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Informist, Wednesday, Oct. 1, 2025

MUMBAI – Though the El Nino-Southern Oscillation is currently neutral, the tropical Pacific is likely to face a brief La Nina event during spring before returning to El Nino-Southern Oscillation neutral conditions during the summer, the Australian Bureau of Meteorology said in its fortnightly report. The bureau's model prediction is consistent with all international models assessed, which indicate some further cooling is likely, reaching the threshold of a La Nina event, it said.

Sub-surface waters in the central tropical Pacific remain cooler than average, while atmospheric indicators such as pressure, wind, and cloud patterns remain mostly neutral, 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 El Nino-Southern Oscillation stood at (-)0.80 degrees Celsius as of Sunday, the bureau said. 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.

Currently, a negative phase of the Indian Ocean Dipole is underway, the bureau said. The negative Indian Ocean Dipole event is likely to continue through spring with a return to neutral in early summer, the bureau said. As of Sunday, the Indian Ocean Dipole value was (-)1.24 degrees Celsius.

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 lesser 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.

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 is a 71% chance of La Nina conditions forming during Oct-Dec.  End

Reported by Shreya Shetty

Edited by Nishant Maher

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