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 26 September 2021 were cooler than average for some parts of the eastern equatorial Pacific Ocean, and generally close to average or slightly cooler than average in the central equatorial Pacific.

These cool SST anomalies are mostly located just to the south of the equator, rather than directly along the equator, which is reflected in sub-surface anomaly patterns. However, anomalies in the eastern tropical Pacific are closer to the equator than they were two weeks ago.

Warmer than average SSTs continue over the far western Pacific Ocean, including parts of the Maritime Continent, and waters to the north and north-west of Australia. Compared to two weeks ago, warm anomalies have increased to the north-west of Australia, but have decreased around the Top End and northern Queensland.

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 26 September 2021 were: NINO3 −0.4 °C, NINO3.4 −0.3 °C, and NINO4 −0.2 °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 26 September 2021 were close to average across most of the tropical Pacific, and stronger than average in some of the area 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) is currently over the Maritime Continent, but weak. It is expected to remain relatively weak as it moves across Australian longitudes in the coming fortnight, but there is some disagreement between models as to the strength of the MJO during that period.

If the MJO remains moderately strong over Australia longitudes, it would likely lead to increased rainfall over the tropics to Australia's north.

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) for most of the past month and a half. The latest weekly value of the Indian Ocean Dipole (IOD) index to 26 September 2021 was −0.29 °C.

Three of the five international climate models surveyed by the Bureau indicate the weak negative IOD pattern could persist through 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. Residual warm SST anomalies are likely to persist as the event declines, and are is still expected to influence rainfall patterns.

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 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 23 September) shows cool anomalies have developed in the sub-surface of the central to eastern equatorial Pacific over September to date. Waters were up to three degrees cooler than average across a large region, and reach up to four degrees cooler than average in the central equatorial Pacific around 150°W and 150 m depth.

Weak warm anomalies continue across parts of the column depth west of the Date Line.

For the five days ending 27 September 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. Over the past two weeks a small volume of cooler than average water has emerged in the sub-surface, reaching more than three degrees cooler than average in the region around 150°W and 150 m depth.

Overall, sub-surface temperatures remain consistent with an ENSO-neutral state.

The El Niño–Southern Oscillation (ENSO) is currently neutral. However, recent model outlooks and cooling in the tropical Pacific Ocean mean the Bureau's ENSO Outlook status is at La Niña WATCH. In the past when La Niña WATCH has been reached, a La Niña event has subsequently developed around 50% of the time; this is approximately double the normal likelihood. La Niña events increase the chances of above-average rainfall for northern and eastern Australia during spring and summer.

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 to three 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.

The negative Indian Ocean Dipole (IOD) event has weakened, with latest IOD values falling shy of negative thresholds. Should these values persist, it is likely that the negative IOD is near its end. However, the pattern of sea surface temperatures in the Indian Ocean remains likely to influence Australian rainfall over the coming months. A negative IOD increases the chances of above-average spring rainfall for much of southern and eastern Australia.

The Madden–Julian Oscillation (MJO) is currently weak, located over the western Maritime Continent, north of Australia. Most models forecast movement of the MJO towards the eastern Maritime Continent in the coming fortnight, but there is some disagreement between models as to the strength of the MJO during that period. Some models suggest a moderate strengthening while others maintain a weak MJO. If the MJO strengthens over the Maritime Continent region, it would encourage enhanced rainfall over the tropics to the north of Australia.

The Southern Annular Mode (SAM) index has mostly been positive for the past five weeks. It is forecast to remain positive for October to December. This forecast is supported by a combination of a strengthened polar vortex over Antarctica, as well as the likelihood of La Niña development. 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 most of September. 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 forecast positive phase of SAM, as well as the increased likelihood of La Niña.

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.

As we move into summer, the influence of positive SAM towards above average rainfall increases over mainland southeastern Australia, while the tendency to reduced rainfall decreases for western Tasmania, and there is an increased chance of above average rainfall in eastern 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, with around a third of models predicting La Niña thresholds to be met in spring.

Three of the seven surveyed models clearly sustain NINO3.4 above La Niña thresholds until at least early 2022, 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. 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.

La Niña typically enhances spring rainfall in northern and eastern Australia. This pattern of cooler sea surface temperatures in parts of the tropical Pacific Ocean is likely to be contributing to the wetter than average outlooks for parts of the country.