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

Sea surface temperatures (SSTs) for November 2023 were warmer than average across almost all the equatorial Pacific Ocean. Across much of the tropical Pacific between 10°S and 10°N, SST anomalies were more than 0.8 °C warmer than the long-term (1961–1990) average, increasing to up to 3 °C warmer than the long-term average east of the Date Line.

The extent of warm anomalies across the equatorial Pacific has slightly decreased compared to October.

Cool anomalies off the west coast of Java, Indonesia have also decreased in magnitude compared to October.

Warm SST anomalies also continued in the western Tasman Sea, including off most of the south-eastern coast of Australia, and have strengthened across other parts of the Tasman Sea. Warm anomalies to the west of Australia decreased in magnitude compared to October, with waters warmer than 0.8 °C above average surrounding much of Western Australia.

The Bureau's long-range forecasts indicate warmer than average SSTs (up to around 2 °C warmer than average) off the coast of south-east Australia will likely continue through the southern hemisphere summer 2023–24. Forecast unusually warm sea surface temperatures (SSTs) in the Tasman Sea may also be contributing to a chance of above median summer rainfall over parts of Australia. Warm anomalies in the Coral Sea are forecast to persist into early 2024.

Globally, in the ERSSTv5 dataset, SSTs for April to November 2023 were warmest on record (since 1900) for their respective months. August and September SSTs were also globally warmest and second-warmest, respectively, for any month.

For the week ending 3 December 2023, sea surface temperatures (SSTs) were warmer than average across the equatorial region of the tropical Pacific Ocean. Anomalies were more than 1.2 °C warmer than average east of the Date Line, excluding regions along the Peruvian coast south of the equator, which have cooled to 0.8 °C warmer than average. Anomalies were more than 2 °C warmer than average in some areas of the central and eastern tropical Pacific. Compared to last fortnight, warm anomalies have generally decreased, particularly along the equator and the far eastern Pacific.

Closer to Australia, warm anomalies up to 3 °C above average persist off the south-east coast, with anomalies up to 2 °C above average extending into the southern Tasman Sea. There were also warm SST anomalies off the north-west and south-west coasts of Australia, mostly up to about 1.2 °C above average. Compared to last fortnight, warm anomalies have returned to the Coral Sea and the warm anomalies in the Tasman Sea have slightly increased.

The latest values of the three NINO indices for the week ending 3 December 2023 were: NINO3, +1.69 °C; NINO3.4, +1.77 °C; and NINO4, +1.60 °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 3 December 2023 were −8.1, −7.2 and −9.1, respectively.

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

Trade winds for the 5 days ending 3 December 2023 were close to average over the equatorial Pacific.

Trade winds for November 2023 have been slightly weaker to weaker than average across the western equatorial Pacific. Trade winds were stronger than average over the central and western Pacific during the second half of November. Trade winds have been weaker than average over the central equatorial Pacific for most of September to November.

During El Niño, there is typically 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.

A moderate to strong pulse of the Madden–Julian Oscillation (MJO) is currently over the Maritime Continent. International climate models suggest it will move across the Maritime Continent and into the western Pacific over the coming fortnight. This would typically have a wetting influence on parts of northern and central Australia.

The positive Indian Ocean Dipole (IOD) event continues. The IOD index is +1.20 °C for the week ending 3 December 2023. The weekly IOD index values for this event have been the second-highest on record since the Bureau SST dataset began in 2001, with the highest on record being during the strong positive IOD event of 2019. To date, the highest weekly IOD index for the current event was +1.92 °C for the week ending 15 October.

Sea surface temperatures (SSTs) for the week ending 3 December show warmer than average waters across much of the western half of the tropical Indian Ocean and south of 15°S. Conversely, the eastern pole of the IOD was cooler than average, with a small area of cooler waters extending off the coast of Java, Indonesia. A gradient between the western and eastern tropical Indian Ocean that is typical of a positive IOD is still apparent but compared to last fortnight, the strength and extent of the cooling in the eastern pole has reduced and there has been a slight decrease in the strength of the anomalous warmth in the western pole.

IOD events typically breakdown as the monsoon trough shifts south into the southern hemisphere, typically at the end of spring. Given the current strength of this event and the active El Niño, the breakdown this year is likely to be slightly later than usual. All international climate models surveyed by the Bureau suggest the positive IOD event is likely to ease in December.

December rainfall is likely to be below median for much of northern Australia, southern WA and along the coast in SA. When the positive IOD persists into December, the usual dry and warm signal associated with spring tends to persist as well, though typically to a lesser degree. However, the small number of years where the IOD has persisted into December means there is greater uncertainty on its typical impacts.

Outgoing Longwave Radiation (OLR) around the equatorial Date Line is currently below average (indicating increased cloudiness). OLR has been mostly below average since mid-September 2023. 

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 30 November 2023) shows warm anomalies across most of the top 100 m of the equatorial Pacific band, with the exception of the western Pacific. Anomalies increase in magnitude eastwards across the equatorial Pacific band with the central Pacific up to 2.5 °C warmer than average, and the eastern Pacific more than 2.5 °C warmer than average.

Warm anomalies have persisted in the central and eastern Pacific between the surface and 200 m depth. Compared to September, the depth of warm anomalies above 1 °C in the central Pacific have increased to depths of 200 m. The depth of warm anomalies in the eastern Pacific has decreased over the past 4 months. West of the Date Line, cool anomalies continue to strengthen between 100 and 300 m depth, undercutting warm anomalies above them in the central Pacific.

For the 5 days ending 3 December 2023, sub-surface temperatures were warmer than average across the upper levels of the equatorial Pacific east of the Date Line, between the surface to around 100 m depth in the eastern Pacific and between the surface to around 180 m depth in the central Pacific. Much of this region was more than 2 °C warmer than average, with anomalies more than 5 °C warmer than average between 125 °W and 95 °W at around 50 m depth.

Compared to last fortnight, the magnitude of warm anomalies in the central Pacific have increased. Cool anomalies have increased in magnitude and extent between 100 to 250 m below the surface in areas west of the Date Line.

The long-range forecast for Australia indicates December to February rainfall is likely to be below median across much of northern and western Australia. Warmer days and nights are very likely for almost all of Australia.

El Niño continues in the tropical Pacific. Indicators of the El Niño–Southern Oscillation (ENSO), including tropical Pacific sea surface temperatures (SSTs), cloud, wind, and pressure patterns are consistent with El Niño conditions. Climate model forecasts indicate some further warming of central to eastern Pacific SSTs is possible, with SSTs remaining above El Niño thresholds early into the second quarter of 2024.

The influence of El Niño on Australian rainfall usually reduces during summer, especially in the east; however, below median rainfall is still often observed in north-east Australia. Additionally, high-impact rainfall events can occur during El Niño years, particularly during October to April when severe storm frequency peaks.

The positive Indian Ocean Dipole (IOD) event continues. IOD index values have eased from their highest values in October and are unlikely to re-strengthen, meaning the positive IOD event is likely past its peak. All international climate models surveyed by the Bureau suggest the positive IOD is likely to ease in December.

The Southern Annular Mode (SAM) index is currently neutral. Forecasts suggest it is likely to remain mostly neutral over the coming fortnight. A neutral SAM has limited influence on Australian climate.

A moderate to strong Madden–Julian Oscillation (MJO) pulse is currently over the Maritime Continent. International climate models suggest it will move across the Maritime Continent and into the western Pacific over the coming fortnight. When the MJO is over the Maritime Continent, it typically increases rainfall over parts of northern and central Australia.

Global warming continues to influence Australian and global climate. Global sea surface temperatures (SSTs) were highest on record for their respective months during April to November. Forecast unusually warm sea surface temperatures (SSTs) in the Tasman Sea may also be contributing to a chance of above median summer rainfall over parts of Australia.

Australia's climate has warmed by 1.48 ± 0.23 °C since national records began in 1910. There has been an increase in extreme heat and fire weather associated with the warming. There has also been a trend towards a greater proportion of rainfall from high intensity, short duration rainfall events, especially across northern Australia.

The Bureau's climate model includes all influences on Australian climate when generating its forecasts, including the influence of climate change.

The Southern Annular Mode (SAM) is currently neutral. Forecasts indicate it is likely to remain mostly neutral for the coming fortnight. A neutral SAM has limited influence on Australian climate.

International climate models suggest some further warming of the central to eastern tropical Pacific Ocean is possible. All surveyed models indicate SSTs will remain above El Niño thresholds early into the second quarter of 2024. The 2023 El Niño event is tracking around at least moderate strength.

The long-range forecast for southern hemisphere summer (December to February) indicates rainfall is likely to be below median across much of northern and western Australia. This is consistent with historical patterns of reduced summer rainfall associated with El Niño, which typically show a drier summer for north-east Australia. The influence of El Niño on Australian rainfall usually reduces during summer, especially in the east; however, below median rainfall is still often observed in north-east Australia. Additionally, high-impact rainfall events can occur during El Niño years, particularly during October to April when severe storm frequency peaks.