The 1998–99 South-West Indian Ocean cyclone season was a quiet season that had the fourth-lowest number of days with tropical storm or tropical cyclone activity. Most of the storms formed either in the Mozambique Channel or in the far eastern portion of the basin, with five storms crossing from the adjacent Australian basin east of 90° E. As a result, few storms impacted Madagascar, and none made landfall on the African continent. Throughout most of the season, there was below-normal sea surface temperatures in the Indian Ocean east of Madagascar. In February, typically the peak in activity, Réunion island recorded its highest average monthly pressure since 1953. Due to generally unfavorable conditions, there were only six tropical storms tracked by the Météo-France office (MFR) on Réunion. There were only two tropical cyclones – a storm with winds of at least .
Activity began late, with the first tropical storm – Alda – forming on January 16, the third latest ever recorded at the time. Alda formed in the Mozambique Channel, which was one of few favorable areas for tropical cyclogenesis in the season. It brought rainfall to southwestern Madagascar that alleviated previously dry conditions. The next five tropical storms either originated or crossed into the adjacent Australian basin, where storms were monitored by the Bureau of Meteorology (BoM). Both Tropical Storm Chikita and Tropical Cyclone Davina brought beneficial rainfall to the Mascarene Islands. The latter storm caused two drowning deaths on Réunion and caused some crop damage. The strongest storm – Evrina – peaked as a strong cyclone in the Australian but weakened upon crossing 90°E, with 10-minute maximum sustained winds of in the basin. The final storm was unnamed, crossing from the Australian basin on April 21 as a minimal tropical storm before quickly dissipating. There were also several tropical disturbances or depressions, many short-lived. The first of these formed on September 3 in the northeastern portion of the basin, and there was a tropical depression in February in the Mozambique Channel that approached tropical storm status.
Season summary
The Météo-France office (MFR) on Réunion island issued warnings in tropical cyclones within the basin during the season. The agency estimated intensity through the Dvorak technique, and warned on tropical cyclones in the region 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. Most storms formed and dissipated within the tropics, with the exception of Severe Tropical Storm Alda.
During the season, atmospheric conditions shifted from El Niño to La Niña, but despite the shift, the season was similarly inactive as its predecessor. In general, sea surface temperatures were below normal, and atmospheric pressures were above normal, both unfavorable for tropical cyclogenesis. In the month of February, on average the peak time for activity, there was a general lack of convection, or thunderstorms, east of Madagascar. The average monthly pressure on Réunion was the highest since reliable records began in 1953. The low number of storms occurred despite an otherwise active cyclone year in the southern hemisphere. In its summary of the season, the MFR described the lack of activity as "rare and remarkable", possibly related to a Walker circulation. The agency monitored 14 tropical disturbances, of which only eight were tropical depressions for at least 24 hours. Six of these intensified into tropical storms, three less than the average of nine, of which only two attained tropical cyclone status, or half the average. There were 28 days in which there was storm or tropical cyclone activity, less than the average of 42 and at the time the fourth lowest since reliable record-keeping began in 1967 with the advent of satellite imagery. The only seasons with a lower number of storm days were 1982–83, 1986–87, and 1997–98.
Systems
Severe Tropical Storm Alda
After a subtropical depression exited the Mozambique Channel, another area of convection formed in the region on January 8, which fluctuated in intensity for several days. A passing cold front increased convection further on January 12, which split off a cutoff low-pressure area. Two days later, a subtropical disturbance formed offshore Beira, Mozambique, classified due to the extratropical origins and lack of centralized convection. The system remained nearly stationary, with the convection extending well to the east. On January 16, the MFR reclassified the system as a tropical depression after the thunderstorms increased near the center, the increasing organization due to moderate but decreasing wind shear. By that time, the depression had begun an eastward movement, but soon turned to the south in the weakness between two high-pressure areas. Late on January 16, the depression intensified into Tropical Storm Alda after the convection organized into a central dense overcast. Also that day, the JTWC began tracking the system as Tropical Cyclone 12S. This marked an unusually late start for the first named storm, at the time the third-latest on record.
Neither the MFR nor the JTWC anticipated much strengthening after Alda attained tropical storm status. Around the time of its upgrade, Alda passed about 20 km (12 mi) west of Europa Island in the Mozambique Channel, where sustained winds reached and gusts were as strong as . A building ridge southeast of Madagascar turned the storm to the southwest. After an increase in wind shear diminished the convection, the JTWC discontinued advisories on January 17, although the thunderstorms redeveloped following an unexpected decrease in wind shear. Alda turned to the southeast ahead of an approaching trough, and developed a ragged eye feature on January 18. That day, the MFR estimated peak 10-minute winds of , and the JTWC estimated one-minute winds of , equivalent to a minimal hurricane. The latter agency also reissued one advisory when the storm was at its peak. Alda accelerated to the southeast and gradually lost tropical characteristics, becoming extratropical on January 19 before being absorbed by the approaching cold front.
Following rainfall from an earlier subtropical depression, Alda dropped of rainfall in Morombe in southwestern Madagascar. The rainfall caused flooding that affected thousands of people. The rainfall was beneficial in alleviating previously dry conditions that had prevented crops to be planted. The passage of the storm also caused temperatures to decrease in Mozambique and Madagascar.
Moderate Tropical Storm Damien–Birenda
On January 21, a tropical disturbance formed in the Australian region about south of the Indonesian island of Java. Moving west-southwestward, the system intensified into Tropical Storm Damien on January 23, and attained tropical cyclone status two days later while passing south of the Cocos Islands. Subsequently, an increase in wind shear induced steady weakening, causing the circulation to be briefly exposed from the convection on January 26. Damien maintained an area of convection nearby the circulation, and exited into the south-west Indian Ocean on January 28 as a minimal tropical storm; at that time, the Mauritius Meteorological Services renamed the storm as Birenda. The storm turned more to the west and west-northwest due to a building ridge to the south. Increasing wind shear weakened Birenda to tropical depression status on January 29 and into a tropical disturbance the next day. Although convection was intermittent, the circulation dissipated on February 3.
Moderate Tropical Storm Chikita
Soon after Damien-Birenda exited the Australian region, another tropical disturbance formed north of the Cocos on January 29, which was initially weak but gradually organized. The disturbance tracked quickly west-westward due to a powerful ridge to the south, remaining in tandem with Tropical Storm Birenda about to the east. On January 31, the system crossed into the south-west Indian Ocean. The fast forward motion caused the effects of wind shear to diminish as well as increase the circulation's strength, and the depression intensified into Tropical Storm Chikita on January 31. At that time, the storm attained its peak intensity of . Almost immediately thereafter, the convection began weakening due to lukewarm water temperatures, and Chikita weakened to tropical depression status on February 1. Continuing rapidly to the west, the circulation became exposed from the convection on February 3 due to increased wind shear. Around that time, Chikita passed about north of Rodrigues island, and shortly thereafter weakened into a tropical disturbance. On the next day, the circulation passed north of both Mauritius and Réunion, presenting an asymmetric structure with the strongest winds to the south. Chikita dissipated on February 5 off the southeast coast of Madagascar.
On Rodrigues, Chikita produced wind gusts of and a rainfall total of , the rainfall proving beneficial due to previously dry conditions. Despite only being a tropical disturbance, Chikita produced wind gusts of on Mauritius and in the mountainous peaks of Réunion. The rainfall rates in both islands varied greatly; the peak total on Mauritius was compared to the peak of in Bébourg in the heights of Réunion. Also on the latter island, there was a six-hour rainfall total of at Piton de la Fournaise. Wet weather persisted after Chikita dissipated, resulting in four-day totals of in Bébourg on Réunion, and easing drought conditions on Mauritius.
Tropical Depression D1 During an extended period of quiet conditions across much of the basin, an area of low pressure persisted along the eastern coast of Mozambique. Convection fluctuated daily but became more persistent on February 11. That day, the thunderstorms organized into a circulation that had formed less than southeast of Beira, Mozambique, becoming Tropical Disturbance D1. After forming, the system moved southward, developing a central dense overcast with northeasterly outflow. Based on the organization, the disturbance intensified into a tropical depression on February 13. Although there were gale-force winds in the southwest periphery, the circulation was located on the northern edge of the convection. The depression neared tropical storm intensity, but an increase in wind shear from a nearby trough prevented further intensification. The JTWC estimated peak 1-minute winds of , making the system a tropical storm by their assessment. A building ridge to the south turned the depression to the northeast, bringing the system near Europa Island. A station on the island recorded sustained winds of with gusts to about three hours before and after the closest approach; the observations suggested that the depression could have become a tropical storm. Soon thereafter, the associated convection dissipated and the circulation turned westward. After crossing over its former path, the depression dissipated on February 17 very close to where it developed.
Intense Tropical Cyclone Davina