Tropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which mid-latitude cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment. There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.

Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins. Climate cycles such as ENSO and the Madden-Julian Oscillation 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, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale).

Requirements for tropical cyclone formation

There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear. These conditions are necessary for tropical cyclone formation, but 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 the special mesocyclone that is the tropical cyclone. These warm waters are needed to maintain the warm core that fuels tropical systems. This value is well above the global average surface temperature of the oceans, which is 16.1 °C (60.9 °F). However, this requirement can be considered only a general baseline because it assumes that the ambient atmospheric environment surrounding an area of disturbed weather presents average conditions.

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 certain lapse rate is required to force the atmosphere to be unstable 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.

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, roughly at the 500 hPa level, is normally a requirement for development. However, when dry air is found at the same height, the wet bulb temperature normally witnessed at 500 hPa does not promote large areas of thunderstorms due to a lack of instability. At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was -77 °C (-132 °F). Recent examples of tropical cyclones that maintained themselves over cooler waters include Delta, Epsilon, and Zeta of the 2005 Atlantic hurricane season.

Role of Maximum Potential Intensity (MPI)

Kerry Emanuel created a mathematical model 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, 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.

Coriolis force

A minimum distance of 500 km (300 miles) 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 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 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 covergence zone , a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity 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.

Weak vertical wind shear

Vertical wind shear of less than 10 m/s (20 kt, 22 mph) between the surface and the tropopause is required for tropical cyclone development. 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, halting 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.

Favorable trough interactions

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 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. It should be noted that developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet eminating 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 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. This can be explained by the greater tropical cyclone activity across the Northern hemisphere than south of the equator.

In the North Atlantic, a distinct hurricane season occurs from June 1 through November 30, sharply peaking from late August through September. 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 begins on November 1 and ends in late April. Southern Hemisphere activity peaks in mid-February to early March. Virtually all the Southern Hemisphere activity is seen from the southern African coast eastward towards South America. Tropical cyclones are rare events across the south Atlantic ocean and the southeastern Pacific ocean.

Season Lengths and Seasonal Averages
Basin	Season Start	Season End	Tropical Storms (>34 knots)	Tropical Cyclones (>63 knots)	Category 3+ Tropical Cyclones (>95 knots)
Northwest Pacific	January	December	26.7	16.9	8.5
South Indian	October	May	20.6	10.3	4.3
Northeast Pacific	May	November	16.3	9.0	4.1
North Atlantic	June	November	10.6	5.9	2.0
Australia Southwest Pacific	November	May	9	4.5	1.9
North Indian	April	December	5.4	2.2	0.4

Unusual areas of formation

For areas of unusual landfall, please see Unusual Landfalls and Tropical cyclone landfall.

Middle latitudes

Areas farther than 30-32 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 in 2004, 1988, and 1975, storms may form or strengthen in this region. Storms surviving beyond 50 degrees as a tropical cyclone are also quite rare (although it is not uncommon for a storm to become extratropical at high intensity in the high latitudes).

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. In December 2001, however, Typhoon Vamei formed in the southern South China Sea and made landfall in Malaysia. It formed from a thunderstorm formation in Borneo that moved into the South China Sea. Cyclone Agni would come as close as 50 miles from the Equator in 2004.

Southeastern Pacific

Tropical cyclone formation is rare in this region. When tropical cyclones do form, they are frequently linked to El Niño episodes. Most of the tropical cyclones that enter this region formed farther west in the Southwest Pacific. They affect the islands of Polynesia in rare instances. During the 1982/83 El Niño event, French Polynesia was affected by six tropical cyclones in five months. There are no records of a tropical cyclone hitting western South America.

South Atlantic

A combination of wind shear and a lack of tropical disturbances from the Intertropical Convergence Zone (ITCZ) makes it very difficult for the South Atlantic to support tropical activity. Three tropical cyclones have been observed here — a weak tropical storm in 1991 off the coast of Africa near Angola, Cyclone Catarina (sometimes also referred to as Aldonça), which made landfall in Brazil in 2004 at Category 2 strength, and a smaller storm in January 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometer wind measurements.

Mediterranean Sea

Storms that appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Examples of these "Mediterranean tropical cyclones" 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. However, there is debate on whether these storms were tropical in nature. The Black Sea has, on occasion, produced or fueled storms that begin cyclonic rotation, and appear to be similar to cyclones seen in the Mediterranean.

Elsewhere

Vortices have been reported off the coast of Morocco in the past. However, it is debatable if they are truly tropical in character. Tropical activity is also extremely rare in the Great Lakes. However, a storm system that appeared similar to a subtropical or tropical cyclone formed in 1996 on Lake Huron. It formed an eye-like structure in its center, and it may have briefly been a subtropical or tropical cyclone.

Influence of large-scale climate cycles

Influence of ENSO

Warm waters during the El Niño-Southern Oscillation lower the potential of tropical cyclone formation primarily in the Atlantic Basin and around Australia, but tend to cause an increase in activity in the North West Pacific Ocean. Because tropical cyclones in the northeastern Pacific and north Atlantic basins are both generated in large part by tropical waves from the same wave train, decreased tropical cyclone activity in the north Atlantic translates to increased tropical cyclone activity in the Eastern North Pacific. Although El Niño does not impact the number of tropical cyclones in the Western North Pacific, El Niño shifts their formation, as cyclones form farther 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.

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 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.

Influence of equatorial Rossby waves

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.

Seasonal forecasts

Since 1984, Colorado State University has been issuing seasonal tropical cyclone forecasts for the north Atlantic basin, with results that are better than climatology. The university has 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 the university's steps, with some organizations issuing seasonal forecasts for the northwest Pacific and the Australian region. 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 Pacific North American pattern or PNA.

See also

• Tropical cyclone forecasting
• Rossby wave
• Tropical cyclone
• Tropical wave
• Intertropical convergence zone
• Monsoon trough

References

1. Chris Landsea. Subject: A15) How do tropical cyclones form ? Retrieved on 2008-06-08.
2. Chris Landsea. Subject: A15) How do tropical cyclones form ? Retrieved on 2008-06-08.
3. University of Illinois. Hurricanes. Retrieved 2008-08-17.
4. Joint Typhoon Warning Center. Cyclone Agni. Retrieved on 2008-06-08.
5. MetOffice. Miscellaneous Images. Retrieved on 2007-05-11.

External links

• Current AO conditions
• Current ENSO conditions
• Current MJO conditions
• Current NAO conditions
• Current PNA conditions
• Maximum Potential Intensity
• Maximum Potential Intensity Maps Worldwide
• Tropical Cyclone Heat Potential