Negative IOD may be underway if index remains below threshold for another week, says BoM

Australia’s Bureau of Meteorology (BoM) has predicted that negative Indian Ocean Dipole (IOD) or Indian Nino, a climate phenomenon similar to the El Nino, will likely be underway if the IOD index remains below the –0.4 °C for another week. At the same time, possibilities for La Nina weather — which brings heavy rains and floods to India and other parts of India — are remote.

“Should a La Niña develop, it is forecast to be relatively weak (in terms of the magnitude of the sea surface temperature anomaly) and short-lived, with all models forecasting ENSO-neutral by March 2025,” BoM said in its latest climate driver update. The Australian weather agency said the IOD index has been below the negative threshold since mid-October. “If the IOD index remains below the threshold for another week, it would indicate a negative IOD event is underway,” it said.

However, all but one of the surveyed climate models indicate that the IOD index is expected to return to neutral levels in December, in line with the typical IOD event lifecycle. Sea surface temperatures (SSTs) for the week ending November 24 were 0.8–2 °C warmer than the 1991–2020 average across much of the central and eastern Indian Ocean and 0.

4–1.2 °C warmer than average in the western equatorial Indian Ocean. This indicates that the western Indian Ocean is cooler than the eastern Indian Ocean.

“This ocean pattern is typical of a negative IOD phase,” said BoM. The Indian Ocean Dipole (IOD) index for the week ending November 24 was −0.54 °C, having been below the negative IOD threshold (–0.

4 °C) since mid-October. To be classified as a negative IOD event, the IOD index needs to be at values below the negative IOD threshold (–0.4 °C) for at least 8 weeks.

IOD is the fluctuation of sea temperature between western and eastern parts. A positive IOD can result in increased rainfall in parts of India. However, negative IOD will result in lower rainfall on the western parts of India, while the Bay of Bengal will likely witness above-normal cyclonic activity.

According to the India Meteorological Department, data from 720 district show that 66 per cent of the country received deficient or no rainfall during the post-monsoon period between October 1 and November 26. BoM said the El Niño–Southern Oscillation (ENSO) was neutral, with (SSTs) in the central equatorial Pacific Ocean at ENSO-neutral levels. “Atmospheric indices, such as those related to patterns of surface pressure, cloud and trade winds, are broadly consistent with an ENSO-neutral state,” it said.

Ocean temperatures in the central equatorial Pacific have started to warm in recent weeks, away from the La Niña threshold, although they are still cooler than the historical average. 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, 2 models suggest SSTs in the tropical Pacific are likely to exceed the La Niña threshold (below −0.8 °C) throughout December to February.

This is sufficient to be classified as a La Niña event, though this would be considered a very short-lived event compared to the historical record. “All models forecast neutral ENSO values by March,” said the Australian weather agency. Comments.