Dipoles & Oscillators Tune in Rain

Negative Indian Ocean Dipole likely in 2021.

Indian Ocean Dipole (IOD) index values continue to be negative. A negative IOD event is declared when there have been at least 8 weeks below the IOD index threshold of −0.4 °C. A negative IOD increases the chances of above average winter-spring rainfall for parts of southern Australia.

The most recent weekly IOD value is −0.27 °C, which is above the negative IOD threshold (−0.4 °C) but follows on from the IOD index being below the negative IOD threshold for the previous six weeks. Most climate models surveyed by the Bureau predict the IOD to return to below the negative IOD threshold, with a negative IOD event likely for the second half of the southern hemisphere winter and into spring.

The El Niño–Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. Cloudiness near the Date Line has been below average over the past fortnight, although this is weak and a localised pattern. Most climate model outlooks indicate a neutral ENSO state is likely to continue for the remainder of the southern hemisphere winter and spring despite tropical Pacific sea surface temperatures slowly cooling over the coming six months. This may be contributing to the wetter than average climate outlooks for parts of Australia.

The Southern Annular Mode (SAM) has been positive for the past four weeks. Forecasts suggest SAM will return to neutral levels over the coming week. Neutral SAM has little influence on Australian climate, while positive SAM typically means cold fronts and troughs are shifted further south than their usual winter path.

It is likely some of the wetter weather seen during June may have been a result of the developing negative IOD event, while the drier weather for parts of southern WA and western Tasmania may have been influenced by the positive SAM.

The Madden–Julian Oscillation (MJO) is currently over the Indian Ocean. Most climate models indicate the MJO will weaken approaching the Maritime Continent over the coming week. Should the MJO maintain its strength, an MJO near the Maritime Continent 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.