Subtropical Storm Potira

South Atlantic tropical cyclones are unusual weather events that occur in the Southern Hemisphere. Strong wind shear, which disrupts the formation of cyclones, as well as a lack of weather disturbances favorable for development in the South Atlantic Ocean, make any strong tropical system extremely rare, and Hurricane Catarina in 2004 is the only recorded South Atlantic hurricane in history. Storms can develop year-round in the South Atlantic, with activity peaking during the months from November through May. Since 2011, the Brazilian Navy Hydrographic Center has assigned names to tropical and subtropical systems in the western side of the basin, near the eastern coast of Brazil, when they have sustained wind speeds of at least , the generally accepted minimum sustained wind speed for a disturbance to be designated as a tropical storm in the North Atlantic basin. Below is a list of notable South Atlantic tropical and subtropical cyclones.

Theories concerning infrequency of occurrence

It was initially thought that tropical cyclones did not develop within the South Atlantic. Very strong vertical wind shear in the troposphere is considered a deterrent. The Intertropical Convergence Zone drops one to two degrees south of the equator, not far enough from the equator for the Coriolis force to significantly aid development. Water temperatures in the tropics of the southern Atlantic are cooler than those in the tropical north Atlantic.

Although they are rare, during April 1991 the United States' National Hurricane Center (NHC) reported that a tropical cyclone had developed over the Eastern South Atlantic. In subsequent years, a few systems were suspected to have the characteristics needed to be classified as a tropical cyclone, including in March 1994 and January 2004. During March 2004, an extratropical cyclone formally transitioned into a tropical cyclone and made landfall on Brazil, after becoming a Category 2 hurricane on the Saffir–Simpson hurricane wind scale. While the system was threatening the Brazilian state of Santa Catarina, a newspaper used the headline "Furacão Catarina", which was originally presumed to mean "furacão (hurricane) threatening (Santa) Catarina (the state)". After international presses started monitoring the system, "Hurricane Catarina" has formally been adopted.

At the Sixth WMO International Workshop on Tropical Cyclones (IWTC-VI) in 2006, it was questioned if any subtropical or tropical cyclones had developed within the South Atlantic before Catarina. It was noted that suspect systems had developed in January 1970, March 1994, January 2004, March 2004, May 2004, February 2006, and March 2006. It was also suggested that an effort should be made to locate any possible systems using satellite imagery and synoptic data; however, it was noted that this effort may be hindered by the lack of any geostationary imagery over the basin before 1966. A study was subsequently performed and published during 2012, which concluded that there had been 63 subtropical cyclones in the Southern Atlantic between 1957 and 2007. During January 2009, a subtropical storm developed in the basin, and in March 2010, a tropical storm developed, which was named Anita by the Brazilian public and private weather services. In 2011, the Brazilian Navy Hydrographic Center started to assign names to tropical and subtropical cyclones that develop within its area of responsibility, to the west of 20°W, when they have sustained wind speeds of at least .

Known storms and impacts

Pre-2010s

1991 Angola tropical storm

A low-pressure area formed over the Congo Basin on 9 April. The next day it moved offshore northern Angola with a curved cloud pattern. It moved westward over an area of warm waters while the circulation became better defined. According to the United States National Hurricane Center, the system was probably either a tropical depression or a tropical storm at its peak intensity. On 14 April, the system rapidly dissipated, as it was absorbed into a large squall line. This is the only recorded tropical cyclone in the eastern South Atlantic.

Hurricane Catarina

Hurricane Catarina was an extraordinarily rare hurricane-strength tropical cyclone, forming in the southern Atlantic Ocean in March 2004. Just after becoming a hurricane, it hit the southern coast of Brazil in the state of Santa Catarina on the evening of 28 March, with winds estimated near making it a Category 2-equivalent cyclone on the Saffir–Simpson hurricane scale. Catarina killed 3 to 10 people and caused millions of dollars in damage in Brazil.

At the time, the Brazilians were taken completely by surprise, and were initially skeptical that an actual tropical cyclone could have formed in the South Atlantic. Eventually, however, they were convinced, and adopted the previously unofficial name "Catarina" for the storm, after Santa Catarina state. This event is considered by some meteorologists to be a nearly once-in-a-lifetime occurrence.

2010s

Tropical Storm Anita

On 8 March 2010, a previously extratropical cyclone developed tropical characteristics and was classified as a subtropical cyclone off the coast of southern Brazil. The following day, the United States Naval Research Laboratory began monitoring the system as a system of interest under the designation of ''90Q''. The National Hurricane Center also began monitoring the system as Low SL90. During the afternoon of 9 March, the system had attained an intensity of and a barometric pressure of 1000 hPa (mbar). It was declared a tropical storm on 10 March and became extratropical late on 12 March. Anita's accumulated cyclone energy was estimated at 2.0525 by the Florida State University. There was no damage associated to the storm, except high sea in the coasts of Rio Grande do Sul and Santa Catarina. Post mortem, the cyclone was given the name "Anita" by private and public weather centers in Southern Brazil.

Subtropical Storm Arani

Early on 14 March 2011, the Navy Hydrographic Center- Brazilian Navy (SMM), in coordination with the National Institute of Meteorology, were monitoring an organizing area of convection near the southeast coast of Brazil. Later that day a low-pressure area developed just east of Vitória, Espírito Santo, and by 12:00  UTC, the system organized into a subtropical depression, located about east of Campos dos Goytacazes. Guided by a trough and a weak ridge to its north, the system moved slowly southeastward over an area of warm waters, intensifying into Subtropical Cyclone Arani on 15 March, as named by the Brazilian Navy Hydrographic Center. The storm was classified as subtropical, as the convection was east of the center. On 16 March, Arani began experiencing of wind shear because another frontal system bumped it from behind.

Before it developed into a subtropical cyclone, Arani produced torrential rains over portions of southeastern Brazil, resulting in flash flooding and landslides. Significant damage was reported in portions of Espírito Santo, though specifics are unknown. Increased swells along the coast prompted ocean travel warnings.

Subtropical Storm Bapo

On 5 February 2015, a subtropical depression developed about to the southeast of São Paulo, Brazil. During the next day, low-level baroclincity decreased around the system, as it moved southeastwards away from the Brazilian coast and intensified further. The system was named Bapo by the Brazilian Navy Hydrography Center during 6 February, after it had intensified into a subtropical storm. Over the next couple of days the system continued to move south-eastwards before it transitioned into an extratropical cyclone during 8 February.

Subtropical Storm Cari

On 10 March 2015, the Hydrographic Center of the Brazilian Navy began issuing warnings on Subtropical Depression 3 during early afternoon, while the Center for Weather Forecast and Climatic Studies (CPTEC in Portuguese) already assigned the name ''Cari'' for the storm. At 00:00 UTC on 11 March, the Hydrographic Center of the Brazilian Navy upgraded Cari to a subtropical storm, also assigning a name to it. On 12 March, the Brazilian Hydrographic Center downgraded Cari to a subtropical depression, while the CPTEC stated that the storm had become a "Hybrid cyclone". During early afternoon of 13 March, the Brazilian Navy declared that Cari became a remnant low.

Cari brought heavy rainfall, flooding and landslides to eastern cities of Santa Catarina and Rio Grande do Sul states. Rain totals from were observed associated with the storms and wind topped in Cabo de Santa Marta. A Navy buoy registered a wave off the coast of Santa Catarina.

Subtropical Storm Deni

A subtropical depression formed southwest of Rio de Janeiro on 15 November 2016. It intensified into a subtropical storm and received the name ''Deni'' on 16 November. Moving south-southeastwards, Deni soon became extratropical shortly before 00:00 UTC on 17 November.

Subtropical Storm Eçaí

An extratropical cyclone entered the South Atlantic Ocean from Santa Catarina early on 4 December 2016. Later, it intensified quickly and then transitioned into a subtropical storm shortly before 22:00 BRST (00:00 UTC on 5 December), with the name ''Eçaí'' assigned by the Hydrographic Center of the Brazilian Navy. Eçaí started to decay on 5 December, and weakened into a subtropical depression at around 00:00 UTC on 6 December.

Subtropical Storm Guará

According to the Hydrographic Center of the Brazilian Navy, on 9 December 2017, a subtropical storm formed over the southeastern tip of a South Atlantic Convergence Zone, close to the state border between Espírito Santo and Bahia, moving southeastwards away from land. On early 11 December, as it moved more southwardly, Guará attained its peak intensity while transitioning to an extratropical cyclone. Shortly thereafter, Guará became fully extratropical, later on the same day.

Tropical Storm Iba

A tropical depression formed within a monsoon trough on 23 March 2019, off the coast of Bahia. On the next day, the system intensified into a tropical storm, receiving the name ''Iba'' from the Brazilian Navy Hydrographic Center. After moving southwestward for a couple of days, on 26 March, Iba turned southeastward. Afterward, the storm began to weaken due to strong wind shear. On 27 March, Iba weakened into a tropical depression and turned to the east, before dissipating on 28 March.

Iba was the first tropical storm to develop in the basin since Anita in 2010, as well as the first fully tropical system to be named from the Brazilian naming list.

Subtropical Storm Jaguar

On 20 May 2019, a subtropical depression formed east of Rio de Janeiro. Later that day, the system strengthened into a subtropical storm, receiving the name ''Jaguar'' from the Brazilian Navy Hydrographic Center. However, the system did not intensify any further, as it soon encountered unfavorable conditions, and Jaguar eventually dissipated on 22 May.

2020s

Subtropical Storm Kurumí

On 21 January 2020, the Brazilian Navy Hydrographic Center began to monitor an area of persisting thunderstorms near São Paulo for potential subtropical cyclone development. Generally tracking southeastward, the system began to organize within the afternoon hours of 22 January and was designated a subtropical depression in the early hours of 23 January. Several hours later, due to a lack of wind shear, the system intensified into a subtropical storm and was given the name ''Kurumí''. After this bout of intensification, Kurumí moved southward and began to succumb to much more unfavorable conditions. It weakened back to a subtropical depression on 25 January, while also beginning to merge with a large extratropical low to its south. The last advisory was issued on Kurumí later that same day.

The front associated with Kurumí later played a role in the 2020 Brazilian floods and mudslides, dragging behind it heavy rainfall. Over 171.8 mm (6.76 in) of rain fell in the Belo Horizonte metro area on 24 January, triggering a landslide and killing 3 people and leaving 1 missing.

Subtropical Storm Mani

According to the Hydrographic Center of the Brazilian Navy, on 25 October 2020, a subtropical depression formed off the coast of the border between Espírito Santo and Bahia, at 00:00 UTC on 26 October, it was named ''Mani''. On 28 October, Mani weakened to a low pressure area.

The storm caused significant damage in Espírito Santo, with landslides of stones and earth leaving more than 400 people homeless. The storm also impacted almost the entire state of Minas Gerais and the northern region of Rio de Janeiro.

Subtropical Storm Oquira

According to the Hydrographic Center of the Brazilian Navy, late on 27 December 2020, a subtropical depression formed off the coast east of Rio Grande do Sul. Moving southwestward, the system's central pressure dropped to by 00:00 UTC on 28 December. Later that day, the system's winds intensified, and it was named ''Oquira'' by the Brazilian Hydrographic Center. On 29 December, Oquira continued to strengthen, deepening while heading further southwestward away from the Brazilian mainland, and reaching a pressure of . Afterward, Oquira's winds decreased and the storm weakened to a subtropical depression on 30 December, but the storm's pressure continued to drop, bottoming out at a minimum central pressure of 998 hPa (29.47 inHg). On 31 December, Oquira transitioned into an extratropical low, and the Hydrographic Center issued their final advisory on the storm.

Tropical Storm 01Q

On 4 February 2021, an extratropical storm off the coast of Rio Grande do Sul developed into a bomb cyclone. On 6 February, the storm began separating from its weather fronts and developed subtropical characteristics, before fully separating from the frontal zone and transitioning into a fully-tropical storm later that day. As a result, the NOAA classified the system as a tropical storm at 17:30 UTC, with the system being designated as ''Tropical Storm 01Q''. However, the storm was short-lived, as it lost its tropical characteristics several hours later, with the NOAA issuing their final bulletin on the storm at 23:30 UTC that day. The storm dissipated soon afterward. Although the NOAA issued bulletins on the storm, the Hydrographic Center of the Brazilian Navy did not monitor it.

Subtropical Storm Potira

A low south of Rio de Janeiro transitioned into a subtropical depression on 19 April 2021.
On 20 April 2021, the Brazilian Navy raised the category of this system to a subtropical storm and it was named ''Potira''. The storm caused a gale in the Copacabana fort and the gusts of wind went over . In the municipalities of Balneário Camboriú and Florianópolis (SC), the hangover caused by Potira caused flooding in the streets and damage to the sidewalks. The ports of Itajaí and Navegantes were closed for 3 days. No economic or material damage caused by the cyclone has been reported. On 25 April, the Brazilian Navy downgraded it to a low-pressure area.

Subtropical Storm Raoni

An extratropical cyclone about east-southeast of Montevideo, Uruguay started to acquire subtropical characteristics while occluding on 28 June 2021. Early the next day, the system transitioned into a subtropical storm while moving northeastwards; it remained unnamed due to it being outside of the Brazilian Navy's area of responsibility. It continued to intensify overnight, gaining winds of in the system's southern portion, while tracking towards the region's area of authority. By 23:30 UTC on the same day, the Satellite Products and Services Division of the NESDIS declared the system to have become a tropical storm, based on a Dvorak rating of 3.5. At around 12:00 UTC on the next day, as the storm entered the boundary of METAREA V, which the Brazilian Navy is responsible for, the name ''Raoni'' was then assigned to the system. Continuing its trajectory towards the northeast, Raoni further developed an eye feature as well as a robust band to the east of the system. Raoni began to weaken by 30 June, with winds eventually decreasing to around 85 km/h (50 mph). On 2 July, Raoni lost its subtropical characteristics and degenerated into a low-pressure area.

The predecessor extratropical cyclone of Raoni caused heavy rains and strong winds gust up to , downing trees and causing damages to different public and private establishments across Punta del Este. The area's waters were also rough due to the storm. Downpours with continuous gales were also experienced in Uruguay's capital Montevideo. From 24 June to 2 July, Raoni channeled cold air from Antarctica into portions of South America, leading to an unusually potent cold wave across Argentina, Uruguay, Paraguay, Bolivia, and Brazil, with the temperature dropping as much as 15 °C (27 °F) below average in some areas. The combination of the cyclone and the cold wave also produced snowfall across the southern portion of South America, with snowfall observed as far north as southern Brazil.

Subtropical Storm Ubá

On 7 December 2021 a cyclone with subtropical characteristics developed off the coast of Rio de Janeiro. It formed as an extratropical cyclone within the South Atlantic Convergence Zone, which caused heavy rains in Minas Gerais, Espírito Santo and southern Bahia, where heavy precipitation accumulated in Itamaraju and in Monte Formoso, killing fifteen people.
On 10 December 2021, according to the Brazilian Navy, the system transitioned into a subtropical depression. On the morning of the same day, the system was upgraded to subtropical storm status, receiving the name ''Ubá''. Gradually weakening, the system was downgraded to depression status on 12 December, degenerating into a low-pressure area on the next day.

Subtropical Storm Yakecan

On 15 May 2022, an extratropical cyclone moved through the southern region of Brazil and stopped offshore. The low made a retrograde movement and obtained subtropical characteristics, according to Centro de Hydrografia da Marinha (the CHM). On the morning of 17 May 2022, the cyclone transitioned into a subtropical storm, and was given the name ''Yakecan''. During its trajectory, the storm caused snow in the Gaúcha and Catarinense Mountains, setting record lows for this time of year. The cyclone lost its subtropical characteristics and was downgraded late on 19 May 2022 to a low-pressure area.

Two people died in Uruguay and Brazil due to the passage of the cyclone. Yakecan is the first storm to be given the last name from the regular naming list, which has been in use since 2011.

Other systems

Pre-2004

According to a presentation at the Sixth WMO International Workshop on Tropical Cyclones (IWTC-VI), satellite imagery from January 1970 showed that a system with an eyewall had developed behind a cold front and that the system needed further analysis to determine if it was tropical or subtropical. On 27 March 1974, a weak area of low pressure that had originated over the Amazon River started to intensify further. Over the next 48 hours the system quickly developed further and was classified as subtropical, as it developed a banding structure and deep convection near its warm core. On 29 March, a north-westerly flow encroached on the systems environment, which caused the system to rapidly move towards 40S and the cold waters that were present to the south of 40°S.

In March 1994, a system that was thought to be weaker than Catarina was spawned but was located over cool and open waters. According to the Zambia Meteorological Department, Cyclone Bonita moved off the coast of Angola and entered the South Atlantic Ocean on 19 January 1996. By the next day, the system had succumbed to cold waters and days of land interaction, dissipating completely. It was the first tropical cyclone known to have traversed southern Africa from the South-West Indian Ocean to the South Atlantic.

2004–2009

During 2004, the large-scale conditions over the South Atlantic were more conducive than usual for subtropical or tropical systems, with 4 systems noted. The first possible tropical cyclone developed within a trough of low pressure, to the southeast of Salvador, Brazil on 18 January. The system subsequently displayed a small central dense overcast (CDO) and was suspected to be at the peak of its development as either a tropical depression or a tropical storm during the next day. The system was subsequently affected by some strong shear, before it moved inland and weakened along the coast of Brazil before it was last noted during 21 January. Within Brazil the system caused heavy rain and flooding with a state of emergency declared in Aracaju, after the river overflowed and burst its banks which flooded homes, destroyed crops and caused parts of the highway to collapse. However, it was noted that not all of the heavy rain and impacts were attributable to the system, as a large monsoon low covered much of Brazil at the time. The second system was a possible hybrid cyclone that developed near south-eastern Brazil between 15 and 16 March. Hurricane Catarina was the third system, while the fourth system formed off the coast of Brazil on 15 May 2004.

On 22 February 2006, a baroclinic cyclone intensified quickly and was estimated to have peaked with 1-minute sustained wind speeds of , after radar data showed that the system had developed an eye and banding. However, there were questions about how tropical the system was, as it did not separate from the westerlies or the baroclinic zone it was in. Between 11 and 17 March 2006, another system with a warm core developed and moved southward along the South Atlantic Zone, before dissipating.

Two subtropical cyclones affected both Uruguay and Rio Grande do Sul state in Brazil between 2009 and 2010. On 28 January 2009, a cold-core mid to upper-level trough in phase with a low-level warm-core low formed a system and moved eastward into the South Atlantic. The storm produced rainfall in 24 hours of or more in some locations of Rocha (Uruguay) and southern Rio Grande do Sul. The weather station owned by MetSul Weather Center in Morro Redondo, Southern Brazil, recorded in a 24-hour period. The storm caused fourteen deaths and the evacuation of thousands, with an emergency declared in four cities. It lasted until 1 February, when the cyclone became extratropical.

2010–2016

On 16 November 2010, a cold-core mid to upper-level trough in phase with a low-level warm-core low developed a low-pressure system over Brazil, and moved southeastward into the South Atlantic, where it slightly deepened. The system brought locally heavy rains in southern Brazil and northeast of Uruguay that exceeded 200 millimeters within a few hours, in some locations of Southern Rio Grande do Sul, northwest of Pelotas. Damages and flooding were observed in Cerrito, São Lourenço do Sul and Pedro Osório. Bañado de Pajas, departament of Cerro Largo in Uruguay, recorded of rain. The subtropical cyclone then became a weak trough on 19 November, according to the CPTEC.

Between 23 December 2013 and 24 January 2015, the CPTEC and Navy Hydrography Center monitored four subtropical depressions to the south of Rio de Janeiro. The first one lasted until Christmas Day, 2013. Two subtropical depressions formed in 2014: one in late-February 2014 and the other in late-March 2014. A fourth one formed in late January 2015.

On 5 January 2016, the Hydrographic Center of the Brazilian Navy issued warnings on a subtropical depression that formed east of Vitória, Espírito Santo. On the next day, the system strengthened into a tropical depression, and other agencies considered the system an invest, designating it as ''90Q''; however, on 7 January, the tropical depression dissipated.

2021–present

On 3 January 2021, according to the Météo-France, the remnants of Tropical Storm Chalane from the South-West Indian Ocean crossed southern Africa and briefly emerged into the eastern South Atlantic before dissipating.

On 14 February 2021, according to the Brazilian Navy, a subtropical depression formed about off the coast of the state of Rio Grande do Sul. For the next few days, the storm slowly meandered southeastward and then southwestward, until it lost its subtropical characteristics over high seas on 17 February.

Storm names

The following names are published by the Brazilian Navy Hydrographic Center's Marine Meteorological Service and used for tropical and subtropical storms that form in the area west of 20ºW and south of equator in the South Atlantic Ocean. Originally announced in 2011, the list has been extended from ten to fifteen names in 2018. The names are assigned in alphabetical order and used in rotating order without regard to year. The names of significant tropical or subtropical systems will be retired. All regular names were exhausted in 2022.

Retirements

''Kamby'' was replaced by ''Kurumí'' in 2018 without being used.

Climatological statistics

There have been 77 recorded tropical and subtropical cyclones in the South Atlantic Ocean since 1957. Like most southern hemisphere cyclone seasons, most of the storms have formed between November and May.

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See also

* Unusual areas of tropical cyclone formation
* List of South America hurricanes
* Southern Hemisphere tropical cyclone
* Atlantic hurricane (North Atlantic tropical cyclone)
* Atlantic Equatorial mode
* Mediterranean tropical-like cyclone
* Tropical cyclone basins
* 2006 Central Pacific cyclone
* 1996 Lake Huron cyclone
* Subtropical Cyclone Katie
* Subtropical Cyclone Lexi

References

External links

Brazilian Navy Hydrography Center – Marine Meteorological Service




CIMSS Satellite Blog: "Another tropical cyclone in the South Atlantic Ocean?"


* ttps://www.youtube.com/watch?v=FJNNMHoh4-U Satellite animation of the 1991 Angola tropical storm*

{{DEFAULTSORT:South Atlantic Tropical Cyclone

Tropical cyclone meteorology
Climate of Brazil