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Informist, Monday, Dec. 29, 2025

 

MUMBAI – The Indian Ocean Dipole has reverted to a neutral phase from its earlier negative one, the Australian Bureau of Meteorology said in its fortnightly report. Meanwhile, the La Nina event remains active over the tropical Pacific Ocean, the bureau said. Temperatures over the tropical Pacific are likely to remain at La Nina levels till early 2026, it said.

 

The Indian Ocean Dipole is likely to remain neutral at least till the end of autumn 2026, the bureau said. Autumn in the Southern Hemisphere spans from March to May. The Indian Ocean Dipole is usually inactive from December to April, it said.

 

The Indian Ocean Dipole is the difference in surface temperatures between the eastern and western tropical Indian Ocean. A negative Indian Ocean Dipole leads to less rainfall in India, while a positive index leads to more rainfall and a neutral phase signals no significant change in precipitation. For monitoring the index, the bureau considers sustained values above 0.4 degrees Celsius for at least eight weeks as typical of a positive index and values below (−)0.4 degrees Celsius for the same duration as typical of a negative index.

 

Weekly values of the relative values of the El Nino-Southern Oscillation have been fluctuating around the La Nina threshold since mid-to-late September, the bureau said. As of Dec. 21, the El Nino-Southern Oscillation Index was (-)0.91 degrees Celsius. Ocean surface temperatures in the tropical Pacific between (-)0.80 degrees Celsius and 0.80 degrees Celsius are considered to be neutral conditions, while sustained values below (-)0.8 degrees Celsius for at least three months are considered indicative of a La Nina event. Values of the index have been fluctuating around the La Nina threshold since mid-to-late September, the bureau said. 

 

Atmospheric indicators such as trade winds, pressure, and cloud patterns over the equatorial Pacific also show consistent signs of La Nina. As of Dec. 21, the 30-day Southern Oscillation Index dropped to (-)0.1 due to tropical activity over Tahiti, French Polynesia, over the past few weeks, the bureau said. Though the index fell below the La Nina threshold of 7, the effect of transient summer systems on the Southern Oscillation Index is not necessarily a reflection of the large-scale state of the climate system, it said.

 

The 60-day and 90-day Southern Oscillation Index were 7.0 and 6.6, respectively, which are close to the La Nina threshold. Trade wind strength and cloud patterns have been indicative of La Nina since at least mid-to-late September, the bureau said.

 

The Southern Oscillation Index tracks the atmospheric part of the El Nino–Southern Oscillation, while the Oceanic Nino Index tracks the ocean part. The Southern Oscillation Index compares the difference from average air pressure in the western Pacific, measured in Darwin, Australia, to the difference from average pressure in the central Pacific, measured in Tahiti.

 

The El Nino-Southern Oscillation refers to the fluctuation between El Nino and La Nina conditions in the Pacific Ocean. El Nino is linked to drier and hotter weather in India while La Nina results in more favourable rainfall. 

 

In its monthly climate outlook on Dec. 1, the India Meteorological Department forecast La Nina conditions persisting through December to February before transitioning to neutral El Nino-Southern Oscillation conditions. The Indian weather department also forecast negative Indian Ocean Dipole conditions weakening in the coming months.  End

 

Reported by Shreya Shetty

Edited by Deepshikha Bhardwaj

 

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