Tropical cyclogenesis#Warm waters, instability, and mid-level moisture

Image:Depth26Cisotherm.png on October 1, 2006]]

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.

{{Main|Lapse rate}}

Image:Atlantic hurricane graphic.png

Normally, an ocean temperature of {{convert|26.5|C|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.{{cite web|author=Matt Menne |publisher=National Climatic Data Center |url=http://www.ncdc.noaa.gov/oa/climate/research/anomalies/anomalies.html#means |archive-url=https://web.archive.org/web/20021219092357/http://www0.ncdc.noaa.gov/oa/climate/research/anomalies/anomalies.html |url-status=dead |archive-date=December 19, 2002 |title=Global Long-term Mean Land and Sea Surface Temperatures |date=March 15, 2000 |access-date=October 19, 2006 }}

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.{{cite web|last=Kushnir|first=Yochanan|title=The Climate System|url=http://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html|publisher=EESC|access-date=September 24, 2010|archive-date=May 20, 2020|archive-url=https://web.archive.org/web/20200520171925/https://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html|url-status=dead}}

At the 500 hPa level, the air temperature averages {{convert|-7|C|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 wet-bulb temperature at 500 hPa in a tropical atmosphere of {{convert|-13.2|C|F}} is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by {{convert|1|C|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 {{convert|-30|C|F}}, 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.{{cite book|title=Atmospheric Science: An Introductory Survey|author1=John M. Wallace |author2=Peter V. Hobbs |name-list-style=amp |pages=76–77|publisher=Academic Press, Inc|year=1977}}{{cite web| author = Chris Landsea | url = http://www.aoml.noaa.gov/hrd/Landsea/climvari/index.html | title = Climate Variability of Tropical Cyclones: Past, Present and Future | year = 2000 | work = Storms | pages = 220–41 | access-date =October 19, 2006 | publisher = Atlantic Oceanographic and Meteorological Laboratory| author-link = Chris Landsea }} At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was {{convert|-77|C|F}}.{{cite web| author = Dian J. Gaffen-Seidel, Rebecca J. Ross and James K. Angell | title = Climatological characteristics of the tropical tropopause as revealed by radiosondes | url = http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html | publisher = National Oceanic and Atmospheric Administration Air Resources Laboratory |date = November 2000| access-date =October 19, 2006 |archive-url = https://web.archive.org/web/20060508184913/http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html |archive-date = May 8, 2006}} A recent example of a tropical cyclone that maintained itself over cooler waters was Epsilon of the 2005 Atlantic hurricane season.{{cite web|author=Lixion Avila|date=December 3, 2005|title=Hurricane Epsilon Discussion Eighteen|publisher=National Hurricane Center|access-date=December 14, 2010|url=http://www.nhc.noaa.gov/archive/2005/dis/al292005.discus.018.shtml?}}

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.{{cite web| author = Kerry A. Emanuel | url = http://wind.mit.edu/~emanuel/pcmin/pclat/pclat.html | title = Maximum Intensity Estimation | publisher = Massachusetts Institute of Technology | year = 1998 | access-date =October 20, 2006}}

{{Main|Coriolis force}}

Image:Hurricane isabel and coriolis force.jpg) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows]]

A minimum distance of {{convert|500|km|mi|abbr=on}} 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{{cite web| author = Department of Atmospheric Sciences | url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/pgf.rxml | title = Pressure Gradient Force | publisher = University of Illinois at Urbana–Champaign | date=October 4, 1999|access-date =October 20, 2006}}) 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.{{cite web | author = G.P. King | url = http://www.eng.warwick.ac.uk/staff/gpk/Teaching-undergrad/es441/GradientWind.pdf | title = Vortex Flows and Gradient Wind Balance | publisher = University of Warwick | date = November 18, 2004 | access-date = October 20, 2006 | url-status = dead | archive-url = https://web.archive.org/web/20071129132804/http://www.eng.warwick.ac.uk/staff/gpk/Teaching-undergrad/es441/GradientWind.pdf | archive-date = November 29, 2007 | df = mdy-all }} 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.{{cite book|last=Kepert|first=Jeffrey D.|title=Global Perspectives on Tropical Cyclones: From Science to Mitigation|year=2010|publisher=World Scientific|location=Singapore|isbn=978-981-4293-47-1|chapter-url=http://www.worldscibooks.com/etextbook/7597/7597_chap01.pdf|archive-url=https://web.archive.org/web/20110629145312/http://www.worldscibooks.com/etextbook/7597/7597_chap01.pdf|url-status=dead|archive-date=June 29, 2011|editor=Johnny C.L. Chan, Jeffrey D Kepert|access-date=February 2, 2011|chapter=Tropical Cyclone Structure and Dynamics}}

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.{{cite journal|journal=Journal of the Atmospheric Sciences|date=June 2010|page=1745|title=Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part III: Sensitivity to Various Genesis Parameters|author1=Kieu, Chanh Q. |author2=Da-Lin Zhang |name-list-style=amp |doi=10.1175/2010JAS3227.1|volume=67|issue=6|bibcode = 2010JAtS...67.1745K |s2cid=55906577}}

{{See also|Wind shear#Effects on tropical cyclones}}

File:Paulette 2020-09-10 1620Z.jpg in 2020, with its low-level centre partially exposed due to strong windshear.]]

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. 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,{{cite web| url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/grow/home.rxml | title=Hurricanes: a tropical cyclone with winds > 64 knots | author=Department of Atmospheric Sciences (DAS) | year=1996 | publisher=University of Illinois at Urbana–Champaign | access-date=August 9, 2008}} 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 diminish to allow tropical cyclogenesis to continue.

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.{{cite web | author1 = M. E. Nicholls | author2 = R. A. Pielke | name-list-style = amp | url = http://blue.atmos.colostate.edu/publications/pdf/PPR-175.pdf | title = A Numerical Investigation of the Effect of Vertical Wind Shear on Tropical Cyclone Intensification | work = 21st Conference on Hurricanes and Tropical Meteorology of the American Meteorological Society | publisher = Colorado State University | pages = 339–41 | date = April 1995 | access-date = October 20, 2006 | url-status = dead | archive-url = https://web.archive.org/web/20060909224836/http://blue.atmos.colostate.edu/publications/pdf/PPR-175.pdf | archive-date = September 9, 2006 | df = mdy-all }} 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.{{cite web| author = Clark Evans |url = http://flhurricane.com/cyclone/showflat.php?Cat=0&Number=64429&an=0&page=0 | title = Favorable trough interactions on tropical cyclones | publisher = Flhurricane.com | date = January 5, 2006 | access-date =October 20, 2006}}{{Cite journal|author1=Deborah Hanley |author2=John Molinari |author3=Daniel Keyser |name-list-style=amp | title = A Composite Study of the Interactions between Tropical Cyclones and Upper-Tropospheric Troughs |date=October 2001 | journal = Monthly Weather Review | volume = 129 | issue = 10 | pages = 2570–84 | doi = 10.1175/1520-0493(2001)129<2570:ACSOTI>2.0.CO;2| issn = 1520-0493 | bibcode=2001MWRv..129.2570H| doi-access = free }}

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.{{cite web | author1 = Eric Rappin | author2 = Michael C. Morgan | name-list-style = amp | url = http://aurora.aos.wisc.edu/~edrappin/mesoconf.pdf | title = The Tropical Cyclone — Jet Interaction | publisher = University of Wisconsin, Madison | access-date = October 20, 2006 | url-status = dead | archive-url = https://web.archive.org/web/20060907034854/http://aurora.aos.wisc.edu/~edrappin/mesoconf.pdf | archive-date = September 7, 2006 | df = mdy-all }}

Image:WorldwideTCpeaks.gif

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.{{cite web|author=Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division |title=Frequently Asked Questions: When is hurricane season? |publisher=National Oceanic and Atmospheric Administration |access-date=July 25, 2006 |url=http://www.aoml.noaa.gov/hrd/tcfaq/G1.html |url-status=dead |archive-url=https://web.archive.org/web/20090506152735/http://www.aoml.noaa.gov/hrd/tcfaq/G1.html |archive-date=May 6, 2009 }}

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.{{Cite news|last=Kaye|first=Ken|title=Peak of hurricane season|url=http://articles.sun-sentinel.com/2010-09-09/news/fl-peak-hurricane-season-20100909_1_peak-of-hurricane-season-cape-verde-islands-seasonal-predictions|access-date=September 23, 2010|newspaper=Sun Sentinel|date=September 9, 2010|archive-date=May 10, 2012|archive-url=https://web.archive.org/web/20120510194943/http://articles.sun-sentinel.com/2010-09-09/news/fl-peak-hurricane-season-20100909_1_peak-of-hurricane-season-cape-verde-islands-seasonal-predictions|url-status=dead}} 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 occurs between early November and 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.{{cite web|author=Chris Landsea|publisher=NOAA|date=July 13, 2005|access-date=May 14, 2009|title=FAQ: Why doesn't the South Atlantic Ocean experience tropical cyclones?|url=http://www.aoml.noaa.gov/hrd/tcfaq/G6.html}}

{{TC Season Stats}}

Image:Pablo 2019-10-27 1402Z.jpg formed in the extreme northeastern Atlantic during the 2019 season.]]

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,{{cite web| publisher = National Hurricane Center | url =http://www.nhc.noaa.gov/2004alex.shtml? |title = Hurricane Alex Tropical Cyclone Report | author = James L. Franklin | date = October 26, 2004 | access-date =October 24, 2006}} Alberto in 1988,{{cite web | url = http://www.weather.unisys.com/hurricane/atlantic/1988/ALBERTO/track.dat | title = Alberto "Best-track" | publisher = Unisys | access-date = March 31, 2006 | archive-url = https://web.archive.org/web/20080131210622/http://www.weather.unisys.com/hurricane/atlantic/1988/ALBERTO/track.dat | archive-date = January 31, 2008 | url-status = dead | df = mdy-all }} and the 1975 Pacific Northwest hurricane,{{cite web | url = http://www.weather.unisys.com/hurricane/e_pacific/1975/12/track.dat | title = 12" "Best-track | publisher = Unisys | access-date = March 31, 2006 | archive-url = https://web.archive.org/web/20090131223050/http://www.weather.unisys.com/hurricane/e_pacific/1975/12/track.dat | archive-date = January 31, 2009 | url-status = dead | df = mdy-all }} 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.{{cite journal|last=Evans|first=Jenni L.|author-link1=Jenni L. Evans|author2=Hart, Robert E. |title=Objective Indicators of the Life Cycle Evolution of Extratropical Transition for Atlantic Tropical Cyclones|journal=Monthly Weather Review|date=May 2003|volume=131|issue=5|pages=911–913|doi=10.1175/1520-0493(2003)131<0909:OIOTLC>2.0.CO;2|bibcode=2003MWRv..131..909E|s2cid=3744671 }}

{{Main|List of tropical cyclones 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.{{cite journal|last1=Chang|first1=C.-P.|last2=Liu|first2=C.-H.|last3=Kuo|first3=H.-C.|title=Typhoon Vamei: An equatorial tropical cyclone formation|journal=Geophysical Research Letters|date=February 2003|volume=30|issue=3|page=1150|doi=10.1029/2002GL016365|bibcode=2003GeoRL..30.1150C|hdl=10945/36685|doi-access=free|hdl-access=free}} However, a few tropical cyclones have been observed forming within five degrees of the equator.{{cite report|publisher=Fiji Meteorological Service |title=Tropical Cyclone Guidance for Season 2010/11 for the Fiji and the Southwest Pacific |date=October 26, 2010 |access-date=May 19, 2024 |url=http://www.pacificdisaster.net/doc/FMS_2010_TC_Guide2010_2011.pdf |archive-url=https://web.archive.org/web/20240519224155/http://www.pacificdisaster.net/doc/FMS_2010_TC_Guide2010_2011.pdf |url-status=live |archive-date=May 19, 2024}}

{{Main|South Atlantic tropical cyclone}}

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.{{cite web| author = Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division | title = Frequently Asked Questions: Why doesn't the South Atlantic Ocean experience tropical cyclones? | publisher = National Oceanic and Atmospheric Administration | access-date =July 25, 2006 | url = http://www.aoml.noaa.gov/hrd/tcfaq/G7.html}}{{cite web | url = https://www.e-education.psu.edu/public/meteo/upperlevel_lows.html | author = Department of Meteorology, e-Education Institute | publisher = Pennsylvania State University | title = Upper-Level Lows | work = Meteorology 241: Fundamentals of Tropical Forecasting | access-date = October 24, 2006 | url-status = dead | archive-url = https://web.archive.org/web/20060907032236/https://www.e-education.psu.edu/public/meteo/upperlevel_lows.html | archive-date = September 7, 2006 | df = mdy-all }} At least six tropical cyclones have been observed here, including a weak tropical storm in 1991 off the coast of Africa near Angola, Hurricane Catarina in March 2004, which made landfall in Brazil at Category 2 strength, Tropical Storm Anita in March 2010, Tropical Storm Iba in March 2019, Tropical Storm 01Q in February 2021, and Tropical Storm Akará in February 2024.{{cite web|publisher=Brazilian Meteorological Service |date=March 2010 |title=Monitoramento – Ciclone tropical na costa gaúcha |url=http://www.metsul.com/blog/ |language=pt |url-status=dead |archive-url=https://web.archive.org/web/20100309131205/http://www.metsul.com/blog/ |archive-date=March 9, 2010 }}

{{Main|Mediterranean tropical-like cyclone}}

Storms that appear similar to tropical cyclones in structure sometimes occur in the Mediterranean Sea. Notable examples of these "Mediterranean tropical cyclones" include an 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, and Daniel in 2023. However, there is debate on whether these storms were tropical in nature.{{cite web| author = Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division | title = Frequently Asked Questions: What regions around the globe have tropical cyclones and who is responsible for forecasting there? | publisher = NOAA | access-date =July 25, 2006 | url = http://www.aoml.noaa.gov/hrd/tcfaq/F1.html}}

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.{{cite web|url=http://www.metoffice.gov.uk/weather/tropicalcyclone/tcimages/Misc/|title=Miscellaneous Images|publisher=Met Office|access-date=November 21, 2015|archive-url=https://web.archive.org/web/20070929102957/http://www.metoffice.gov.uk/weather/tropicalcyclone/tcimages/Misc/|archive-date=September 29, 2007 }} Two of these storms reached tropical storm and subtropical storm intensity in August 2002 and September 2005 respectively.{{cite report |first1=Pieter |last1=Groenemeijer |first2=Alois M. |last2=Holzer |title=Satellite Based Climatology of (Sub-) Tropical Cyclones in Europe |url=https://www.essl.org/ECSS/2013/programme/presentations/166.pdf |publisher=EUMETSAT |department=European Severe Storms Laboratory |access-date=January 16, 2024}}

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, 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 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.{{cite report|title=Pacific ENSO Update - Quarter 1, 1998 |url=http://www.soest.hawaii.edu/MET/Enso/peu/update.dir/Update-1stQtr98.html |url-status=live |volume=4 |issue=1 |website=Pacific ENSO Update |publisher=The Pacific ENSO Applications Climate Centre |archive-url=https://web.archive.org/web/20160304061151/http://www.soest.hawaii.edu/MET/Enso/peu/update.dir/Update-1stQtr98.html |archive-date=March 4, 2016}} 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.{{cite web|author=Diamond, Howard J|work=Climate Program Office|publisher=National Oceanic and Atmospheric Administration|date=August 25, 2015|access-date=October 16, 2017|title=Review of the 2014/15 Tropical Cyclone Season in the Southwest Pacific Ocean Basin|url=http://cpo.noaa.gov/News/News-Article/ArtMID/6226/ArticleID/476/Review-of-the-201415-Tropical-Cyclone-Season-in-the-Southwest-Pacific-Ocean-Basin}} Another subtropical cyclone was identified at 77.8 degrees longitude west in May 2018, just off the coast of Chile.{{Cite news|url=https://weather.com/storms/hurricane/news/2018-05-08-subtropical-cyclone-chile|title=Extremely Rare Southeast Pacific Subtropical Cyclone Forms Off the Chilean Coast|author=Jonathan Belles|work=The Weather Channel|date=May 9, 2018|access-date=May 10, 2018}} This system was unofficially named Lexi by researchers.{{cite web|url=http://www.australiasevereweather.com/cyclones/2018/trak1805.htm|title=Monthly Global Tropical Cyclone Tracks - May 2018|author=Steve Young|publisher=Australia Severe Weather|date=July 5, 2018|access-date=September 3, 2018}} A subtropical cyclone was spotted just off the Chilean coast in January 2022, named Humberto by researchers.{{Cite web|url=https://www.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=fxsa21&version=2&fmt=reg|title=South American Forecast Discussion|date=January 12, 2022|access-date=January 15, 2022|website=Weather Prediction Center|archive-url=https://archive.today/20220115193506/https://www.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=fxsa21&version=2&fmt=reg|archive-date=January 15, 2022|url-status=live}}{{Cite web|url=https://www.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=fxsa21|title=South American Forecast Discussion|date=January 13, 2022|access-date=January 15, 2022|website=Weather Prediction Center|archive-url=https://archive.today/20220115193252/https://www.wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=fxsa21|archive-date=January 15, 2022|url-status=live}}

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 September 1996 over Lake Huron. The system developed an eye-like structure in its center, and it may have briefly been a subtropical or tropical cyclone.{{Cite journal| title = Hurricane Huron | journal = Bulletin of the American Meteorological Society | pages = 223–36 |author1=Todd Miner |author2=Peter J. Sousounis |author3=James Wallman |author4=Greg Mann |name-list-style=amp | volume = 81 | issue = 2 |date=February 2000 | doi = 10.1175/1520-0477(2000)081<0223:HH>2.3.CO;2 |bibcode = 2000BAMS...81..223M |doi-access = free }}

{{main|Brown ocean effect}}

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. 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.

{{See also|Tropical cyclones and climate change}}

File:Sstaanim.gif (SST) anomalies in the Tropical Pacific]]

File:ENSO effects on Hurricane activity.jpg

{{Main|El Niño–Southern Oscillation}}

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 (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.{{cite web|url=https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-8-3.html|title=Climate Change 2007: Working Group I: The Physical Science Basis|year=2007|publisher=IPCC|access-date=October 9, 2017|archive-url=https://web.archive.org/web/20181102212836/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-8-3.html|archive-date=November 2, 2018|url-status=dead}}

Tropical cyclones in the northeastern Pacific and north Atlantic basins are both generated in large part by tropical waves from the same wave train.{{cite journal|last=Avila|first=Lixion A.|author2=Pasch, Richard J. |title=Atlantic Tropical Systems of 1993|journal=Monthly Weather Review|date=March 1995|page=893|doi=10.1175/1520-0493(1995)123<0887:ATSO>2.0.CO;2|issn=1520-0493|volume=123|issue=3|bibcode=1995MWRv..123..887A|doi-access=free}}

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 (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.{{cite web|author = Bureau of Meteorology Research Centre |publisher = Australian Bureau of Meteorology | url = http://www.bom.gov.au/bmrc/pubs/tcguide/ch5/ch5_2.htm | archive-url = https://archive.today/20121127133255/http://www.bom.gov.au/bmrc/pubs/tcguide/ch5/ch5_2.htm | url-status = dead | archive-date = November 27, 2012 | title = ENSO Relationships with Seasonal Tropical Cyclone Activity | work = Global Guide to Tropical Cyclone Forecasting | access-date =October 20, 2006}} 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.{{Cite journal|doi=10.1175/JCLI3457.1|last=Camargo|first=Suzana J.|author2=Adam H. Sobel |title=Western North Pacific Tropical Cyclone Intensity and ENSO|journal=Journal of Climate|date=August 2005|page=2996|volume=18|issue=15|bibcode = 2005JCli...18.2996C |s2cid=14609267 |doi-access=free}} Tropical cyclogenesis in the Northwestern Pacific is suppressed west of 150°E in the year following an El Niño event.{{Cite journal|doi=10.1175/1520-0493(1985)113<0599:TCAITN>2.0.CO;2|last=Chan|first=J. C. L.|title=Tropical Cyclone Activity in the Northwest Pacific in Relation to the El Niño/Southern Oscillation Phenomenon|journal=Monthly Weather Review|date=April 1985|volume=113|issue=4|pages=599–606|issn=1520-0493|bibcode = 1985MWRv..113..599C |hdl=10945/45699|doi-access=free|hdl-access=free}}

Image:MJO 5-day running mean through 1 Oct 2006.png

{{Main|Madden–Julian oscillation}}

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.{{Cite journal|author1=John Molinari |author2=David Vollaro |name-list-style=amp | title = Planetary- and Synoptic-Scale Influences on Eastern Pacific Tropical Cyclogenesis | journal = Monthly Weather Review | volume = 128 | issue = 9 |date=September 2000 | pages = 3296–307 | doi = 10.1175/1520-0493(2000)128<3296:PASSIO>2.0.CO;2| issn = 1520-0493|bibcode = 2000MWRv..128.3296M |s2cid=9278279 | doi-access = free }} 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.{{Cite journal| doi = 10.1175/1520-0469(2001)058<2545:TMJOBD>2.0.CO;2| title = The Madden–Julian Oscillation, Barotropic Dynamics, and North Pacific Tropical Cyclone Formation. Part I: Observations |author1=Maloney, E. D. |author2=D. L. Hartmann |name-list-style=amp | journal = Journal of the Atmospheric Sciences |date=September 2001 | volume = 58 | issue = 17 | pages = 2545–2558 | issn = 1520-0469|bibcode = 2001JAtS...58.2545M | citeseerx = 10.1.1.583.3789 | s2cid = 35852730 }}

{{Main|Rossby wave}}

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.{{cite web| author = Kelly Lombardo | url = http://ams.confex.com/ams/pdfpapers/75682.pdf | title = Influence of Equatorial Rossby Waves on Tropical Cyclogenesis in the Western Pacific | publisher = State University of New York at Albany | access-date =October 20, 2006 }}

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. 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.{{cite web | author1 = Mark Saunders | author2 = Peter Yuen | name-list-style = amp | url = http://tsr.mssl.ucl.ac.uk/for_typh.html | publisher = Tropical Storm Risk | title = Tropical Storm Risk Group Seasonal Predictions | access-date = October 20, 2006 | url-status = dead | archive-url = https://web.archive.org/web/20060504024010/http://tsr.mssl.ucl.ac.uk/for_typh.html | archive-date = May 4, 2006 | df = mdy-all }} The predictors are related to regional oscillations in the global climate system: the Walker circulation which is related to the El Niño–Southern Oscillation; the North Atlantic oscillation (NAO); the Arctic oscillation (AO); and the Pacific North American pattern (PNA).{{cite web|author1=Philip J. Klotzbach |author2=Willam Gray |author3=Bill Thornson |name-list-style=amp | url = http://typhoon.atmos.colostate.edu/forecasts/2006/april2006/ | title = Extended Range Forecast of Atlantic Seasonal Hurricane Activity and U.S. Landfall Strike Probability for 2006 |publisher = Colorado State University | date = October 3, 2006 | access-date =October 20, 2006}}

• Invest (meteorology)
• Monsoon trough
• Tropical cyclone forecasting

{{Reflist|2}}

{{Good article}}

• [http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml Current AO conditions]
• [http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml Current ENSO conditions]
• [http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml Current MJO conditions]
• [http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml Current NAO conditions]
• [http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.shtml Current PNA conditions]
• [http://wind.mit.edu/~emanuel/pcmin/pclat/pclat.html Maximum Potential Intensity]
• [http://wxmaps.org/pix/hurpot.html Maximum Potential Intensity Maps Worldwide]
• [http://www.aoml.noaa.gov/phod/cyclone/data/ Tropical Cyclone Heat Potential]

{{DEFAULTSORT:Tropical Cyclogenesis}}

Category:Tropical cyclone meteorology