Tropical Climate Update

El Niño and positive Indian Ocean Dipole underway

The Bureau has declared an El Niño and a positive Indian Ocean Dipole (IOD) are underway. 

Sea surface temperature (SST) patterns across the Pacific Ocean have been consistent with El Niño thresholds since at least mid-winter in the southern hemisphere, but the overlying atmosphere has been slower to respond. More recently, the atmosphere has shown clearer signs of responding to the warm SSTs over the central and eastern tropical Pacific. The broad-scale pressure pattern for the last 30 days has reflected an El Niño, with low pressure anomalies over the central and eastern Pacific and high-pressure anomalies over Australia. The 90-day Southern Oscillation Index (SOI) is −7.8, meeting criteria indicative of El Niño (sustained values below −7). Recent trade wind strength has been generally close to average, but was slightly weaker than average across the Pacific in August 2023 for the first time since January 2020. Overall, there are signs of coupling of the Pacific Ocean and atmosphere. This coupling is a characteristic of an El Niño event and is what strengthens and sustains an event for an extended period. El Niño typically leads to reduced spring rainfall for eastern Australia. 

A positive IOD has also developed, with the Dipole Mode Index (DMI), also known as the IOD index, being above the positive IOD threshold (+0.4 °C) for the last five weeks. The IOD index for week ending 17 September was +1.25 °C. The longevity of this trend, combined with the strength of the dipole being observed and forecast, indicate a positive IOD event is underway. Cooler than average SSTs have been developing off the south-west coast of Indonesia since late August, while SSTs in the western Indian Ocean have remained warmer than average. 

All models indicate that El Niño will be sustained to at least until the end of summer while the positive IOD will be sustained to at least the end of spring. A combined El Niño and positive IOD can have significant impacts on northern Australian weather early on in the northern wet season (October to April). Some of the main outcomes that have historically been observed are lower than average rainfall from October to December, a later monsoon onset and low tropical cyclone activity at the start of the tropical cyclone season (November to April). Concurrent El Niño and positive IOD events can potentially lead to these impacts being exacerbated and extended further into the wet season. 

Recent concurrent events were in 2015, when a strong El Niño coincided with a positive IOD during spring of that year, and prior to that, 2006. Spring 2015 was 8^th-warmest on record (since 1910) for northern Australia, with rainfall around 27% below the long-term (1961–1990) average.

Madden–Julian Oscillation weak

A weak pulse of the MJO is currently over the Maritime Continent. Climate models surveyed by the Bureau indicate the MJO will weaken as it continues to move over the Maritime Continent in the coming week.