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

SSTs for September 2021 were close to average across most of the equatorial Pacific Ocean, but warmer than average SSTs were present in waters around the north and south-east of Australia, and weak cool SST anomalies were present in parts of the central to eastern equatorial Pacific.

Values of the three key NINO indices for September 2021 were: NINO3 −0.2 °C, NINO3.4 −0.3 °C, and NINO4 −0.2 °C.

Sea surface temperatures (SST) for the tropical Pacific Ocean for the week ending 10 October 2021 were cooler than average along much of the central and eastern equatorial Pacific Ocean, although anomalies were within 0.8 degrees of average. Negative anomalies cover a much greater area than they did two weeks ago. Cool SST anomalies continue to be centred just to the south of the equator, rather than directly along the equator, which is reflected in sub-surface anomaly patterns.

Weak warm SST anomalies continue over the far western Pacific Ocean, including parts of the Maritime Continent, and waters to the north of Australia. Compared to two weeks ago, warm anomalies have decreased to the north-west of Australia, but have increased across the Arafura Sea north of the Top End.

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 10 October 2021 were: NINO3 −0.4 °C, NINO3.4 −0.5 °C, and NINO4 −0.4 °C.

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

Trade winds for the 5 days ending 10 October 2021 were slightly stronger than average across much of the central and western tropical Pacific, and close to average in the east.

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

The Madden–Julian Oscillation (MJO) has been active over the Maritime Continent, but is expected to weaken in the coming week and is unlikely to influence Australian rainfall patterns. While the MJO is over the Maritime Continent region, it encourages enhanced rainfall over the tropics to the north of Australia.

The negative Indian Ocean Dipole (IOD) event has strengthened again over the past fortnight, with index values again meeting negative IOD thresholds (i.e. −0.4 °C or cooler). The latest weekly value of the Indian Ocean Dipole (IOD) index to 10 October 2021 was −0.53 °C.

However, all five international climate models surveyed by the Bureau indicate the monthly IOD value for November will be within the neutral range, indicating a return to neutral weekly values in the coming weeks. This is consistent with the typical life cycle of an IOD event, which sees events dissipate in late spring or early summer with the arrival of the Australian monsoon. When the monsoon trough shifts south over the tropical Indian Ocean, it changes wind patterns and prevents an IOD event from forming. This is why IOD events are unable to form (and therefore influence Australian climate) during December to April.

Weekly sea surface temperatures (SSTs) in the Indian Ocean were warmer than average across much of the eastern half and north of the basin. Residual warm SST anomalies are likely to persist as the event declines, and are still expected to influence rainfall patterns. A negative IOD, and a warmer eastern Indian Ocean generally, increase the chances of above average spring rainfall for much of southern and eastern Australia. Additionally, daytime temperatures are typically cooler than average for southern Australia, but warmer than average in the north.

Cloudiness near the Date Line has been consistently below average (positive OLR anomalies) since June.

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

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to September) shows cool anomalies have developed in the sub-surface of the central to eastern equatorial Pacific, with a marked increase during September compared to previous months. 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 10 October 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. A volume of cooler than average water reaching more than three degrees cooler than average is present between 50 and 100 m depth extending from 140°W to 115°W. Compared to two weeks ago, this cool water has shifted from the central Pacific to the eastern Pacific.

Overall, sub-surface temperatures remain consistent with an ENSO-neutral state. However, if this volume of cooler than average water continues to shoal (rise towards the surface) and progress eastward, it is likely to lead to further cooling of surface waters in the eastern equatorial Pacific Ocean, and may support further development of La Niña.

The Bureau's ENSO Outlook has been raised to La Niña ALERT. This is due to continued cooling in the tropical Pacific Ocean and an increase in the number of climate models showing sustained La Niña conditions over summer. Historically, when La Niña ALERT criteria have been met, La Niña has subsequently developed around 70% of the time. A 70% chance of an event is approximately triple 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 are currently within the ENSO-neutral range, but some have shifted towards a La Niña-like state. Sea surface temperatures in the central tropical Pacific Ocean are neutral, but have cooled over the past three months and are supported by cooler than average waters beneath the surface. Some atmospheric indicators, such as the Southern Oscillation Index (SOI) and cloudiness near the Date Line, are approaching La Niña levels. Six of the seven international climate models surveyed by the Bureau meet La Niña criteria from November.

A weak negative Indian Ocean Dipole (IOD) event continues. Most models suggest the negative IOD event will ease to neutral levels in late spring. A negative IOD increases the chances of above-average spring rainfall for much of southern and eastern Australia, while a neutral IOD has little influence on Australian climate.

The Madden–Julian Oscillation (MJO) has been active over the Maritime Continent since late September. The MJO is forecast to progress eastwards over the coming week and weaken as it approaches the western Pacific. While the MJO is over the Maritime Continent region, it encourages enhanced rainfall over the tropics to the north of Australia.

The Southern Annular Mode (SAM) index has been neutral for the past week after 5 to 6 weeks at positive levels. While it is forecast to remain neutral for the coming week, it is expected to return to generally positive levels from October to December. A positive SAM during spring typically brings wetter weather to eastern parts of Australia, but drier than average conditions for western Tasmania.

Climate change continues to influence Australian and global climate. Australia's climate has warmed by around 1.44 °C for the 1910–2019 period. Rainfall across northern Australia during its wet season (October–April) has increased since the late 1990s. In recent decades there has been a trend towards a greater proportion of rainfall from high intensity short duration rainfall events, especially across northern Australia.

The Southern Annular Mode (SAM) index is neutral and likely to remain neutral this week. SAM is expected to return to generally positive levels from 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.

However, all of the seven international climate models surveyed by the Bureau anticipate further cooling of tropical Pacific sea surface temperatures. All models surveyed indicate La Niña thresholds may be met during November. Six of the seven models indicate this cooling will be sustained at La Niña levels until at least January 2022—long enough to meet minimum La Niña event criteria (i.e. at least three months). Four models continue the event into February, but by March only two models continue to meet the threshold.

La Niña typically enhances spring rainfall in northern and eastern Australia.