El Niño continues; positive IOD likely past its peak.
BOM, 5 December 2023.
According to the Bureau of Meteorology (BOM) Australia, 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.
Indian Ocean
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.
