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

SSTs for May 2023 were warmer than average over the eastern, southern and far west of the tropical Pacific Ocean. Warm anomalies up to 2 °C warmer than average were present over parts of these regions, increasing to more than 4 °C warmer than average off small, isolated parts of the South American coast.

Compared to April, warm anomalies in the east tropical Pacific Ocean have extended westwards. Large parts of the basin have cooled, especially in the southern central and eastern parts of the South Pacific Ocean. A band of warm anomalies in the south-west Pacific persists from the south-east of Papua New Guinea south-eastwards to around New Caledonia, Vanuatu, Fiji and the east of New Zealand.

Warm SST anomalies also continued in the southern Tasman Sea, between south-east Australia and New Zealand, as well as to the south-west of Western Australia.

Cool anomalies off the east coast of New South Wales increased in strength and spatial coverage during May.

Globally, the April and May 2023 SSTs were the warmest on record for their respective months. In the ERSSTv5 dataset, the global area-average SST for April was 0.69 °C above the 1961-1990 average, average, exceeding the previous April record of 0.67 °C in 2019. Global SST for May was 0.70 °C above average (previous May record was 0.63 °C in 2020.

For the week ending 4 June 2023, sea surface temperatures (SSTs) were warmer than average by up to 2 °C over parts of the south-west, south and central Pacific Ocean increasing to up to 3° C warmer than average in the eastern tropical Pacific and more than 4 °C warmer than average off the South American coast. Compared to two weeks ago, warm anomalies in the central and eastern tropical Pacific have remained similar in strength but have increased in spatial extent, reaching further westwards.

Anomalously warm SSTs in the western tropical Pacific Ocean and in the Coral Sea have marginally decreased in strength and spatial coverage in the last two weeks.

Warm SST anomalies persist in the southern Tasman Sea, from south-east Australia to around New Zealand, and near south-west Western Australia.

Cold SST anomalies have formed around much of the north-western Australian coastline, while the cold SST off the NSW coast has decreased in strength and spatial coverage.

The latest values of the three NINO indices for the week ending 4 June 2023 were: NINO3 +1.2 °C, NINO3.4 +0.8 °C, and NINO4 +0.6 °C.

Persistent NINO3 or NINO3.4 values cooler than +0.8 °C are typical of El Niño, while persistent values warmer than −0.8 °C typically indicate La Niña.

The 30-day Southern Oscillation Index (SOI) for the 30 days ending 4 June 2023 was −20.5, while the values for the 60-day SOI was −10.6 and the 90-day SOI was −7.1. Both the 30-day and the 90-day SOI have seen a steady decreasing trend over the past month.

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

Trade winds for the 5 days ending 4 June 2023 were close to average over most of the tropical Pacific.

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

A weakening Madden-Julian Oscillation (MJO) pulse lies over the western hemisphere. Most models indicate the signal will become weak or indiscernible in the coming days. While some models indicate a pulse will strengthen over the eastern Indian Ocean or western Maritime Continent in about a week, others maintain a weak or indiscernible signal. The MJO has little influence on Australian rainfall patterns at this time of the year.

The Indian Ocean Dipole (IOD) is currently neutral. The IOD index for the week ending 4 June 2023 was +0.32 °C, which is within neutral bounds (between −0.40 °C and +0.40 °C).

Weekly sea surface temperatures (SSTs) are above average across most of the tropical Indian Ocean, with warmer SSTs in the west than those in the east. Warm anomalies also exist over much of the southern half of the basin, and to the south-west of Australia. Small areas of cool anomalies exist off the north-west coasts of Western Australia.

All international climate models surveyed by the Bureau suggest a positive IOD event may develop in coming months. A positive IOD can supress winter and spring rainfall over much of central and south-east Australia, and if combined with El Niño, the drying effect is typically stronger and more widespread across Australia.

Cloudiness near the Date Line has been close to average since late April 2023.

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

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 31 May 2023) shows warm anomalies warm anomalies are present for most of the top 175m of the equatorial Pacific band. Anomalies reached more than 2 °C warmer than average across much of this region.

Compared to previous months, cool anomalies have steadily decreased in extent, and are now absent. During April, warm anomalies expanded in extent to cover the whole basin, strengthening in the eastern Pacific during May.

For the five days ending 4 June 2023, sub-surface temperatures were warmer than average across the equatorial Pacific, at depths between about 25 to 200 m in the western and central Pacific, and between the surface to 100 m depth in the eastern Pacific. Anomalies were more than 1 °C warmer than average across the basin, increasing to more than 3 °C in the west and 5 °C in the far east.

Compared to two weeks ago, warm anomalies have increased marginally in strength in the west, while cooling slightly in the central Pacific.

The Bureau's ENSO Outlook has been shifted to El Niño ALERT, indicating a 70% chance of El Niño forming this year. This equates to roughly three times the normal chance of an El Niño forming. Central and eastern Pacific sea surface temperatures (SSTs) have warmed to El Niño thresholds. All models surveyed by the Bureau are forecasting the likelihood of further warming and that these SSTs will remain above El Niño thresholds at least into the southern hemisphere spring. Some atmospheric indicators such as the Southern Oscillation Index (SOI) have shifted towards El Niño thresholds, but wind, cloud and broad-scale pressure patterns indicate the Pacific Ocean and atmosphere are yet to reinforce each other, as occurs during El Niño events. El Niño typically suppresses rainfall in eastern Australia during the winter and spring months.

The current status of the ENSO outlook does not change the Bureau's long-range forecast for drier and warmer conditions across much of Australia for winter. The Bureau's climate model takes into account all influences from the oceans and atmosphere when generating its long-range forecasts.

The Indian Ocean Dipole (IOD) is currently in a neutral phase, with the IOD index at +0.32 °C. All models suggest positive IOD event thresholds may be reached in winter. A positive IOD typically supresses winter and spring rainfall over much of Australia, and if it coincides with El Niño, it can exacerbate El Niño's drying effect. Long-range forecasts of IOD made at this time of the year have generally had low accuracy and thus should be viewed with caution beyond June.

A weakening Madden–Julian Oscillation (MJO) pulse lies over the western hemisphere. Most models indicate the signal will become weak or indiscernible in coming days. While some models indicate a pulse will strengthen over the eastern Indian Ocean or western Maritime Continent region, north of Australia, in about a week, others maintain a weak or indiscernible signal. The MJO has little influence on Australian rainfall patterns at this time of the year.

The Southern Annular Mode (SAM) index is currently neutral and is expected to hover around positive thresholds for the coming three weeks. During winter, a positive SAM often has a drying influence for parts of south-west and south-east Australia.

Climate change continues to influence Australian and global climates. Global SSTs were the highest on record for the months of April and May. The Australian continent has warmed by around 1.47 °C over the period 1910–2021. 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 of 10 to 20% in cool season (April–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 at neutral levels (within 1 standard deviation), with forecasts hovering close to positive SAM index values for the coming three weeks.

During winter, positive SAM typically has a drying influence for parts of south-west and south-east Australia.

Central and eastern Pacific Sea surface temperatures (SSTs) have warmed to El Niño thresholds. International climate models suggest further warming of the central and eastern tropical Pacific Ocean is likely. By the end of June, six out of seven surveyed models indicate El Niño thresholds for SSTs will be reached with the seventh reaching the threshold during July. All models persist at these levels until at least the end of the southern spring. If the atmosphere responds to this warming, an El Niño event would be expected to develop.

El Niño typically suppresses rainfall in eastern Australia during the winter and spring months.