Recent cooling of the surface of the tropical Pacific Ocean, changes in tropical weather patterns, and continued ocean cooling forecast by climate models suggest La Niña could become established in spring 2020.

The Bureau’s ENSO Outlook remains at La Niña ALERT. This means the chance of La Niña forming in 2020 is around 70%—roughly three times the average likelihood.

While most key indicators remain within ENSO-neutral range, there have been further signs of La Niña development in the past fortnight. The central tropical Pacific Ocean has continued to cool and trade winds remain stronger than average, while the Southern Oscillation Index (SOI) has exceeded La Niña thresholds in recent days. Equatorial cloudiness near the Date Line also remains below average.

All the surveyed international climate models surveyed anticipate further cooling of the tropical Pacific Ocean. Five of the eight models reach or exceed La Niña thresholds during October, with six models indicating that if La Niña forms it is likely to persist into December.

Large parts of the Indian Ocean are warmer than average, with some weak cool anomalies in the west of the basin. The Indian Ocean Dipole (IOD) index has now been at negative IOD levels for three consecutive weeks. More than half of the surveyed models indicate a negative IOD is likely for spring. To be considered a negative IOD event, these values would need to be sustained for at least eight weeks.

Both La Niña and negative IOD typically increase the chance of above average rainfall across much of Australia during spring.

The Southern Annular Mode (SAM) is negative, and is expected to become neutral for the remainder of September. At this time of year negative SAM is typically associated with above-average rainfall across far southern parts of the country, and decreased rainfall further north.

The Madden–Julian Oscillation (MJO) is currently in the Indian Ocean, and is expected become weak or indiscernible as it approaches the Pacific.

Climate change is also influencing the Australian climate. Australia’s climate has warmed by around 1.4 °C since 1910, while southern Australia has seen a 10–20% reduction in cool season (April–October) rainfall in recent decades.