The Australian Bureau of Meteorology recently announced that a La Niña cycle has emerged and will persist until at least the end of 2020. The Indian Ocean Dipole is also expected to be negative throughout October and November, further increasing the likelihood of wetter conditions across the region. As a result, the Pacific is likely to experience wetter than average conditions between now and March.
In the western Pacific (from Papua New Guinea to southern French Polynesia) La Niña conditions have historically brought higher rainfall. In the eastern regions of the Pacific (in Kiribati, Tuvalu and the northern Cook Islands) La Niña usually results in drier than average weather conditions. During severe La Niña events, that can contribute to severe droughts in the eastern Pacific and large floods in the western part of the ocean. Cyclones are less likely to form in the South Pacific during La Niña weather events, but they are more likely to form closer to the east coast of Australia.
Cyclone Yasi made landfall in Queensland during the 2010-11 summer. It was the largest cyclone to have made landfall in that state since at least 1918 and caused $3.5 billion in damage and destroyed 75 per cent of the banana crop. Floods associated with the La Niña were estimated to have caused more than $10 billion in damage across the country, with the federal government introducing a temporary flood levy and budget cuts to fund the recovery. The La Niña cycle re-emerged in the 2011-12 summer. Rain from those two events resulted in 2010-12 being the wettest two-year period on record in Australia.
Fiji experienced the most severe floods in decades during the 2011-12 summer, caused in part by the La Niña. The Fijian Meteorological Service is not expecting a severe cyclone season this summer, but has not updated its official statement to reflect the announcement of a La Niña cycle.
It is unlikely that the 2020-21 La Niña will cause that kind of damage, as it is expected to be weaker than those of 2010-12. It is expected to result in higher than average rainfall in Queensland, New South Wales and Victoria, however, which could bring a decisive end to the multiyear drought, reduce the summer fire risk and lift urban water supplies.
Water storage levels in urban dams have increased significantly since the lows of 2018-19. WaterNSW reports that dam levels are currently at 94 per cent of capacity. The Warragamba Dam, which is the largest dam that supplies water to Sydney, reached full capacity in August. The Sydney Desalination Plant is running at 20 per cent of capacity, in case there are any supply disruptions from the dam caused by water quality issues (parts of the dam are covered with ash and debris from the 2019 bushfire season, which could affect water quality). A water management review is expected to occur in December. The dams that supply Melbourne are at 73 per cent of capacity and, like Sydney, desalination will continue to supply water to the city during 2021. Desalination and water recycling are likely to provide a greater share of urban water supplies into the future.
The La Niña cycle is likely to bring wetter than average conditions to eastern Australia and the western Pacific. It is unlikely, however, that it will cause the kind of devastation left in the wake of the 2010-12 La Niña cycle.