Negative IOD likely to emerge this month, says Australian weather agency

There are no signs of the La Nina weather pattern emerging at least until February, while the Indian Ocean Dipole (IOD) may turn negative this month, Australia’s Bureau of Meteorology (BoM) has said. 

Some of the atmospheric indices are displaying La Nina signals over the recent months but a consistent and sustained shift in the atmosphere has not been observed, BoM said in its latest Climate Driver Update on Tuesday.

However, all models indicate that the IOD (Indian Ocean Dipole) index will meet or exceed negative IOD thresholds in November, with all but one returning to neutral levels in December, it said.

Lower rainfall in west

A negative IOD results in the western Indian Ocean turning cooler than normal. This could result in lower rainfall for India, particularly in the western parts, besides resulting in weak cyclonic activity in the Bay of Bengal. 

The latest Indian Ocean Dipole (IOD) index value is −0.69 °C for _the week ending 10 November, marking the fifth week below the negative IOD threshold (−0.40 °C). To be classified as a negative IOD event, the index needs to be at values below the negative IOD threshold for a sustained period of at least 6 to 8 weeks.

SSTs for the week ending November 10, 2024, were 0.8–2 °C warmer than the 1991-2020 average across much of the central and eastern Indian Ocean and close to average in the western equatorial Indian Ocean (with a small part of this area being up to -0.8 °C cooler than average). This pattern is typical of the ocean pattern associated with a negative IOD phase, said BoM.

IOD forecast skill has historically been moderate for forecasts out to 1 month ahead, the Australian weather agency said. 

ENSO-neutral conditions to continue

“Atmospheric indices, such as those related to patterns of surface pressure, cloud and trade winds, are broadly consistent with an ENSO-neutral state,” the weather agency.  

Global SSTs remain at near record levels as of  October 31, with temperatures since July being just short of the record temperatures observed during 2023, yet above all other years since observations began in 1854, it said. 

“The sustained nature of this significant global ocean heat suggests that climate indicators such as ENSO and IOD may not behave or evolve as they have in the past,” said BoM.

The El Nino–Southern Oscillation (ENSO) was neutral, with sea surface temperatures (SSTs) in the central equatorial Pacific Ocean at ENSO-neutral levels, said BoM.

The Bureau’s model suggests SSTs are likely to remain within the ENSO-neutral thresholds (−0.8 °C to +0.8 °C) throughout the forecast period to February 2025. 

“Of the 6 other climate models surveyed, only one model suggests SSTs in the tropical Pacific are likely to exceed the La Nina threshold (below −0.8 °C) throughout December to February, which is sufficient time to be classified as a La Nina event. All models forecast neutral ENSO values by March,” said the Australian weather agency.