The 2019–20 South-West Indian Ocean cyclone season was a slightly above-average season in tropical cyclone and subtropical cyclone formation west of 90°E. The season officially began on 15 November, however, the formation of the first system—Zone of Disturbed Weather 01—occurred on 22 July 2019, well before the official start of the season. This was the earliest start to a season since the 2016–17 season. The season then officially ended on 30 April 2020, with the exception of Mauritius and the Seychelles, for which it officially ended on 15 May 2020. These dates conventionally delimit the period of each year when most tropical and subtropical cyclones form in the basin, which is west of 90°E and south of the Equator. Tropical and subtropical cyclones in this basin are monitored by the Regional Specialized Meteorological Centre in Réunion.
For the second consecutive year in a row, the first system formed before the official start of the season. Afterwards, three storms formed in the month of December: Tropical Cyclone Belna on 2 December, which brought flooding and strong wind to northwestern Madagascar, Very Intense Tropical Cyclone Ambali, which formed a day later and became the first very intense tropical cyclone in the basin since Fantala in 2016, and Tropical Cyclone Calvinia which stalled near Mauritius and brought heavy rain and moderate flooding to the island nation towards the end of the month, crossing into 2020. The rest of the year would remain quite weak until Intense Tropical Cyclone Herold formed in March, becoming the first major cyclone of 2020. Just weeks after, Intense Tropical Cyclone Irondro in April would become the third intense tropical cyclone of the season. The season would later conclude with the dissipation of Moderate Tropical Storm Jeruto on 16 April. The season had little impact on land with the fact that all storms that formed with the exception of Belna, Diane and Francisco did not make any direct impacts on land and stayed out to sea. __TOC__
Seasonal forecasts The season began with one of the strongest positive phases of the Indian Ocean Dipole (IOD) in the past 40 years. Météo-France Réunion expected that the positive anomalies would decay slowly throughout austral summer, having a prominent impact on the cyclone season through mid-February. As a result, storm activity was not forecast to begin until December—the month in which the monsoonal flow becomes established in the western half of the basin (the dry conditions in the central and eastern Indian Ocean induced by the IOD would prevent typical early-season storms). With warm and wet conditions expected in the western Indian Ocean, cyclone formation was expected to be enhanced west of 70°E.
A near-average eight to eleven storms were expected throughout the course of the season, with the possibility of a higher than average number of these attaining Tropical Cyclone strength with winds of or greater as a result of favorable upper-level divergence in the western region of the basin. Activity was expected to be focused west of the Chagos Archipelago, presenting an increased threat to land. A wide range of track types and motions were expected, although predominantly southerly storm motion were expected to be favored.
In November, the Mauritius Meteorological Services forecasted eight to ten named storms and emphasized that storm formation would be more likely west of Diego Garcia.
Seasonal summary
On 22 July, Zone of Disturbed Weather 01 formed and started the cyclone season. The disturbance failed to organise into a tropical depression and dissipated on 25 July.
Tropical activity came to a halt until 2 December when Tropical Disturbance 02 formed. A day later, Tropical Depression 03 formed and was later named Ambali. On 5 December, Tropical Storm Ambali rapidly intensified, becoming the first very intense tropical cyclone since Fantala in 2016. Tropical Disturbance 02 then became Belna, and Belna continued to intensify into a Severe Tropical Storm. Belna then intensified into a cyclone, equivalent to a category 3 hurricane before making landfall in northwestern Madagascar as a category 2 hurricane-equivalent storm, leaving significant damages and 9 deaths. In late December, tropical storm Calvinia formed. It persisted into 2020. Activity was quiet for three weeks. On 19 January, a tropical depression formed, but soon dissipated without being named.
Two tropical storms formed in late January, Diane and Esami. They also both dissipated without reaching tropical cyclone status. In early February, another tropical depression formed. It eventually intensified into a tropical storm and was named Francisco. Francisco degenerated into a remnant low several days after formation. In mid-February, the storm regenerated into a tropical depression. At the same time, another disturbance formed, and would strengthen into Tropical Storm Gabekile. It wasn't long until Gabekile strengthened into a tropical cyclone. However, it quickly weakened. By 19 February, Gabekile had dissipated. Nearly a month later, Moderate Tropical Storm Herold formed. On 14 March, Herold strengthened into a severe tropical storm. Herold then rapidly intensified to become the season's 2nd Intense Tropical Cyclone. Shortly after, it rapidly weakened as it moved southwards before dissipating. In early April, Moderate tropical storm Irondro formed. Irondro was also an intense tropical cyclone, but it dissipated some time later, on 6 April. A tropical low crossed over on 12 April, and was named Jeruto. However, Jeruto rapidly entered unfavorable conditions, and quickly dissipated on 16 April.
Systems Zone of Disturbed Weather 01
In mid-July, a broad wind circulation developed over the central Indian Ocean, accompanied by deep convection. Some computer models suggested the possibility of a southern segment of this system organising into a tropical cyclone; Météo-France (MFR) initially estimated a "very low" chance of a moderate tropical storm materialising from the large circulation near Diego Garcia. Due to strong wind shear, the environment remained unfavourable for tropical development. On 22 July, Météo-France began monitoring the system as a zone of disturbed weather; the system's forward motion was initially southward. A temporary decrease in wind shear on 23 July provided a brief period conducive for tropical development, and the system strengthened to its peak winds of that day. However, the disturbance's centre of circulation remained ill-defined and lacking showers and thunderstorms. Although the disturbance was forecast to initially strengthen into a tropical storm, an increase in wind shear prevented the storm from consolidating further about the centre of circulation and caused the overall wind field to disorganise. The system curved towards the west on 24 July and eventually degenerated into a remnant circulation northeast of Rodrigues by the 25 July; these remnants persisted for another day before dissipating entirely.
Tropical Cyclone Belna
Météo-France began highlighting the potential for tropical cyclone development in their daily bulletins on 25 November, noting an increase in shower activity west of the Seychelles. Aided by the passage of a Kelvin wave and a favourable window in the Madden–Julian oscillation, a broad trough of low pressure began to take shape within the storm activity, extending across the equator. Projections from computer models remained in disagreement over the system's future, complicated by the concurrent development of a tropical disturbance in the northwestern Indian Ocean along the same trough. A loosely defined wind circulation was detected south of Mahe, Seychelles on 29 November, offset from convection. Over the following days, this circulation tightened within an environment moderately conducive for tropical development. Météo-France declared the system as a zone of disturbed weather on 2 December; at the time the system had drifted west from its point of origin. Although the storm was better organised and the environment conducive for intensification, the storm's wind field initially remained elongated and rainfall remained north of the storm's centre. The disturbance become a tropical depression on 5 December, attended by an increase in rainbands and the return of convection at the center of circulation; at 18:00 UTC that day, the system was upgraded to Moderate Tropical Storm Belna. Belna was upgraded further to severe tropical storm status early on 6 December. Around the same time, a cloud-obscured eye briefly became apparent in microwave satellite imagery. Due to a strengthening area of high pressure to its east, Belna began to curve from its initial westward drift to a more directed southwestward trajectory. After a brief period of strengthening, Belna's central dense overcast remained largely unchanged throughout 6 December before signs of resumed intensification emerged by the day's end, followed by the development of another eye. With the storm's eye becoming better defined, MFR upgraded Belna to a tropical cyclone early on 7 December. Hot towers were detected atop and within the storm's radius of maximum winds, suggesting the onset of a more accelerated rate of intensification. On 9 December, Belna made landfall near Mayotte and soon began to rapidly weaken, with the winds dropping below tropical-storm-force on the next day. The system dissipated late on 11 December over Haute Matsiatra.
Météo Madagascar first issued green alerts for the Madagascan districts of Diana, Sava, and Sofia on 4 December based on a high probability of Belna impacting northeastern Madagascar. Accordingly, cyclone response measures were activated by the National Office for Risk and Disaster Management and humanitarian organisations across northern Madagascar. Green alerts were later extended to encompass five districts. A cyclone pre-alert was issued for Mayotte on 6 December, succeeded by an orange alert the following day. Civil security personnel from mainland France and Reunion, some from the National Gendarmerie, were sent to Mayotte to aid storm preparation efforts there. Shelters were opened in several Mayotte communes on 7 December. The National Office for Risk and Disaster Management and 11 humanitarian agencies were active in northern Madagascar by 9 December. Belna's effects in Mayotte were minimal as the storm passed to the east. The worst effects of Belna in Madagascar occurred in Soalala, where the storm made landfall. The roofs of 80% of residences and government buildings in the city were damaged by Belna's winds. Damage to homes displaced 1,400 people in Soalala and another 900 throughout Madagascar. Extensive flooding also affected both Soalala and Antsiranana. Belna's impacts in Madagascar killed nine people and caused at least US$25 million in economic losses.
Very Intense Tropical Cyclone Ambali