The El Niño–Southern Oscillation (ENSO) is currently neutral. However, strengthening model outlooks and recent cooling in the tropical Pacific Ocean has raised the chance of La Niña forming in 2021. Consequently, the Bureau has lifted its ENSO Outlook status to La Niña WATCH, meaning around a 50% chance of La Niña forming. This is approximately double the normal likelihood.
Most oceanic and atmospheric indicators of ENSO remain within the ENSO-neutral range. However, sea surface temperatures in the central tropical Pacific Ocean have cooled over the past two months, supported by cooler than average waters beneath the surface. Climate models continue this cooling trend over the coming months, with three of the seven models surveyed by the Bureau meeting La Niña criteria, while two additional models briefly touch La Niña thresholds.
La Niña events increase the chances of above-average rainfall for northern and eastern Australia during spring and summer.
The negative Indian Ocean Dipole (IOD) has weakened, with IOD values at marginal negative IOD levels. However, the pattern of sea surface temperatures in the Indian Ocean remains likely to influence Australian rainfall over the coming months. Models suggest this weak negative IOD pattern could persist at borderline levels through October before easing further. A negative IOD increases the chances of above-average spring rainfall for much of southern and eastern Australia.
The Madden–Julian Oscillation (MJO) has recently stalled over the eastern Indian Ocean. The MJO’s influence on the wind patterns across the Maritime Continent region, north of Australia, may have contributed to weakening of the IOD. Most models indicate the MJO is likely to move across Australian longitudes in the coming fortnight, but there is divergence in the forecast strength of the MJO. If the MJO remains moderately strong over Australia longitudes, it would likely lead to increased rainfall over the tropics to Australia’s north, and strengthen the trade flow across the central Pacific region.
The Southern Annular Mode (SAM) index has been positive over the past three weeks, and is forecast to remain positive for several weeks to come. This is supported by a strengthened polar vortex over Antarctica. A positive SAM during spring typically brings wetter weather to eastern parts of Australia, but may be drier for western Tasmania.
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. Rainfall across northern Australia during its wet season (October–April) has increased since the late 1990s with a greater proportion of high intensity short duration rainfall events.