The 2020–21 Australian region cyclone season was a below average but very deadly season when most tropical cyclones formed in the Southern Indian Ocean and Pacific Oceans between 90°E and 160°E. It produced 8 tropical cyclones with 3 strengthening into severe tropical cyclones. However, it featured the region's third-deadliest cyclone on record—Cyclone Seroja, which brought severe floods and landslides to southern Indonesia and East Timor. The season officially began on 1 November 2020 and started with the formation of Tropical Low 01U on 24 November within the basin, which would later become Tropical Storm Bongoyo in the South-West Indian Ocean, and ended with the dissipation of a tropical low on 24 April, 6 days before the season ended on 30 April. However, a tropical cyclone could form at any time between 1 July 2020 and 30 June 2021 and would count towards the season total. During the season, tropical cyclones were officially monitored by one of the three tropical cyclone warning centres (TCWCs) for the region which are operated by the Australian Bureau of Meteorology, National Weather Service of Papua New Guinea and the Indonesian Agency for Meteorology, Climatology and Geophysics. The United States Joint Typhoon Warning Center (JTWC) and other national meteorological services including Météo-France also monitored the basin during the season.
Season forecasts In October 2020, Australia's Bureau of Meteorology (BOM) issued its tropical cyclone outlook for the 2020–21 season, and in the same month, the agency contributed towards the Southwest Pacific Tropical Cyclone Outlook, along with New Zealand's MetService, NIWA and the Fiji Meteorological Service (FMS). Each of the outlooks accounted for analogue seasons and the effects of various climate drivers, which included the state of the El Niño Southern Oscillation (ENSO). Below average sea surface temperatures in the eastern tropical Pacific Ocean and atmospheric conditions indicated a La Niña pattern, and it was declared that a La Niña had emerged on 29 September 2020. The BOM predicted that the La Niña pattern will persist into early 2021, and waters north of Australia and the Southwestern Pacific Ocean will be warmer than average in the coming three months. The bureau also advised that cyclone kits include a supply of face masks and hand sanitisers, owing to the concurrent COVID-19 pandemic.
The BOM expected an average to slightly above average number of tropical cyclones for the season. The agency predicted a 66% chance of above average activity for the region as a whole, compared with the average of 11 tropical cyclones. They also predicted that the Western and Northwestern sub-regions both had a 63% chance of experiencing more tropical cyclones than usual, the Northern region had a 57% chance of above average activity, and the Eastern region had a 67% chance of more tropical cyclones than the average. The Northwestern sub-region had a probability of two tropical cyclones crossing the Western Australian coast, with a significant risk of at least one of these to be a severe tropical cyclone.
The Southwest Pacific Tropical Cyclone Outlook covered a portion of the Australian region that comprised waters surrounding Papua New Guinea, the Solomon Islands, the Coral Sea, and the northern Tasman Sea including Norfolk Island. The outlook called for a reduced level of tropical cyclone activity in the early season, from November to January, but an increased level of activity in the late season, from February to April. Near normal activity during the season was expected for Papua New Guinea and the Solomon Islands, while an elevated level of activity was expected in the Coral Sea and the northern Tasman Sea, especially during the late season.
Season summary
Systems Tropical Low 01U (Bongoyo)
During mid to late November 2020, a moderate strength pulse of the Madden–Julian Oscillation (MJO) tracked eastwards across the equatorial Indian Ocean towards the Maritime Continent and Australian longitudes. The presence of the MJO brought an increase in tropical moisture to the atmospheric environment, as well as aiding the formation of Tropical Cyclone Alicia in the South-West Indian Ocean region in mid November. On 24 November, the Bureau of Meteorology (BOM) reported that a tropical low had developed within a trough in the far northwestern corner of the Australian region, located approximately northwest of the Cocos Islands. The system was assigned the identifier code 01U by the BOM. The Joint Typhoon Warning Center (JTWC) assessed environmental conditions as being only marginally conducive for tropical cyclogenesis, with good poleward outflow and sea surface temperatures near 30 °C (86 °F) being offset by the effects of moderate vertical wind shear. Consequently, the tropical low was expected to have only a low chance of developing into a tropical cyclone prior to moving westwards out of the Australian region later in the week.
Deep convection persisted over the system as the tropical low tracked gradually southwards over the following days, and by early 27 November, a curved convective rainband had developed on the eastern side. Despite this, the system remained mostly disorganised in the marginal environment, with an elongated low-level circulation centre. On 29 November, the tropical low began to be steered slowly towards the west-northwest on the northern side of the subtropical high pressure belt. The system encountered somewhat weaker wind shear along its new track, allowing deep convection to develop closer to the low-level circulation centre. Before any significant intensification could occur, however, the tropical low exited the Australian region on 30 November, passing into the area of responsibility of Météo-France's tropical cyclone warning centre in Réunion. The system later intensified into Severe Tropical Storm Bongoyo in the South-West Indian Ocean cyclone region on 7 December. At its peak intensity in the Australian region, the BOM estimated the system's minimum atmospheric pressure as 1005 hPa (29.68 inHg) on 25 November. Satellite scatterometer data revealed maximum sustained winds of approximately at 02:30 UTC on 30 November, a few hours prior to the system exiting the Australian region.
Tropical Low 01U made its closest approach to the Cocos Islands on 27–28 November, passing a little more than to the west. Increased winds, as well as occasional showers and thunderstorms, occurred across the islands while the system was located nearby. Maximum 10-minute sustained winds of with gusts to were observed at the airport on West Island at 04:30 UTC on 27 November. A total of of rainfall was also recorded on 24–28 November.
Tropical Low 02U
On 6 December, the Bureau of Meteorology (BOM) reported that a tropical low had developed within a monsoon trough near the western end of Java, approximately north-northeast of Christmas Island. The system was assigned the official identifier code 02U upon formation. Initially located in an unfavourable environment for intensification, the tropical low began to encounter somewhat improved conditions as it tracked towards the southeast. On 7 December, warm sea surface temperatures and strong diffluence in the upper troposphere fuelled the development of deep convection around the system's low-level circulation centre. Referencing the tropical low's quickly improving structure and the presence of a weak eye feature in microwave satellite imagery, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert for the system at 05:30 UTC. After tracking in a clockwise path around the northeastern side of Christmas Island, Tropical Low 02U became quasi-stationary on 8 December. Deep convection continued to develop near the centre during the overnight period, and as the low began to track eastwards, the JTWC upgraded the system to a tropical storm on the Saffir–Simpson hurricane wind scale at 18:00 UTC on 8 December.
Tropical Low 02U soon assumed a southeastward course, steered by the influence of a subtropical ridge situated to the east. Environmental conditions remained only marginally favourable for development, however, and the system's deep convection became displaced to the southwest of the centre due to northeasterly vertical wind shear, leaving the low-level circulation centre fully exposed. During this period, the JTWC reported that the tropical low's one-minute sustained winds had decreased to approximately , from the earlier peak of . By 12:00 UTC on 10 December, the system had begun to accelerate towards the southeast, moving into an environment more favourable for intensification. The interaction with an upper-level trough enhanced the tropical low's poleward outflow channel, allowing deep convection to rebuild over the centre of the system. This enabled the system to strengthen as it rapidly approached the coast of Western Australia at a speed of approximately , although vertical wind shear once again began to increase.
Tropical Low 02U made landfall between Port Hedland and Whim Creek on the Pilbara coastline just after 03:00 UTC on 11 December. About three hours prior to landfall, the JTWC reported that maximum one-minute sustained winds had increased to . Sustained gale-force winds were observed at Bedout Island and Port Hedland around the time of landfall, peaking at at both locations. Ten-minute sustained winds also reached on Legendre Island at 00:00 UTC. Despite these wind readings, the BOM did not classify the low as a Category 1 tropical cyclone because they determined that gale-force winds did not extend more than halfway around the centre. They attributed the strong winds on the eastern side of the system to a combination of enhanced northwesterly monsoonal flow and the system's fast translational velocity, rather than the low having intensified into a tropical cyclone. The tropical low continued rapidly inland across Western Australia over the next day, accelerating to about soon after landfall. Despite tracking over land, the BOM indicated that sustained winds to gale force, accompanied by gusts of up to , could still occur near the centre of the system, particularly on the eastern side. The weakening tropical low was last mentioned by the BOM at around 08:00 UTC on 12 December, while located in the southeastern corner of Western Australia.
Tropical Low
The presence of the Madden–Julian Oscillation and an equatorial Rossby wave in the Australian region contributed to the formation of another broad low-pressure system in the monsoon trough over the far eastern Indian Ocean during early December. Initially lacking organised deep convection, the system proceeded to track in a generally southwards direction towards the Kimberley region of Western Australia. By late on 7 December, infrared satellite imagery indicated an improvement in the structure of the low, with the system displaying an increase in flaring deep convection, accompanied by rotation evident in the lower troposphere. Owing to the increase in organisation, the Bureau of Meteorology (BOM) classified the system as a tropical low by 21:00 UTC, when it was located approximately northwest of Broome. Environmental conditions around the system were conducive for tropical cyclogenesis, with low vertical wind shear, good upper-level outflow and sea surface temperatures near 30 °C (86 °F). Due to the system's proximity to the coast, however, both the BOM and the Joint Typhoon Warning Center (JTWC) forecast only a low chance of development into a tropical cyclone before making landfall.