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

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

Peaks of activity worldwide
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

Global Tropical Cyclone Tracks between
1985 and 2005, indicating the areas where tropical cyclones usually
develop
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

Loop of SST anomalies in the Tropical
Pacific
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.

5-day running mean of MJO.
Note how it moves eastward with time.
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
References
- Chris
Landsea. Subject: A15) How do tropical cyclones form ?
Retrieved on 2008-06-08.
- Chris
Landsea. Subject: A15) How do tropical cyclones form ?
Retrieved on 2008-06-08.
- University of Illinois. Hurricanes. Retrieved 2008-08-17.
- Joint Typhoon Warning Center.
Cyclone Agni. Retrieved on 2008-06-08.
- MetOffice. Miscellaneous Images. Retrieved on
2007-05-11.
External links