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 21 November 2021 were cooler than average along the equator across most of the Pacific Ocean. Compared to two weeks ago, cool anomalies have strengthened, particularly in the eastern equatorial Pacific.

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 increased to the north-west and north-east of Australia but decreased around the Gulf of Carpentaria.

While SSTs analysed for the three NINO region boxes remain close to but just shy of threshold values, the temperature gradient across the Pacific is clear, with anomalies taking the characteristic shape of cool tongue along the equator and warm horseshoe extending from the Maritime Continent into the north-west and south of the basin. As seen in the response of atmospheric indicators, this SST pattern has been sufficient to initiate coupling of the ocean and atmosphere.

All three NINO indices are within the ENSO-neutral range, but have generally strengthened over the last fortnight. The latest values of the three NINO indices in the tropical Pacific for the week ending 21 November 2021 were: NINO3 −0.7 °C, NINO3.4 −0.6 °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 21 November 2021 were stronger than average across the western half of the tropical Pacific, and close to average in the 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 at weak to moderate strength over the Maritime Continent at present, but is expected to intensify over the western Pacific in the coming weeks. If this occurs, it would increase chances of above average rainfall across north-east Australia between now and early December.

The latest weekly value of the Indian Ocean Dipole (IOD) index to 21 November 2021 −0.34 °C, within neutral bounds.

While the negative IOD event is near its end, a residual atmospheric pattern continues and weak warm SST anomalies and increased cloudiness persist around the coastlines of Indonesia and north-west Australia in the west of the Indian Ocean basin. A negative IOD, and a warmer eastern Indian Ocean generally, contributed to increased chances of above average spring rainfall for much of southern and eastern Australia, warmer than average days in the north, and cooler than average days in the south. It is possible some of this influence may continue as these patterns diminish further.

All five international climate models surveyed by the Bureau indicate the IOD will remain neutral for the coming months, 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. A neutral IOD has little influence on Australian climate.

Cloudiness near the Date Line has been consistently below average (positive OLR anomalies) since June, and across the Pacific more generally cloud patterns are consistent with those typical of La Niña.

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 18 November 2021) shows cool anomalies have persisted across the sub-surface of the central to eastern equatorial Pacific over the past three months. For November to date, waters were more than three degrees cooler than average across a large region east of 140°W, and reach up to four degrees cooler than average in some areas.

Weak warm anomalies continue across parts of the column depth in the far west of the equatorial Pacific.

For the five days ending 21 November 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. Due to the sparsity of observations in the eastern half of the equatorial Pacific (point observations are indicated by Xs on the sub-surface temperature plot), it is hard to be confident as to what has occurred with the volume of cooler than average water present below the surface. However, cooling of surface waters would indicate it is likely this relatively cool sub-surface water has risen closer to the surface, with shoaling in the eastern equatorial Pacific contributing to surface level changes.

In the west a volume of slightly warmer than average water persists, reaching more than two degrees warmer than average between 100 and 150 m depth between the Date Line and the west of the basin. The strength of these warm anomalies remains similar to two weeks ago.

La Niña has become established in the tropical Pacific. The Bureau's ENSO Outlook has been raised to LA NIÑA. Climate models suggest this La Niña will be short-lived, persisting until the late southern hemisphere summer or early autumn 2022. La Niña events increase the chance of above average rainfall across much of northern and eastern Australia during summer.

Several indicators of the El Niño–Southern Oscillation (ENSO) now show clear La Niña patterns. Sea surface temperatures in the tropical Pacific are close to La Niña thresholds, with climate model outlooks expecting them to cool further. In the atmosphere, cloud and wind patterns are typical of La Niña, indicating the atmosphere is now responding to, and reinforcing, the changes observed in the ocean.

The negative Indian Ocean Dipole (IOD) is approaching its end, with oceanic index values in the neutral range. However, cloud and wind patterns across the eastern Indian Ocean suggest some IOD influence remains. All models indicate the IOD will remain neutral for the coming months, consistent with its typical seasonal 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 currently over the Maritime Continent region at weak to moderate strength. The MJO is forecast to progress eastwards across the Maritime Continent and into the western Pacific over the coming fortnight, increasing the chances of above average rainfall across northern Australia and the Maritime Continent, to Australia's north.

The Southern Annular Mode (SAM) has generally been positive for several weeks. It 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, although recent SAM index values have dipped briefly into neutral. It is expected to remain generally positive until the end of the year.

A positive SAM at this time of the year typically brings above average rainfall 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.

La Niña is active in the tropical Pacific Ocean.

Over recent weeks coupling has been seen between the ocean and the atmosphere, meaning cloud and wind patterns in the atmosphere are responding to and reinforcing changes in ocean temperatures below. Once this positive feedback loop is established, it is what sustains these changes in ocean and atmosphere and helps maintain La Niña conditions for an extended period.

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 December and January. Four models continue the event into February, but by March only one model continues to meet the threshold.

In general, the models anticipate that this will be short-lived, and of weak to moderate strength though there is some variation in predicted peak strength between the various models. For 2021–22 to be considered a La Niña year, the event will have to be sustained for at least three months.

Regardless of whether La Niña thresholds are sustained for three months or for a shorter period, the presence of La Niña-like patterns in the Pacific increases the chances of above-average rainfall for northern and eastern Australia during spring and the coming summer.