BOM: La Niña anticipated to ease
BOM, 4 Jan 2023
La Niña continues in the tropical Pacific, but is slowly weakening. However, while ocean temperatures have warmed in recent weeks, atmospheric indicators are largely unchanged, remaining at La Niña levels. La Niña typically increases the chance of above average rainfall for northern and eastern Australia during summer.
Long-range forecasts suggest that tropical Pacific Ocean temperatures will continue to warm towards ENSO-neutral levels over the coming weeks, with some change in atmospheric patterns towards neutral levels also possible. As long-range forecast accuracy is generally lower at this time of year, ENSO outlooks that extend through autumn should be viewed with caution.
The Indian Ocean Dipole (IOD) is neutral and has little influence on Australian climate while the monsoon trough is in the southern hemisphere (typically December to April).
The Southern Annular Mode (SAM) is strongly positive and is likely to be positive until at least mid-January. During summer, a positive SAM increases the chance of above average rainfall for eastern New South Wales, eastern Victoria and north-east Tasmania and below average rainfall for western Tasmania.
The Madden–Julian Oscillation (MJO) has been in the Western Pacific since late December and is forecast to move into the Atlantic in the coming days. This may cause an easing in trade wind strength and increase cloudiness near the dateline, potentially weakening some indicators of La Niña.
Sea surface temperatures remain warmer than average in the western Pacific, much of the Maritime Continent, and around northern Australia. Warmer Australian waters, especially in the tropics, can result in greater evaporation, humidity, cloudiness, and rainfall.
Climate change continues to influence Australian and global climates. Australia’s climate has warmed by around 1.47 °C in 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.
Pacific Ocean Nino3.4 Forecast
A La Niña is under way in the tropical Pacific. While Pacific Ocean temperatures have warmed in recent weeks, atmospheric indicators have yet to respond. The Bureau’s ENSO Outlook continues at LA NIÑA.
All of the seven international climate models surveyed by the Bureau anticipate central Pacific sea-surface temperatures will decline over summer, with only one model continuing above La Niña thresholds (−0.8 °C or cooler) during January, and all models indicating ENSO-neutral values during February.
ENSO events typically peak in late (southern hemisphere) summer and decay during the autumn; current outlooks indicate this La Niña may decay slightly earlier than usual.
La Niña events increase the chance of above-average rainfall for northern and eastern Australia during summer. Even as La Niña weakens, it can continue to influence global weather and climate.
ENSO is not the only driver influencing Australia’s climate. Other factors such as warmer than average SSTs to Australia’s north-east are also contributing to current outlooks. The Bureau’s long-range forecast indicates some parts of eastern and northern Australia are likely to be wetter than average over the next three months.
Indian Ocean Forecast
The Indian Ocean Dipole (IOD) is neutral and has little influence on Australian climate while the monsoon trough is in the southern hemisphere (typically December to April). The IOD index value for the week ending 1 January 2023 was +0.07 °C; within neutral bounds (between −0.4 °C and +0.4 °C).
Sea surface temperatures (SSTs) are generally close to average across most of the Indian Ocean basin, although weak warm anomalies are present over some areas to the north of the basin, as well as off the coast of Western Australia and in parts of the Maritime Continent.
All five international climate models surveyed by the Bureau anticipate neutral values of the IOD index will continue through summer and into the southern hemisphere autumn.
Source: BOM Climate Driver Update
