Wind and Rain and Sleet and Hail

Negative Indian Ocean Dipole established, supporting wetter winter-spring outlook

A negative Indian Ocean Dipole (IOD) is underway. A negative IOD increases the likelihood of above average winter–spring rainfall for much of southern and eastern Australia. Maximum temperatures are also more likely to be below average across southern Australia, while maximum and minimum temperatures are more likely to be above average for the northern tropics of Australia.

The most recent weekly IOD index value is −0.62 °C, with the IOD index having been below the negative IOD threshold for eight of the past nine weeks. Most climate models surveyed by the Bureau predict a negative IOD pattern is likely to persist until at least mid-spring.

The El Niño–Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. The past fortnight has seen the Southern Oscillation Index (SOI) increase to +13.9 for the 30 days ending 18 July 2021, although this is mostly due to local sea surface pressure variations at Darwin and Tahiti. Additionally, trade winds are stronger than average in the western half of the Pacific, which may encourage some temporary cooling of the ocean surface. Most climate model outlooks indicate the central tropical Pacific Ocean is likely to cool over the coming months, with three of seven models surveyed by the Bureau indicating this cooling will be enough to reach La Niña thresholds in spring, with the remaining four models staying neutral. This forecast cooling may also be contributing to the outlook for above median rainfall for much of Australia in the coming months.

The Southern Annular Mode (SAM) index is neutral and is forecast to remain neutral for at least the next fortnight. A neutral SAM has little influence on Australian climate.

The Madden–Julian Oscillation (MJO) has recently strengthened over the Maritime Continent, to Australia’s north. Most climate models indicate the MJO will continue moving eastwards into the western Pacific over the coming week. An MJO in these regions typically leads to stronger westerly winds over the Indian Ocean, which can reinforce the negative IOD.

Climate change continues to influence Australian and global climate. Australia’s climate has warmed by 1.44 ± 0.24 °C over 1910–2019, while southern Australia has seen a reduction of 10–20% in cool season (April–October) rainfall in recent decades.