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Informist, Wednesday, Oct. 29, 2025

 

MUMBAI – The El Nino Southern Oscillation remains neutral with some signs of a possible La Nina development in the tropical Pacific Ocean, though these signals have not been sustained at sufficient strength or duration to meet the criteria for an active La Nina event yet, the Australian Bureau of Meteorology said in its fortnightly report Tuesday. However, further cooling of the tropical Pacific Ocean, which remains cooler than average, is likely to meet La Nina levels during November and December, the report stated.

 

Currently, some atmospheric indicators such as trade winds, pressure, and cloud patterns over the equatorial central Pacific show some signs of La Nina development. The 30-day Southern Oscillation Index has recently exceeded 7 degrees Celsius, although the 90-day Southern Oscillation remains neutral. As of Sunday, the Madden–Julian Oscillation is enhancing trade winds over the western Pacific, which may provide a short-term boost to La Nina conditions, 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.

 

As of Sunday, the El Nino-Southern Oscillation stood at (−) 0.86 degrees Celsius. The weekly values of the relative index have been fluctuating around the La Nina threshold since mid-to-late September. 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, the bureau said.

 

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, hotter weather in India, while La Nina results in more favourable rainfall.

 

The negative phase of the Indian Ocean Dipole remains active, with the latest index value reaching (-)1.61 degrees Celsius for the week ended Sunday, which is "the most negative Indian Ocean Dipole value recorded since the Australian bureau's dataset began in August 2008," it said. The negative Indian Ocean Dipole event is likely to continue through spring with a return to neutral in early summer, the bureau said.

 

Spring in the Southern Hemisphere spans from Sept. 22 to Dec. 20, while summer is from Dec. 21 to Mar. 19. 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. 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.

 

The advent of both La Nina and a negative Indian Ocean Dipole points to different variabilities in the intensity of rainfall over India. However, when both events occur simultaneously, La Nina is the dominating feature, A. Suryachandra Rao, director, Indian Institute of Tropical Meteorology, Pune, told Informist in an interview in September.

 

This is supported by the India Meteorological Department's outlook of above-normal rainfall over India in October. The weather department has forecast above-normal rainfall for the country at 115% of the long-period average. The department also said there was a 71% chance of La Nina conditions forming during Oct-Dec.  End

 

Reported by Shreya Shetty

Edited by Nishant Maher

 

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