Climate Driver Update history
Climate drivers in the Pacific, Indian and Southern oceans and the Tropics

SSTs for August 2021 were close to average across most of the equatorial Pacific Ocean, although warmer than average SSTs continued in the eastern equatorial Pacific Ocean and in waters around the north and east to south-east of Australia.

Values of the three key NINO indices for August 2021 were: NINO3 0.0 °C, NINO3.4 −0.1 °C, and NINO4 +0.2 °C.

Sea surface temperatures (SST) for the tropical Pacific Ocean for the week ending 12 September 2021 were close to average across much of the equatorial Pacific Ocean, with small areas of slightly cooler than average SSTs in the central to eastern equatorial Pacific.

These cool SST anomalies are generally located just to the south of the equator, rather than directly along the equator, which is reflected in sub-surface anomaly patterns.

Warmer than average SSTs continue over the far western Pacific Ocean, including parts of the Maritime Continent. Warm anomalies close to the Australian continent have decreased compared to two weeks ago.

All three NINO indices are within the ENSO-neutral range. The latest values of the three NINO indices in the tropical Pacific for the week ending 12 September 2021 were: NINO3 −0.2 °C, NINO3.4 −0.2 °C, and NINO4 −0.1 °C.

Persistent NINO3 or NINO3.4 values warmer than +0.8 °C are typical of El Niño, while persistent values cooler than −0.8 °C typically indicate La Niña.

Trade winds for the 5 days ending 12 September 2021 were close to average across most of the tropical Pacific, and stronger than average around and west of the Date Line.

During El Niño there is a sustained weakening, or even reversal, of the trade winds across much of the tropical Pacific. Conversely, during La Niña, there is a sustained strengthening of the trade winds.

The Madden–Julian Oscillation (MJO) has stalled over the Indian Ocean. 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. If on the other hand it remains weak or indiscernable, it is unlikley to have a significant influence on Australian rainfall patterns.

The negative Indian Ocean Dipole (IOD) event has weakened, with index values falling just shy of the negative IOD threshold (i.e. −0.4 °C) in recent weeks. The latest weekly value of the Indian Ocean Dipole (IOD) index to 12 September 2021 was −0.34 °C.

Three of the five international climate models surveyed by the Bureau anticipate the IOD index will return to negative IOD levels during October. All models indicate a return to a neutral IOD pattern by December. This is consistent with the typical life cycle of an IOD event, which sees events dissipating in early summer with the arrival of the Australian monsoon.

Weekly sea surface temperatures (SSTs) in the Indian Ocean were warmer than average across much of the eastern half of the basin. Compared to two weeks ago, SSTs in the Arabian Sea and around the Horn of Africa have cooled and are generally close to average.

A negative IOD, and a warmer eastern Indian Ocean generally, increase the chances of above average winter–spring rainfall for parts of southern and eastern Australia. Negative IOD events also makes below average maximum temperatures more likely across southern Australia, while maximum and minimum temperatures are more likely to be above average for the northern tropics of Australia.

Cloudiness near the Date Line has generally been close to or slightly below average (positive OLR anomalies) since April, and tended to be a little more consistently below average since June.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies).

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to August) shows a pattern consistent with a neutral ENSO state, despite the emergence of cool anomalies in the sub-surface of the central to eastern equatorial Pacific during August. Waters were up to two and a half degrees cooler than average across a region of the eastern equatorial Pacific between about 75 m and 125 m depth during August.

The stronger sub-surface anomalies seen in this plot compared to the weekly plot aligns with the slightly off-equatorial position of cool surface anomalies—the month sub-surface plot spans 5° north and south of the equator, while the weekly plot spans a slightly narrower band 2° north and south of the equator.

Weak warm anomalies continue across parts of the column depth west of the Date Line, but were weaker during August than they were in July.

For the five days ending 13 September 2021, sub-surface temperatures were close to average across the equatorial Pacific.

Sub-surface temperatures remain consistent with an ENSO-neutral state.

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.

The Southern Annular Mode (SAM) index is positive, and has been so for the past three weeks. SAM is forecast to remain positive for the next three weeks, and also more generally for October to December.

A strengthened polar vortex over Antarctica is currently assisting this positive phase of SAM.

A positive SAM during spring typically brings wetter weather to eastern parts of Australia, but typically has a drying influence on south-westerly exposed coasts such as western Tasmania.

The El Niño–Southern Oscillation (ENSO) is currently neutral. Most of the international climate models surveyed by the Bureau anticipate tropical Pacific sea surface temperatures will cool during the coming months.

Model outlooks have increased their forecast liklihood of La Niña forming in 2021. Along with cooling in the Pacific, this has led the Bureau to raise its ENSO Outlook status to La Niña WATCH. This means that while ENSO currently remains neutral, the chance of La Niña forming during the southern hemisphere spring has increased to around 50%—twice the normal likelihood.

Five of the seven surveyed models anticipate La Niña thresholds will be met or surpassed for some or all of the months from October to January. Three of the seven surveyed models clearly sustain NINO3.4 above La Niña thresholds for long enough to indicate a full-fledged event (i.e. at least three months). However, only one model currently forecasts La Niña will continue into February, with all other models returning to neutral values by then.

La Niña typically enhances spring rainfall in northern and eastern Australia, and the shift towards cooler forecast values of NINO3.4 may already be contributing to the wetter than average rainfall outlooks for parts of the country.