2005–06 South-West Indian Ocean cyclone season

The 2005–06 South-West Indian Ocean cyclone season was the fifth least-active on record. The Météo-France office on the island of Réunion tracked 13 tropical disturbances, of which six intensified into a moderate tropical storm. Three of these systems proceeded to attain tropical cyclone status – reaching 10 minute maximum sustained winds of at least . The American-based Joint Typhoon Warning Center also tracked eight storms in the basin. Activity was below normal due to a powerful Walker circulation, which increased convection over the neighboring Australian basin, but suppressed activity in the western Indian Ocean. As a result, most of the storms developed near or entered from the Australian basin, crossing 90°E to enter the South-West Indian Ocean.

A series of four short-lived systems occurred from September to November in the northeastern portion of the basin. These were followed by the first named storm – Alvin – which was renamed after it crossed from the Australian region as Tropical Cyclone Bertie in late November. After another short-lived disturbance in late December, there was a tropical disturbance in the Mozambique Channel in January that killed 26 people when it brought heavy rainfall to Mozambique. Later in the month, Tropical Cyclone Boloetse took an erratic track across Madagascar, killing six people when it brushed the island's southwest coast. In February, there was a small, short-lived unnamed tropical storm that presented difficulties to warning agencies in determining its structure. Intense Tropical Cyclone Carina was the strongest system of the season, attaining peak 10 minute winds of in the open waters of the eastern portion of the basin. Sprawling Tropical Storm Diwa brought six months' worth of rainfall to the drought-ridden island of Réunion, reaching in the mountainous peaks. The rains led to flooding and landslides that killed 10 people directly or indirectly. Two of the deaths occurred when a saturated cliff collapsed onto a coastal road. The final storm, Elia, dissipated on April 17 after previously entering from the Australian basin.

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Seasonal summary

Météo-France's meteorological office in Réunion (MFR) – the official Regional Specialized Meteorological Center for the South-West Indian Ocean – tracked and named all tropical cyclones from the east coast of Africa to 90° E, and south of the equator. The Joint Typhoon Warning Center (JTWC), which is a joint United States Navy – United States Air Force task force that issues tropical cyclone warnings for the region, also issued advisories for storms during the season.

There were 13 tropical disturbances in the season that were monitored by the MFR. Since the agency began operations in the early 1990s, this season had the second-fewest disturbances that received warning, only behind the 2000–01 season. Since the advent of satellite imagery in 1967, this season was the fifth least-active in terms of storm days and the number of cyclones. There were six systems that attained the intensity of a moderate tropical storm, which has 10 minute sustained winds of at least , below the average of nine. There were 30 days in which a moderate tropical storm was active, less than the average of 48. Three tropical storms attained tropical cyclone status, or 10 minute winds of at least , and there were 10 days in which these systems were active; this is half the average of 20 days, continuing the trend since the 2000–01 season of fewer stronger systems. The season was very similar to the 1998–99 season; both had late starts for the first named storm, drought conditions over much of the basin, and low activity. The third named storm, Carina, did not occur until late February, which at the time was the latest such date since naming began in 1960.

In general, storms in the season formed in the basin's periphery or north of the Mascarene Islands. No systems developed in the Mozambique Channel, an unusual occurrence. The overall lack of activity was due to a strong Walker circulation over Indonesia, which increased convection over the Australian region, but suppressed convection over the Indian Ocean. The Intertropical Convergence Zone – typically a major contributor to tropical cyclogenesis – was rarely active. Conditions became generally more favorable in February and March, when four of the six tropical storms occurred. The general lack of thunderstorms over the basin caused drought-like conditions. Pierrefonds Airport on Réunion recorded just of rainfall from November to January, a record minimum. The island also recorded its third-highest average atmospheric pressure from November to April.

Systems

Tropical Disturbance 01

At the beginning of September 2005, the ITCZ was active in the northeastern portion of the basin, accompanied by a small circulation and scattered convection. Located in the presence of strong wind shear, the circulation was exposed from the convection, a sign of unfavorable conditions. However, warm waters fueled an increase in convection, and the MFR classified the system as Tropical Disturbance 1 on September 5. That day, the JTWC issued a TCFA, but continued shear caused weakening as the disturbance tracked southeastward. By September 8, the MFR had discontinued advisories on the system, after the circulation had become exposed. After turning to the west-northwest, the circulation began dissipating on September 12.

Tropical Depression 02

The second disturbance was first classified by the MFR on October 12 about east of Diego Garcia. The system had enough of a circulation and associated convection, and proceeded southwestward for its duration. After the convection consolidated more and developed curved rainbands, the MFR upgraded the system to a tropical depression on October 14. This period of organization occurred during a brief decrease in wind shear. At 12:00 UTC on October 14, the JTWC classified the system as Tropical Cyclone 01S, estimating peak 1 minute winds of . Increasingly unfavorable conditions caused the convection to diminish. Late on October 15, both the JTWC and MFR discontinued advisories due to the increasing disorganization of the disturbance. The circulation remained well-organized but devoid of convection, and the MFR last monitored the center on October 21.

Tropical Depression 03

Tropical Depression Three formed on November 6 while east-southeast of Diego Garcia; it moved generally southward, reaching peak winds of before dissipating on November 8.

Tropical Depression 04

Simultaneous to Tropical Depression Three was Tropical Depression Four, which entered the basin on November 7 from the Australian region, and was also classified by the JTWC as Tropical Cyclone 02S with winds of . The depression was weakening at the time, and both the JTWC and MFR discontinued advisories on November 8.

Intense Tropical Cyclone Bertie–Alvin

In the middle of November, a westerly wind burst produced an area of convection southwest of Sumatra, which spawned a circulation at 2º S on November 16. Two days later, the BoM classified the system as a tropical low to the north of the Cocos Islands. The low moved southwestward, quickly intensifying, prompting the BoM to name it Bertie. The storm moved southwestward and intensified due to favorable water temperatures and atmospheric conditions, reaching winds of on November 22 while just east of 90º E. The track shifted nearly due south, and the eye moved along the dividing line between the Australian and south-west Indian Ocean basins. Early on November 24, the cyclone crossed 90º E and was renamed Alvin; at the time, the system was beginning to weaken due to cooler waters from upwelling and increased wind shear.

The MFR estimated peak 10 minute winds of within the basin, making it an intense tropical cyclone; it was the third consecutive year in which there was a November storm of such intensity. The JTWC, which designated Alvin as Tropical Cyclone 03S, estimated 1 minute winds of . A building ridge to the south turned the storm to the west-northwest. By November 25, the increased wind shear had exposed the circulation from the convection, indicative of rapid weakening, and on that day Alvin was downgraded below tropical cyclone status. On the next day, the storm weakened to tropical depression status after nearly all of the convection was gone, prompting the JTWC to cease issuing advisories. The residual circulation remained well-defined with only temporary increases in convection. The MFR stopped issuing warnings on December 3, after Alvin had passed south of Diego Garcia, although the circulation continued westward and was still visible north of Madagascar on December 10.

Tropical Depression 06

On December 21, the BoM began monitoring a tropical low in the western portion of the Australian basin, which had moved eastward from the south-west Indian Ocean basin. The system moved southwestward and later to the southwest, once again crossing 90º E and causing the MFR to designate it Tropical Depression 6. At the time, the circulation was partially exposed from the convection, although it organized over the next day. This spurred the JTWC to designate it as Tropical Cyclone 04S on December 24 with winds of . As with other systems in the season, strong shear caused the system to weaken; the JTWC discontinued advisories on December 25, and the MFR followed suit the next day. However, the system continued to the southwest, reorganizing enough on December 27 for the MFR to reissue advisories. Shear again weakened the convection, and the MFR discontinued advisories again on December 29 while the system was a short distance west-northwest of Rodrigues.

Tropical Disturbance 07

The Intertropical Convergence Zone (ITCZ) produced an area of convection northeast of Madagascar on December 29, which had an association circulation. On January 1, the system moved across northern Madagascar and subsequently entered the Mozambique Channel. It continued quickly to the southwest, passing north of Europa Island, and was classified as Tropical Disturbance 7 late on January 3. It continued intensifying and organizing until moving ashore Mozambique near Vilankulo, and the system nearly attained tropical depression stage. The system followed the country's coastline, bending southward toward the capital Maputo. On January 7, the disturbance moved offshore, but the system soon moved back overland and dissipated later that day over Swaziland. The system brought heavy rainfall to Inhambane Province, reaching in Inhambane. The rains resulted in flooding but also alleviated drought conditions in Mozambique. The rains also caused the Mutamba River to exceed its banks in Inhambane, flooding roads up to a meter (3.3 ft) deep and halting traffic. Across Mozambique, 26 people died due to the floods.

Tropical Cyclone Boloetse