The 2005–06 Australian region cyclone season was an above average tropical cyclone season. It began on 1 November 2005 and ended on 30 April 2006. The regional tropical cyclone operational plan also defines a tropical cyclone year separately from a tropical cyclone season, which runs from 1 July 2005 to 30 June 2006.
Tropical cyclones in this area are monitored by four Tropical Cyclone Warning Centres (TCWCs): the Australian Bureau of Meteorology in Perth, Darwin, and Brisbane; and TCWC Port Moresby in Papua New Guinea.
Systems
Tropical Low 02S
An area of convection persisted on 4 November about north of the Cocos Islands, associated with a tight low-level circulation. With good outflow and moderate but decreasing wind shear, conditions favored development. By early on 5 November, the system consisted of a partially exposed circulation with deep convection on its western side, tracking southwestward. At 0400 UTC that day, the Tropical Cyclone Warning Centre (TCWC) in Perth issued a gale warning in association with the disturbance. Later in the day, the Joint Typhoon Warning Center (JTWC) classified it as Tropical Cyclone 02S, and both the JTWC and TCWC Perth anticipated further strengthening.
Throughout its duration, TCWC Perth maintained the system as a tropical low. However, the JTWC assessed the storm as reaching peak winds of , based on observations indicating the consolidation of convection over the center. The agency forecast the storm to strengthen further; however, by late on 6 November it had begun weakening. Around the same time, the storm crossed into the area of responsibility of Météo-France in Réunion (MFR); as such, it was re-designated as Tropical Depression 04. The depression continued southwestward as it slowly weakened. Early on 8 November, the JTWC issued its final warning, and six hours later, the MFR discontinued advisories while it was located about southeast of Diego Garcia.
Severe Tropical Cyclone Bertie–Alvin
A weak tropical low persisted on 17 November at a low latitude about west-southwest of the Indonesia island of Sumatra. Early on 18 November, the TCWC Perth issued a gale warning on the system; at the time, the system consisted of cycling convection in association with several small circulations. The circulations organized into a well-defined center of circulation, and at 0000 UTC on 19 November it was classified as Tropical Cyclone Bertie about northwest of the Cocos Islands. The storm tracked southwestward around the western periphery of a mid-level ridge between Australia and Indonesia. Environmental conditions generally favored further strengthening, with good outflow though moderate wind shear, and late on 19 November the JTWC initiated advisories on Bertie.
Tropical Cyclone Bertie gradually strengthened and became a Severe Tropical Cyclone on 21 November. The next day, TCWC Perth issued its last advisory on the cyclone, as it was expected to cross 90ºE, which separates the TCWC Perth and the Météo-France (MFR) areas of responsibility. However, Bertie turned due southward, and the MFR issued advisories for 30 hours as the cyclone remained just east of 90ºE, during which the MFR estimated Bertie attained peak winds of . Early on 23 November, the JTWC assessed the cyclone as reaching winds of , which is the equivalent to a marginal Category 4 on the Saffir–Simpson hurricane scale. On 24 November, the cyclone crossed into the area of responsibility of the MFR, and it was renamed Intense Tropical Cyclone Alvin. Shortly thereafter the storm turned to the northwest and weakened as it encountered unfavourable conditions The JTWC issued its last advisory on 26 November after it weakened to a tropical depression, and two days later the MFR discontinued advisories. The remnants of the storm continued northwestward until turning west-southwestward and dissipating on 3 December.
Tropical Low 04S
A tropical low persisted on 19 December just west of 90ºE, which is just outside the area of responsibility of TCWC Perth. The system initially consisted of pulsating convection in association with an elongated and partially exposed low-level circulation. TCWC Perth began issuing gale warnings on the system on 22 December as the low tracked southeastward. Moderate wind shear persisted over the system, preventing immediate development The low turned southwestward, and on 23 December it entered the area of responsibility of Météo-France in Réunion (MFR).