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

SSTs for April 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 these regions, increasing to more than 4 °C warmer than average off parts of the South American coast.

Compared to March, large parts of the basin have warmed, especially in the east and south-west of the tropical Pacific.

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. Warm anomalies off the west coast of Western Australia have eased to near-average values.

Globally, the April 2023 SSTs were the warmest on record. In the ERSSTv5 dataset, the global area-average SST was 0.71 °C above the 1961-1990 average, exceeding the previous April record of 0.67 °C in 2019.

For the week ending 21 May 2023, sea surface temperatures (SST) were warmer than average over the eastern equatorial Pacific Ocean, from around 140°W to the South American coastline. Compared to two weeks ago, these warm anomalies have remained similar in strength but increased marginally in spatial extent, reaching further westwards. Warm anomalies also exist across much of the Pacific south of 10°S.

Anomalously warm sea surface temperatures continue to persist in the western tropical Pacific Ocean increasing in spatial coverage in the last two weeks, meaning there is little temperature gradient along most of the equator barring near the South American coastline, and hence contributing to the limited opportunity for an atmospheric response to the eastern Pacific warming.

A band of warm anomalies also stretches from the northern Maritime Continent, into the Coral Sea and to the north-east of New Zealand.

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

A cold SST anomaly off the NSW coast has increased in strength and spatial coverage over the last two weeks.

The latest values of the three NINO indices for the week ending 21 May 2023 were: NINO3 +1.0 °C, NINO3.4 +0.6 °C, and NINO4 +0.4 °C.

The 30-day Southern Oscillation Index (SOI) for the 30 days ending 21 May 2023 was −9.3, while the 90-day SOI value was -2.9. The 30-day SOI has maintained a negative value over the past fortnight, and both the 30-day and the 90-day SOI continue to demonstrate a gradual decreasing trend.

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 21 May 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 moderately strong Madden-Julian Oscillation (MJO) pulse is moving over the Western Pacific region and is forecast to move into the central Pacific region in the coming week. There is some variation amongst climate models, with some indicating marginal strengthening and continued eastwards progression across the tropical Pacific until the start of June, while others suggest the MJO pulse will rapidly weaken and become indiscernible. However, an MJO pulse over the western Pacific would likely weaken trade winds across the equatorial Pacific Ocean. This, in turn, would result in further warming of the equatorial Pacific Ocean and hence drive further development towards El Niño.

The Indian Ocean Dipole (IOD) is currently neutral. The IOD index value for the week ending 21 May 2023 was −0.15 °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, reaching up to 2 °C above average near the western Maritime Continent and in the far east. Warm anomalies also exist over much of the southern half of the basin, and off the southwest of Australia.

Small areas of cool anomalies exist in the central Indian Ocean and off the Pilbara and south-west coasts of Western Australia.

All international climate models surveyed by the Bureau suggest a positive IOD event may develop in the coming months. A positive IOD can supress winter and spring rainfall over much of central and southeast Australia, and if combined with El Niño, the drying effect is typically stronger and more widespread across Australia. However, model accuracy has historically been low at this time of year, and therefore outlooks for beyond June should be viewed with caution.

Cloudiness near the Date Line has recently been around average since late April 2023.

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

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 23 May 2023) shows warm anomalies across the basin, between 50 m and 250 m in the western Pacific, between 75 m and 175 m depth in the central Pacific and above 125 m depth in the eastern Pacific. Near the South American coastline, the depth of the anomaly increases again to about 200 m. 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 21 May 2023, sub-surface temperatures were warmer than average across the equatorial Pacific between about 50 m depth and 200 m depth in the western and central Pacific, and between the surface and 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 in strength over the far-east of the basin and also marginally in the west, whilst cooling slightly in the central Pacific.

The Pacific Ocean is currently ENSO-neutral (neither La Niña nor El Niño). Sea surface temperatures are warmer than average in the west and the east of the tropical Pacific. While all international climate models indicate it is very likely that tropical Pacific Ocean temperatures will reach El Niño thresholds during the southern hemisphere winter, an atmospheric response is also required for an El Niño to be declared. Thus far, little shift has been observed in atmospheric ENSO indicators with trade winds and cloudiness patterns in the Pacific remaining indicative of ENSO-neutral conditions. The 30-day SOI has dropped below the El Niño threshold, but sustained values are required for it to be considered a part of an El Niño response.

The ENSO Outlook remains at El Niño WATCH. This indicates there is an increased risk of an El Niño occurring this year, at least double the usual chance. History shows that when the ENSO Outlook has reached El Niño WATCH, El Niño has subsequently developed in about half of those years. El Niño typically suppresses rainfall in eastern Australia during the winter and spring months. The 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 forecast.

The Indian Ocean Dipole (IOD) is currently neutral. All five models suggest that a positive IOD event could develop in winter. A positive IOD typically supresses winter and spring rainfall over much of Australia, and if it occurs with El Niño, it can exacerbate El Niño's drying effect.

A moderately strong Madden-Julian Oscillation (MJO) pulse is moving over the Western Pacific region and is forecast to move into the central Pacific region in the coming days. There is some variation amongst climate models, with some indicating marginal strengthening and continued eastwards progression across the tropical Pacific until the start of June, while others suggest the MJO pulse will rapidly weaken and become indiscernible. However, if the MJO pulse maintains its strength and continues to track over the western or central Pacific, it would likely weaken trade winds across the equatorial Pacific Ocean. This, in turn, would result in further warming of the equatorial Pacific Ocean and hence drive further development towards El Niño.

The Southern Annular Mode (SAM) index is currently positive and is expected to return to neutral towards the end of May. During autumn SAM typically has a weaker influence on Australian rainfall, but as we approach winter, 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. In April 2023, the global sea surface temperature (SST) was the highest on record for that month, while for the Coral Sea SSTs were the 2nd-highest on record. 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.

The Southern Annular Mode (SAM) index is currently positive and is expected to return to neutral around the end of May.

During autumn SAM typically has a weaker influence on Australian rainfall, but as we approach winter, positive SAM often has a drying influence for parts of south-west and south-east Australia.

The El Niño–Southern Oscillation (ENSO) is currently neutral (neither La Niña nor El Niño).

The Bureau's ENSO Outlook is at El Niño WATCH. An El Niño WATCH is not a guarantee that El Niño will occur, rather it is an indication that some of the typical precursors of an event are currently in place. An El Niño WATCH indicates there is an increased risk of an El Niño occurring this year, at least double the usual chance.

Oceanic warming is continuing in the central and eastern tropical Pacific. However, the western Pacific also remains warmer than average. There are indications of some changes in the atmosphere but this needs to be sustained over several weeks or months.

While models are forecasting a very high likelihood that ocean temperatures in the central and eastern Pacific will reach El Niño thresholds in early spring, an accompanying shift in atmospheric circulation is necessary for an El Niño event to be declared. Due to the persistence of neutral atmospheric indicators over the tropical Pacific, our ENSO outlook remains at El Niño WATCH.

International climate models suggest further warming of the central and eastern tropical Pacific Ocean is likely. By July, all surveyed models indicate El Niño thresholds for sea surface temperatures will be met or exceeded. El Niño typically suppresses rainfall in eastern Australia during the winter and spring months.