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

SSTs for October 2021 show weak cool SST anomalies were present across much of the central to eastern equatorial Pacific, while generally weak warmer than average SSTs were present in waters around the north of Australia and the Maritime Continent.

Values of the three key NINO indices for October 2021 were: NINO3 −0.4 °C, NINO3.4 −0.6 °C, and NINO4 −0.3 °C.

Sea surface temperatures (SST) for the tropical Pacific Ocean for the week ending 7 November 2021 were cooler than average along the equator in the central Pacific Ocean. Compared to two weeks ago, cool anomalies have shifted slightly westward, and areas of warm anomalies have emerged in parts of the equatorial Pacific east of 120°W.

Weak warm SST anomalies continue over the far western Pacific Ocean, including parts of the Maritime Continent, and waters around northern Australia. Compared to two weeks ago, warm anomalies have decreased close to Australia.

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 7 November 2021 were: NINO3 −0.3 °C, NINO3.4 −0.6 °C, and NINO4 −0.3 °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 7 November 2021 were stronger than average across the far western tropical Pacific, and close to average in the central and eastern tropical Pacific.

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) is forecast by a minority of climate models to strengthen and move eastwards across the Maritime Continent and into the western Pacific over the coming fortnight. If the MJO strengthens this would increase chances of above average rainfall across north-east Australia.

The negative Indian Ocean Dipole (IOD) event has weakened over the past fortnight. The latest weekly value of the Indian Ocean Dipole (IOD) index to 7 November 2021 was −0.13 °C, which is firmly within neutral bounds.

All five international climate models surveyed by the Bureau indicate the IOD will be neutral by December. A return to neutral weekly IOD values in late spring or early summer is consistent with the typical life cycle of an IOD event. As 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.

There were weak positive sea surface temperatures (SSTs) for the week ending 7 November 2021 across the tropical Indian Ocean, including areas around Indonesia, where cloudiness remains increased. Residual warm SST anomalies are likely to persist as the event declines and are 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 October 2021) shows cool anomalies have developed in the sub-surface of the central to eastern equatorial Pacific, with a marked increase during September and October 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.

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

For the five days ending 7 November 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. A volume of cooler than average water is present between 50 and 100 m depth in the eastern equatorial Pacific, reaching more than three degrees cooler than average between 135°W to 115°W. The strength of these cool anomalies has decreased slightly compared to two weeks ago.

Overall, sub-surface temperatures remain consistent with an ENSO-neutral state. However, if this volume of cooler than average water shoals (rises towards the surface) and progresses 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 remains at La Niña ALERT, meaning around a 70% chance of La Niña forming in the coming months. Several climate drivers are combining to produce the current wet outlook for Australia.

International climate models have strengthened their forecast likelihood of La Niña forming before the end of the year. However, atmospheric and oceanic observations have yet to consistently reach La Niña levels. The latest tropical Pacific Ocean temperatures, while cooler than average, are at similar levels to a fortnight ago and do not meet La Niña thresholds. Similarly, in the atmosphere, the Southern Oscillation Index (SOI) has eased back slightly from La Niña levels. Regardless of whether La Niña thresholds are met, a La Niña-like pattern in the Pacific may still increase the chances of above-average rainfall for northern and eastern Australia at times during spring and summer.

The negative Indian Ocean Dipole (IOD) has weakened, with the weekly index rising to neutral values. However, cloud patterns in the eastern Indian Ocean suggest the atmosphere is still responding to warmer than average ocean temperatures in the region. All models indicate the IOD will remain neutral for the coming months, typical of its annual cycle. A negative IOD increases the chances of above-average spring rainfall for much of southern and eastern Australia.

The Madden–Julian Oscillation (MJO) is forecast by a minority of climate models to strengthen and move eastwards across the Maritime Continent and into the western Pacific over the coming fortnight. If the MJO strengthens this would increase chances of above average rainfall across north-east Australia.

The Southern Annular Mode (SAM) has been positive for the past three weeks, and is forecast to remain at positive levels to the end of the year. A positive SAM during summer 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 currently positive and is expected to remain generally positive until the end of the year.

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 south-eastern 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.

All seven of the international climate models surveyed by the Bureau anticipate further cooling of tropical Pacific sea surface temperatures. All models surveyed indicate La Niña thresholds are likely to 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 one model continues to meet the threshold.

Regardless of whether La Niña thresholds are met, La Niña-like patterns in the Pacific may still increase the chances of above-average rainfall for northern and eastern Australia at times during spring and summer.