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

Sea surface temperatures (SSTs) for August 2023 were warmer than average over almost all of the equatorial Pacific Ocean. SST anomalies more than 0.8 °C warmer than the long term average (1961–1990) were present over much of the tropical Pacific, increasing to more than 3 °C warmer than average in the eastern tropical Pacific. Much of the southern Pacific was also warmer than average for August.

Compared to July, warm anomalies have strengthened across the central and western equatorial Pacific.

Warm SST anomalies also continued in the southern Tasman Sea, between south-east Australia and New Zealand, but have slightly decreased compared to last month. Warm anomalies have spread to surround most of eastern and south-eastern Australia. Compared to last month, SSTs have warmed, but remain close to average across much of the waters surrounding Western Australia.

Globally, April to August 2023 SSTs were warmest on record (since 1900) for their respective months. In the ERSSTv5 dataset, the global area-average SSTs for April, May, June, July and August were above their long term  (1961–1990) average by 0.69 °C, 0.70 °C, 0.71 °C, 0.80 °C and 0.75 °C, respectively. August 2023 SSTs were also the warmest globally for any month since observational records began in 1854.

For the week ending 24 September 2023, sea surface temperatures (SSTs) were warmer than average across almost all the equatorial region of the tropical Pacific Ocean. Anomalies were more than 2 °C warmer than average in the eastern tropical Pacific. Compared to last week, the strength and spread of warmth in the tropical Pacific remains similar. Atypical to a normal El Niño pattern, warm anomalies continue to persist in the south-west Pacific from the east of the Solomon Islands extending south-eastwards to Tahiti.

Closer to Australia, warmer than average waters persist along the east Australian coastline, with waters up to 2 °C above average along the north-and south-east coasts. There are also warm SST anomalies in patchy areas south-west of WA and in the southern Tasman Sea, from south-east Australia to the South Island of New Zealand. The past fortnight has seen warmer than average water appear off the northern WA coastline.

The latest values of the three NINO indices for the week ending 24 September 2023 were: NINO3, +1.82 °C; NINO3.4, +1.45 °C; and NINO4, +1.15 °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-, 60- and 90‐day Southern Oscillation Index (SOI) values for the period ending 24 September 2023 were −16.0, −13.1 and −10.2, respectively.

Sustained negative values of below −7 typically indicate El Niño, while sustained positive values above +7 typically indicate La Niña.

Trade winds for the 5 days ending 24 September 2023 were weaker than average in the far western equatorial Pacific, but generally close to average elsewhere.

Trade winds for August 2023 were slightly weaker than average across the equatorial Pacific for the first time since January 2020.

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

The Madden–Julian Oscillation (MJO) is currently weak or indiscernible. About half the models indicate it is likely to remain weak or indiscernible, while the other half suggest the MJO may strengthen and progress into the Maritime Continent and western Pacific over the coming fortnight.

A positive IOD event is underway. The Indian Ocean Dipole (IOD) index was +1.45 °C for week ending 24 September. This is its sixth week above the positive IOD threshold (+0.40 °C).

Weekly sea surface temperatures (SSTs) for the week ending 24 September show warmer than average waters close to the Horn of Africa. Conversely, the eastern pole of the IOD was cooler than average, with a notable area of cooler waters extending southwards from the coast of Java. This shows a clear gradient between the western and eastern tropical Indian Ocean that is typical of a positive IOD. Compared to last week, the cooling has expanded westwards from Java and the warm anomalies have slightly cooled over the western pole of the IOD.

All international climate models surveyed by the Bureau suggest the positive IOD event is likely to continue for the remainder of the southern hemisphere spring. A positive IOD typically leads to reduced spring rainfall for central and south-east Australia. When a positive IOD and El Niño occur together, their drying effect is typically stronger and more widespread across Australia.

Outgoing Longwave Radiation (OLR) near the Date Line has been below average for the past 30 days ending 24 September.

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

The 4-month sequence of equatorial Pacific sub-surface temperature anomalies (to 25 September 2023) shows warm anomalies across most of the top 150 m of the equatorial Pacific band, except in the far west. Anomalies increase in magnitude eastwards across the equatorial Pacific band, with the far west close to average and the eastern Pacific more than 2 °C warmer than average.

The past 3 months have seen sub-surface heat shift towards the eastern Pacific, between the surface and 150 m depth. During September, there has been a movement of warmth away from the South American coastline. However, warm anomalies in the eastern Pacific have decreased slightly. In the western Pacific, temperatures have decreased such that most of the water column is now close to average.

For the 5 days ending 24 September 2023, sub-surface temperatures were warmer than average across the upper levels of the equatorial Pacific between the surface to around 125 m depth. Much of this region was more than 2 °C warmer than average, with anomalies more than 5 °C warmer than average in a small area of the eastern Pacific.

Compared to last week, warm anomalies in the eastern and central Pacific have increased.

An El Niño and a positive IOD are underway.

Oceanic indicators firmly exhibit an El Niño state. Central and eastern Pacific sea surface temperatures (SSTs) continue to exceed El Niño thresholds. Models indicate further warming of the central to eastern Pacific is likely.

Broadscale pressure patterns over the tropical Pacific reflect El Niño, with the 90-day Southern Oscillation Index (SOI) at −10.2. Trade wind strength over the past week has weakened in the far western Pacific, but is close to normal elsewhere. Overall, there are signs that the atmosphere is responding to the warm SSTs over the Pacific and coupling of ocean and atmosphere is occurring. This coupling is a characteristic of an El Niño event and is what strengthens and sustains an event for an extended period. El Niño typically leads to reduced spring rainfall for eastern Australia.

Climate models indicate this El Niño is likely to persist until at least the end of February. El Niño typically leads to reduced spring and early summer rainfall for eastern Australia, and warmer days for the southern two-thirds of the country.

A positive Indian Ocean Dipole (IOD) is underway. The IOD index is +1.45 °C for week ending 24 September. This is its sixth week above the positive IOD threshold (+0.40 °C). All models predict this positive IOD will persist to at least the end of spring. A positive IOD typically leads to reduced spring rainfall for central and south-east Australia.

When a positive IOD and El Niño occur together, their drying effect is typically stronger and more widespread across Australia.

The Madden–Julian Oscillation (MJO) is currently weak or indiscernible. About half the models indicate it is likely to remain weak or indiscernible, while the other half suggest the MJO may strengthen and progress into the Maritime Continent and western Pacific over the coming fortnight.

The Southern Annular Mode (SAM) index is currently neutral with forecasts indicating it is likely to remain neutral over the coming fortnight.

The long-range forecast for Australia indicates warmer and drier than average conditions are likely across most of Australia from October to December. The Bureau's climate model takes into account all influences from the oceans and atmosphere when generating its long-range forecasts.

Global warming

Global warming continues to influence Australian and global climate. Global sea surface temperatures (SSTs) were warmest on record for their respective months during April to August 2023. August 2023 SSTs were also the warmest globally for any month since observational records began in 1850.

Australia's climate has warmed by an average of 1.48 ± 0.23 °C since national records began in 1910. There has also been a trend towards a greater proportion of rainfall from high intensity, short duration rainfall events, especially across northern Australia. Southern Australia has seen a reduction, by 10 to 20%, in cool season (April to October) rainfall in recent decades. This is due to a combination of natural variability on decadal timescales and changes in large-scale circulation caused by an increase in greenhouse gas emissions.

The Southern Annular Mode (SAM) index is currently neutral with forecasts indicating it is likely to remain neutral for the coming fortnight. During spring, a neutral SAM is associated with typical climate conditions for Australia.

Central and eastern Pacific sea surface temperatures (SSTs) currently exceed El Niño thresholds. International climate models suggest some further warming of the central and eastern tropical Pacific Ocean is likely. All surveyed models indicate SSTs will remain above El Niño thresholds until at least the end of the 2023–24 southern hemisphere summer.

Bureau long-range forecasts are for SSTs up to 2.5 °C warmer than average off eastern Tasmania and in the eastern Tasman Sea from October to the end of 2023.

El Niño typically increases the chance of below average winter–spring rainfall in eastern Australia and warmer than average days for the southern two-thirds of Australia.