Negative Indian Ocean Dipole likely for the coming months, BOM

MELBOURNE, Australia – According to the latest Climate Driver Update from the Bureau of Meteorology of the Australian Government (BOM), the Indian Ocean Dipole (IOD) is currently neutral. The IOD index has been below zero over the past four weeks, with two of those weeks exceeding the negative IOD threshold value (−0.4 °C). All climate model outlooks surveyed suggest a negative IOD is likely to form the coming months.

Outlook accuracy for Indian Ocean Dipole forecasts significantly improves from June, while model consistency over the coming weeks and months adds confidence to this outlook. A negative IOD increases the chances of above average winter–spring rainfall for much of Australia. It also increases the chances of warmer days and nights for northern Australia.

The 2021–22 La Niña event is slowly weakening in the tropical Pacific. Compared to two weeks ago, tropical Pacific sea surface temperatures have warmed, particularly in the western half of the tropical Pacific, returning to near-average values.

However, some atmospheric indicators continue to show a La Niña signal, including cloudiness along the equator and the Southern Oscillation Index (SOI), while trade winds have shifted more firmly towards a more neutral ENSO pattern (neither La Niña nor El Niño).

Most climate models surveyed by the Bureau indicate a return to neutral ENSO during the southern hemisphere winter. Two of the seven models maintain La Niña conditions through the southern winter.

Warmer than average sea surface temperatures around much of Australia are likely to be contributing to wetter outlooks over the coming months, and the forecast sea surface temperature pattern in the tropical Pacific still favours average to above average winter rainfall for eastern Australia.

The Madden–Julian Oscillation (MJO) is currently at moderate strength over the western hemisphere, and is expected to strengthen in the coming weeks. The MJO is contributing to the enhanced westerly wind anomalies currently observed in the western Pacific, which typically act to weaken La Niña events.

The Southern Annular Mode (SAM) index is currently positive, but is expected to return to neutral and remain so for much of June. Neutral SAM has little influence on Australian rainfall. Longer-term SAM is tending towards positive which typically has a drying influence for parts of south-west and south-east Australia during winter.

Climate change continues to influence Australian and global climate. Australia’s climate has warmed by around 1.47 °C for the 1910–2020 period. Southern Australia has seen a reduction of 10–20% in cool season (April–October) rainfall in recent decades. There has also been a trend towards a greater proportion of rainfall from high intensity short duration rainfall events, especially across northern Australia.