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 29 August 2021 were close to average across much 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 in the central to eastern equatorial Pacific. Compared to two weeks ago, the size of these cool anomalies has increased.

These cool SST anomalies are generally located just to the south of the equator, rather than directly along the equator, which is reflected in sub-surface anomaly patterns.

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. Warm anomalies in this region have increased compared to two weeks ago.

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 29 August 2021 were: NINO3 −0.1 °C, NINO3.4 −0.3 °C, and NINO4 0.0 °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 29 August 2021 was +3.5. The 90-day SOI value was +7.9.

The 30-day value has been relatively flat over the past two weeks, remaining within neutral values.

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 29 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 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 direct influence on Australian rainfall patterns, but typically acts to increase cloudiness and rainfall over Southeast Asian parts of the Maritime Continent and eastern Indian Ocean. The MJO may have acted to weaken the negative IOD by strengthening easterly winds over the Maritime Continent, however, this is most likely to be temporary.

The negative Indian Ocean Dipole (IOD) event is likely to persist during spring, although current values have dipped below the threshold. The latest weekly value of the Indian Ocean Dipole (IOD) index to 29 August 2021 was −0.35 °C. The IOD index has generally been below the negative IOD threshold (−0.4 °C) since mid-May.

All of the five international climate models surveyed by the Bureau anticipate the IOD index will remain within negative thresholds during September, four for October, and three for November, with all five models indicating neutral values by 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 remain generally close to average.

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 26 August) shows a pattern consistent with a neutral ENSO state, despite the emergence of cool anomalies in the sub-surface of the central to eastern equatorial Pacific during August to date. Water in part of this region is more than three degrees cooler than average for August.

The stronger sub-surface anomalies seen in this plot compared to the weekly plot aligns with the slightly off-equatorial position of cool surface anomalies—the month sub-surface plot spans 5° north and south of the equator, while the weekly plot spans a slightly narrower band 2° north and south of the equator.

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

For the five days ending 29 August, sub-surface temperatures were close to average across the equatorial Pacific.

The subsurface remains consistent with an ENSO-neutral state.

The El Niño–Southern Oscillation (ENSO) is currently neutral. Sea surface temperatures (SSTs) in the central tropical Pacific Ocean are near average, despite slowly cooling over the past two months. Waters beneath the surface are slightly cooler than average.

While most models indicate SSTs in the central tropical Pacific are likely to continue cooling over the coming months, ENSO-neutral is favoured to persist. Only two of the seven models surveyed suggest SSTs will remain cool for long enough to be considered a La Niña event. This cooling of the tropical Pacific may be contributing to the wetter than median climate outlooks in Australia.

Most climate models predict the negative Indian Ocean Dipole (IOD) event will persist through the southern hemisphere spring, despite current values rising marginally above the −0.4 °C threshold. Stronger-than-usual easterly winds across the tropical eastern Indian Ocean over the last three weeks have weakened the IOD event, however, this is most likely to be temporary. A majority of the five climate models surveyed by the Bureau predict the negative IOD will persist into November. A negative IOD increases the chance of above average winter–spring rainfall for much of southern and eastern Australia.

The Madden–Julian Oscillation (MJO) is currently in the western Indian Ocean, where it has remained since early August. This phase of the MJO typically sees increased cloudiness over the Southeast Asian parts of the Maritime Continent and eastern Indian Ocean, along with enhanced easterly wind anomalies, which may have acted to weaken the negative IOD. Forecasts generally indicate a weak MJO will progress eastwards across the Indian Ocean in the coming week.

The Southern Annular Mode (SAM) index has been positive over the past week and is forecast to remain so for the next fortnight. A positive SAM typically has a drying influence on south-west and south-east Australia during winter, and enhances chances of above average rainfall over much of New South Wales during spring.

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 is positive. It is expected to slowly decline, return to neutral values during the second half of September.

While positive during winter, SAM tends to decrease the chance of rainfall over much of the south-east and south-west of Australia.

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, but vary in how much cooling they expect.

Four of the seven surveyed models anticipate a continued ENSO-neutral state for the coming months, despite some cooling.

Two of the seven surveyed models sustain NINO3.4 at La Niña thresholds for at least three months during spring to early summer (i.e. long enough to indicate a full-fledged event), with one more anticipating La Niña thresholds are likely to be exceeded during November and December.

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 even if a full-fledged event does not eventuate.