Tropical Climate Update

La Niña Watch—50% chance of La Niña forming in 2021

The El Niño–Southern Oscillation (ENSO) is currently neutral. However, strengthening model outlooks and recent cooling in the tropical Pacific Ocean have raised the chance of La Niña forming in 2021. Consequently, the Bureau has lifted its ENSO Outlook status to La Niña WATCH, meaning there is around a 50% chance of La Niña forming. This is approximately double the normal likelihood. It is not unusual for La Niña to form in consecutive years, with a follow-up La Niña occurring nearly half of the time since 1900.

A weak negative Indian Ocean Dipole (IOD) remains the primary seasonal climate driver influencing weather patterns across northern Australia. The combined influence of the negative IOD, warmer-than-average waters around northern Australia and the sea surface temperature pattern across the tropical Pacific Ocean is contributing to the outlook for above-average rainfall across much of northern Australia in the coming months.

While the current forecasts indicate the influence of the negative IOD will diminish during November, La Niña can influence weather patterns across northern Australia well into the wet season (October-April). Rainfall outlooks indicate above-average rainfall for the November to January period, and if La Niña does develop in the coming months, there is an increased probability of above-average rainfall across northern Australia for the remainder of wet season 2021-22.

La Niña is also usually associated with an earlier monsoon onset date at Darwin, and an increased chance of average to above-average tropical cyclone (TC) numbers across the Australian region. While a negative IOD does not normally have a significant influence on seasonal tropical cyclone numbers, the average date of the first TC to form in the Australia region during the 8 negative IOD events since 1970 was 3 weeks earlier than average—mid-November instead of early December.

Read more about the current climate drivers in the Climate Driver Update

Madden–Julian Oscillation to weaken

A moderately strong pulse of the Madden–Julian Oscillation (MJO) is currently located on the western edge of the Maritime Continent, just outside of Australian longitudes. International climate models forecast this pulse to weaken and move across Australian longitudes in the coming fortnight. When an MJO moves over the Maritime Continent at this time of the year, its influence on rainfall patterns is usually confined to the tropics north of Australia. Typically, much of the Maritime Continent region along with South-East Asia have an increased likelihood of observing above-average cloudiness and rainfall if an MJO is active in the region.

An MJO pulse over the Maritime Continent can also strengthen trade winds across the central Pacific region. In the current climate context this could lead to further cooling of the equatorial Pacific Ocean and aid the development of La Niña.

Read more about the Madden–Julian Oscillation

Typhoon forms over western North Pacific Ocean

The first typhoon since July 2021 developed east of the Philippines in the past fortnight. Typhoon Chanthu (Kiko) passed just to the north of the Philippines, continued tracking northwards just off the east coast of mainland China and Taiwan, and finally made landfall on southern Japan. Chanthu (Kiko) reached typhoon intensity (equivalent of an Australian tropical cyclone category 3 or higher) on 8 September, and peaked at a strength equivalent to a category 5 Australian TC before passing over southern Japan at tropical storm strength (comparable to an Australian category 1 or 2 storm) on 17 September. With peak mean (10-minute average) wind speeds estimated by the Japan Meteorological Agency to be in excess of 215 km/h, and wind gusts to 300 km/h, Chanthu (Kiko) was the strongest typhoon since Surigae in April this year.

Closer to Australia, a tropical low has formed in the southern Indian Ocean and is rated as having less than 50% chance of strengthening to tropical cyclone intensity in the next 2 days, according to the La Réunion tropical cyclone forecasting agency. Based on current forecasts, it is expected to remain well west of the Australian mainland.