Tropical Climate Update

Bureau releases Tropical Cyclone Outlook for 2020–21

The Australian tropical cyclone outlook for season 2020–21 was released Monday 12 October. The outlook for the coming tropical cyclone season, which officially commences on 1 November, is influenced strongly by the La Niña underway in the tropical Pacific Ocean. La Niña often means more tropical cyclones than in non-La Niña years, and the current outlook indicates a 66% chance of more than average cyclone numbers across the Australian Region this season.

The long-term average of tropical cyclone numbers across the Australian region, since reliable satellite imagery became available in 1970, is 11 tropical cyclones per season. When the tropical cyclone outlook talks of above average numbers, this is the number it refers to. However, recent decades have seen a decline in seasonal tropical cyclone numbers, such that the average since 2000 currently stands at 9 per season. Only once since 2000 have more than 11 tropical cyclones been observed in a season; in 2005, when 12 tropical cyclones affected the Australian Region.

The timing of the first tropical cyclone of a season is often earlier in La Niña years—instead of an average date around 1 January, mid-December is more likely. The date of the first landfalling tropical cyclone is typically also earlier, usually impacting the Australian coast in early January.

La Niña active in the tropical Pacific Ocean

The Bureau’s ENSO Outlook is at LA NIÑA, indicating that an event has become established in the tropical Pacific. Both atmospheric and oceanic indicators are at levels consistent with a mature La Niña event.

All surveyed international climate models predict La Niña will persist until at least February 2021. Climate models have recently indicated the peak strength of the current La Niña, based on sea surface temperatures (SST) across the tropical Pacific Ocean, may be relatively strong. While La Niña and its associated impacts are unique from event to event, there is normally a relationship between the strength of the event and the severity of its impacts. As such, there is the potential for significant impacts across much of Australia in the coming months if the current La Niña strengthens at the predicted rate.

The Indian Ocean Dipole (IOD) has risen in the past two weeks and now only two of the surveyed climate models indicate negative IOD thresholds may be reached in November.

Read more about the El Niño-Southern Oscillation (ENSO)

Northern Australia likely to be wetter than average in coming months

The latest climate outlook from the Bureau indicates a high chance of above-average rainfall across most of Australia in the November 2020–January 2021 period, with similar chances of above-average rainfall across northern Australia in the December 2020–February 2021 period.

La Niña is typically associated with above-average rainfall across most of northern Australia during spring. During summer, above-average rainfall is mostly confined to eastern Queensland, although much of the Northern Territory and parts of far northern Western Australia typically observe marginally above-average rainfall. As a result, the 7-month period of the northern wet season (October to April) normally sees above-average rainfall totals during La Niña.

In contrast to La Niña, the influence of the IOD is typically confined to spring, and by early summer the IOD normally breaks down as monsoonal conditions return to the southern hemisphere.

Monsoon onset (at Darwin) is normally earlier during La Niña, with an average date around mid-December, compared to the average date of around 28 December.