The 1996–97 South-West Indian Ocean cyclone season was the longest on record, with both an unusually early start and unusually late ending. Most activity was from November through February. According to the Météo-France office (MFR) at Réunion, there were 21 tropical disturbances, 14 of which intensified into tropical depressions. There were 12 named storms, beginning with Antoinette and proceeding sequentially until Lisette. In addition, the Joint Typhoon Warning Center also warned on storms in the region, which identified five other tropical storms. Five of the storms attained tropical cyclone status, or with 10–minute maximum sustained winds of at least ; of these, three strengthened further into intense tropical cyclones, with Daniella and Helinda tied for strongest storm of the season.
In August, a tropical depression developed in the south-west Indian Ocean for the first time 27 years, and a month later, a rare September tropical disturbance formed. The first named storm, Antoinette, was the first of several to originate in the neighboring Australian basin, or east of 90° E; the subsequent two named storms also formed in the Australian region. In early December, Cyclone Daniella likely developed out of the remnants of previous Tropical Storm Chantelle. After reaching peak 10–minute winds of , Daniella weakened and passed just southwest of Mauritius; there, the storm left heavy crop damage and indirectly caused three deaths. In early January, Tropical Storm Fabriola was the first in a succession of three storms to move over Madagascar. The next – Cyclone Gretelle – killed 152 people when it struck southeastern Madagascar. Between January and February, Cyclone Pancho-Helinda lasted about 20 days between both the Australian and south-west Indian basins. Also in February, Tropical Storm Josie killed 36 people in western Madagascar after causing severe flooding. The final named storm was Tropical Storm Lisette, which dissipated on March 3 after striking Mozambique, killing three people. Despite the early end to the named storms, there were two additional disturbances, one of which became the first July tropical depression in 25 years.
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Season summary
During the season, the Météo-France office (MFR) based on Réunion island issued warnings in tropical cyclones within the basin, as part of its role as a Regional Specialized Meteorological Center. The agency estimated intensities via the Dvorak technique, which uses images from two satellites by the American National Oceanic and Atmospheric Administration. On certain days, the satellites had limited coverage in the eastern portion of the region, which limited warning capability. At the time, the MFR issued warnings on tropical cyclones in the Indian Ocean from the coast of Africa to 90° E, south of the equator. The Joint Typhoon Warning Center – a joint United States Navy – United States Air Force task force – also issued tropical cyclone warnings for the region. Wind estimates from Météo-France and most other basins throughout the world are sustained over 10 minutes, while estimates from the United States-based Joint Typhoon Warning Center are sustained over 1 minute. 10 minute winds are about 1.14 times the amount of 1 minute winds.
The season was active, about 50% above normal, and was the fourth consecutive above-normal season. There was a total of 100 days on which disturbances, depressions, storms, or cyclones were active, compared to the average of 68. There were 12 named storms, above the normal of 9. Five intensified into tropical cyclones, also above the normal of 4. The season was the longest on record, with the first depression in August 1996 and the final depression in July 1997. Despite the long season, there was minimal activity from March to May, which usually represents about 25% of total activity. In addition, February was relatively quiet despite being the climatological peak. Majority of the storms developed in the northeast portion of the basin, and the storms took a variety of tracks during their durations.
Systems
Tropical Depression A1 For several weeks in early August – the middle of winter in the southern hemisphere – an area of warm water temperatures of over persisted in the northeast portion of the basin. The region was unaffected by the progression of troughs that would usually decrease the water temperatures. On August 13, an area of convection persisted east of the Chagos, in association with the near-equatorial trough, later developing a low-pressure area and well-defined circulation. On August 16, the system organized into a tropical disturbance, becoming Tropical Depression A1 the next day; this made it the first such storm in the month of August since Tropical Depression Aline in August 1969. Also on August 17, the JTWC initiated advisories on Tropical Cyclone 2S. The depression moved southward, developing a well-defined central dense overcast, despite the presence of northerly wind shear. On August 18, an anticyclone turned the depression eastward; that day, the JTWC estimated peak 1–minute winds of , and the MFR estimated a peak 10–minute intensity of . The depression later began to weaken under the influence of an approaching trough, and was dissipating as it crossed into the Australian region on August 19. The JTWC tracked the circulation for another two days, estimating that the system turned sharply to the west, and the circulation dissipated on August 23.
Tropical Disturbance A2 Less than a month later, another area of convection developed within the near-equatorial trough near the Chagos archipelago, due to persistent warm water temperatures. On September 6, the MFR classified it as Tropical Disturbance A2, and the next day, the JTWC initiated advisories on Tropical Cyclone 3S while the storm was over the northeastern portion of the basin. This was a fairly unusual event, as there is only an average of 0.4 storms in the month of September across the southern hemisphere. Located along the northern side of a ridge, the storm tracked westward, but failed to intensity beyond 1–minute winds of due to wind shear. On September 10, the storm dissipated to the west-southwest of Diego Garcia.
Severe Tropical Storm Antoinette
In the middle of October, the Intertropical Convergence Zone (ITCZ) was active over the northeast portion of the basin and into the adjacent Australian region, spawning a low-pressure area on October 15 northwest of the Cocos Islands. With a large ridge to the south, the system tracked southwestward, crossing into the south-west Indian Ocean as a tropical depression on October 17. Also on that day, the JTWC began issuing warnings on the system as Tropical Cyclone 4S. On October 18, the depression intensified into Tropical Storm Antoinette after developing a well-defined central dense overcast, and began turning more to the west-southwest. That day, Antoinette strengthened to a peak intensity of just below tropical cyclone status, or with 10–minute winds just below . Around that time, the storm developed an eye feature, but that quickly dissipated. Antoinette began weakening steadily on October 20 due to increasing wind shear, and by that time it was moving quickly westward. After the circulation became largely devoid of deep convection, the storm weakened into a tropical depression on October 21. The next day, Antoinette passed about north of the northern tip of Madagascar, and dissipated on October 24 in the Mozambique Channel.
Intense Tropical Cyclone Melanie–Bellamine
In late October, the ITCZ developed a persistent area of convection west of the Indonesian island of Sumatra. A tropical low formed on October 28 within the Australian region, which initially failed to develop much due to persistent wind shear. The Australian Bureau of Meteorology (BoM) named the system Melanie on October 31 to the north of the Cocos Islands. The newly upgraded storm tracked generally westward due to a ridge to the south, and entered the south-west Indian Ocean on November 1 as a strong tropical storm. At that time, Melanie was renamed Bellamine. After the ridged moved farther away, the storm turned more to the southwest and began quickly intensifying. Early on November 3, Bellamine attained tropical cyclone status, and the next day reached peak 10–minute winds of ; this made it an intense tropical cyclone.
After reaching peak winds, Bellamine slowed its motion on November 4 while moving between two high-pressure areas, and weakened slightly. After another ridge built, the cyclone resumed a steady west-southwest track, although its 10–minute winds decreased to by November 6. That day, Bellamine began restrengthening as the eye became increasingly well-defined, in diameter. On November 7, the storm re-attained intense tropical cyclone status, as well as its previous peak intensity according to MFR. This was a rare occurrence of an early November storm becoming an intense tropical cyclone on two occasions. Meanwhile, the JTWC estimated peak 1–minute winds of . A strong approaching trough turned Bellamine to the southeast on November 8 and incurred rapid weakening due to wind shear. After the trough passed the cyclone, Bellamine turned back to the southwest on November 10 as a weakening tropical depression. Two days later, the circulation dissipated without having affected land.
Severe Tropical Storm Chantelle
Similar to previous storms Antoinette and Bellamine, Tropical Storm Chantelle originated from the ITCZ in the Australian region. On November 23, the MFR began tracking the system as a tropical disturbance off the west coast of Sumatra. The system fluctuated in intensity while moving westward, entering the south-west Indian as a tropical depression on November 24. Around that time, the JTWC had begun classifying it as Tropical Cyclone 7S. On November 26, the depression intensified into Tropical Storm Chantelle, and the convection continued to organize despite the presence of wind shear. That day, the JTWC estimated peak 1–minute winds of . On November 27, the MFR estimated peak 10–minute winds of . An approaching trough imparted significant weakening, and Chantelle weakened to tropical depression status 18 hours after it was at peak intensity. A ridge behind the trough steered the weakening system to the northwest, causing Chantelle to reach the low latitude of 5° S. Widespread cloudiness made it difficult for MFR to continue tracking the circulation, and the agency issued the final advisory on November 30 northwest of Diego Garcia. The JTWC continued tracking the storm for several more days, and the remnants of Chantelle likely contributed to the formation of the subsequent Tropical Cyclone Daniella.
Intense Tropical Cyclone Daniella