andTropical Storm Akará in February 2024.[40]
Storms that appear similar to tropical cyclones in structure sometimes occur in theMediterranean Sea. Notable examples of these "Mediterranean tropical cyclones" includean unnamed system in September 1969,Leucosia in 1982,Celeno in 1995,Cornelia in 1996,Querida in 2006,Rolf in 2011,Qendresa in 2014,Numa in 2017,Ianos in 2020, andDaniel in 2023. However, there is debate on whether these storms were tropical in nature.[41]
TheBlack Sea has, on occasion, produced or fueled storms that begincyclonic rotation, and that appear to be similar to tropical-like cyclones observed in the Mediterranean.[42] Two of these storms reached tropical storm and subtropical storm intensity in August 2002 and September 2005 respectively.[43]
Tropical cyclogenesis is extremely rare in the far southeastern Pacific Ocean, due to the cold sea-surface temperatures generated by theHumboldt Current, and also due to unfavorablewind shear; as such,Cyclone Yaku in March 2023 is the only known instance of a tropical cyclone impacting western South America. Besides Yaku, there have been several other systems that have been observed developing in the region east of120°W, which is the official eastern boundary of theSouth Pacific basin. On May 11, 1983, a tropical depression developed near110°W, which was thought to be the easternmost forming South Pacific tropical cyclone ever observed in the satellite era.[44] In mid-2015,a rare subtropical cyclone was identified in early May, slightly nearChile, even further east than the 1983 tropical depression. This system was unofficially dubbedKatie by researchers.[45]Another subtropical cyclone was identified at 77.8 degrees longitude west in May 2018, just off the coast of Chile.[46] This system was unofficially namedLexi by researchers.[47]A subtropical cyclone was spotted just off the Chilean coast in January 2022, namedHumberto by researchers.[48][49]
Vortices have been reported off the coast ofMorocco in the past. However, it is debatable if they are truly tropical in character.[42]
Tropical activity is also extremely rare in theGreat Lakes. However,a storm system that appeared similar to a subtropical or tropical cyclone formed in September 1996 overLake Huron. The system developed aneye-like structure in its center, and it may have briefly been a subtropical or tropical cyclone.[50]
In March 1977, a cyclone formed in theBering Sea with peak winds of 60 m/s (130 mph) and a measured pressure of 970 mbar (29 inHg). The storm exhibited many characteristics of tropical cyclones, included a well-defined eye, similar thermodynamics, and a symmetrical cloud structure.[51]
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 thebrown ocean effect. 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.
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 constantaccumulated cyclone energy (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.[52]
Tropical cyclones in the northeastern Pacific and north Atlantic basins are both generated in large part bytropical waves from the same wave train.[53]
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, between150°E and theInternational Date Line (IDL).[54] Coupled with an increase in activity in the North-Central Pacific (IDL to140°W) and the South-Central Pacific (east of160°E), there is a net increase in tropical cyclone development near the International Date Line on both sides of the equator.[55] 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.[56] Tropical cyclogenesis in the Northwestern Pacific is suppressed west of 150°E in the year following an El Niño event.[54]
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.[57] 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.[58]
Research has shown that trapped equatorialRossby wave packets can increase the likelihood of tropical cyclogenesis in the Pacific Ocean, as they increase the low-levelwesterly 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.[59]
Since 1984,Colorado State University has been issuing seasonal tropical cyclone forecasts for the north Atlantic basin, with results that they claim are better than climatology.[60] 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.[61] The predictors are related to regional oscillations in the global climate system: theWalker circulation which is related to theEl Niño–Southern Oscillation; theNorth Atlantic oscillation (NAO); theArctic oscillation (AO); and the Pacific North American pattern (PNA).[60]