Tropical Cyclogenesis

Tropical cyclogenesis is 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 temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

Tropical cyclogenesis requires six main factors: sufficiently warm sea surface temperatures (at least ), atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing 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. Of those, 47 reach strength higher than , and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson 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 sustain a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear. 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. 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 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, 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 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 that maintained itself over cooler waters was Epsilon 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. This does not take into account vertical wind shear.

Coriolis force

A minimum distance of from the equator (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 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 Convergence Zone (ITCZ), 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. Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone 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 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, 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. 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 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 after recurving polewards, and typically become fully extratropical after reaching 45–50˚ of latitude. The majority of extratropical cyclones 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 and a lack of tropical disturbances from the Intertropical Convergence Zone (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, Hurricane Catarina, 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 wind measurements.

Mediterranean and Black Seas

Storms that appear similar to tropical cyclones in structure sometimes occur in the Mediterranean Sea. 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, 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 has, on occasion, produced or fueled storms that begin cyclonic rotation, 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, and also due to unfavorable wind shear; 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, even further east than the 1983 tropical depression. This system was unofficially dubbed ''Katie'' by researchers.Katie