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

SSTs for July 2021 were close to average across most of the equatorial Pacific Ocean, although warmer than average SSTs continued in the eastern Pacific Ocean close to South America.

SSTs were warmer than average in waters around the north and south-east of Australia.

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

Sea surface temperatures (SST) for the tropical Pacific Ocean for the week ending 15 August 2021 were close to average across the majority of the equatorial Pacific Ocean, with small areas of slightly warmer than average SSTs in the east of the basin and small areas of slightly cooler than average SSTs between 140°W and 110°W in the central Pacific.

Warmer than average SSTs continue over the far western Pacific Ocean, including parts of the Maritime Continent, around the coast of northern and eastern Australia, and across the Tasman Sea.

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 15 August 2021 were: NINO3 −0.1 °C, NINO3.4 −0.1 °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.

The 30-day Southern Oscillation Index (SOI) for the 30 days ending 15 August 2021 was +6.3. The 90-day SOI value was +6.0.

The 30-day value has decreased over the past week, with pressure patterns once again becoming more neutral.

Sustained negative values of the SOI below −7 typically indicate El Niño while sustained positive values above +7 typically indicate La Niña. Values between +7 and −7 generally indicate neutral conditions.

Trade winds for the 5 days ending 15 August 2021 were close to average across the tropical Pacific.

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 Indian Ocean, and is expected to remain at moderate or weak strength over the Indian Ocean during the next week. It is unlikely to influence Australia rainfall patterns this week, but may act to increase Pacific trade winds. An MJO in the western Indian Ocean has little influence on Australian rainfall patterns, but typically acts to increase cloudiness and rainfall across much of the northern Indian Ocean and increase tropical Pacific trade winds.

A negative IOD event is underway.

The latest weekly value of the Indian Ocean Dipole (IOD) index to 15 August 2021 was −0.37 °C. The IOD index has generally been below the negative IOD threshold (−0.4 °C) since mid-May.

Four of the five international climate models surveyed by the Bureau anticipate the IOD index will remain within negative thresholds through spring, with all five models indicating neutral values for December.

Weekly sea surface temperatures (SSTs) in the Indian Ocean were warmer than average across much of the eastern half of the basin, but have weakened compared to past weeks. SSTs around the Horn of Africa and the Arabian Peninsula are mostly close to average, although small areas of cool anomalies persist close to the coastline.

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 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 July) shows a pattern consistent with a neutral ENSO state. Weak warm anomalies were present across much of the column depth west of the Date Line, while weak cool anomalies were present across the central and eastern equatorial Pacific between around 175 m depth and 75 m depth. These cool anomalies developed during July. 

For the five days ending 15 August, sub-surface temperatures were close to average across most of the equatorial Pacific, although a region of slightly cooler than average water is present in the sub-surface of the equatorial Pacific around 100 m depth around 125°W. Anomalies reach more than two degrees cooler than average in this region.

Overall, the subsurface remains consistent with an ENSO neutral state.

The negative Indian Ocean Dipole (IOD) event continues. A negative IOD increases the chances of above average winter–spring rainfall for much of southern and eastern Australia.

The most recent weekly IOD index value is −0.37 °C, with easterly winds across the tropical eastern Indian Ocean over the past week acting to ease the negative IOD values somewhat. However, this is likely to be only temporary, with most climate models surveyed by the Bureau predicting negative IOD conditions are likely to persist during spring.

The El Niño–Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. The Southern Oscillation Index (SOI), which recently had moderately strong La Niña-like values, has returned to neutral values, with the 30-day value to 15 August at +6.3. Most climate model outlooks indicate the central tropical Pacific is likely to cool over the coming months, but remain ENSO-neutral. Three of seven models surveyed by the Bureau are indicating this cooling will be enough to reach La Niña thresholds in spring, but only one of these persists for longer than one month. This cooling of the tropical Pacific may be contributing to the wetter than median rainfall outlooks in Australia, along with the negative IOD.

The Southern Annular Mode (SAM) index has recently dipped to negative values but is forecast to return to neutral levels in the coming days, and remain so for the next fortnight. A neutral SAM has little influence on Australian climate.

The Madden–Julian Oscillation (MJO) is currently in the western Indian Ocean. Most climate models indicate the MJO will remain slow moving in this region this week. An MJO in the western Indian Ocean has little influence on Australian rainfall patterns, but typically acts to increase cloudiness and rainfall across much of the northern Indian Ocean and increase tropical Pacific trade winds.

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.

The Southern Annular Mode (SAM) index has recently dropped to negative levels. It is expected to return to neutral values over the coming week.

While negative during winter, SAM tends to increase the chance of above average rainfall over much of the southeast. A neutral SAM has little influence on Australian climate.

The El Niño–Southern Oscillation (ENSO) is currently neutral. Most of the international climate models surveyed by the Bureau anticipate NINO3.4 will cool during the coming months, but vary in how much cooling they expect. Three of the seven surveyed models anticipate NINO3.4 will meet or exceed La Niña thresholds during November, but only one sustains cooling long enough between October and December to indicate a full-fledged event.

The majority, four models, predict a neutral ENSO pattern to continue through to January 2021.

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