ONI Drops Below Threshold Mark, ENSO-Neutral Likely Soon

El Niño and La Niña typically develop in the Northern Hemisphere’s spring-summer and peak in winter. There is yet no conclusive evidence of the impact of climate change on the frequency and intensity of El Niño/La Niña events. However, these respective events contribute towards ocean warming and cooling. An El Niño event can have the overall effect of increasing global average temperature, whereas La Niña events have a cooling tendency. Yet, the prolonged triple-dip La Niña phase from 1998–2001 and again from 2020–2022 failed to induce notable cooling. The last decade, between 2015–2024 figures among the warmest of all, individually and collectively. Human-induced climate change will continue to impact the intensity and frequency of El Niño/La Niña, triggering extreme weather and chaotic climatic events across the globe.

ENSO: The sea surface temperature anomaly in the Nino 3.4 region during the Dec–Feb 2025 season was -0.56°C, and for February 2025, it was -0.35°C. Further, for March 2025, the average Nino 3.4 index was nearly zero-zero. It also means that the SST anomaly in the Nino 3.4 region during the Jan–Mar 2025 season is likely to rise above the threshold value of -0.5°C. The preliminary estimates point to a quarterly index of -0.4°C, a rise of about 0.1°C from the previous quarter. The warming in the equatorial Pacific across the Nino region hints at the cessation of the La Niña event earlier than the expected timelines. Practically, La Niña conditions dominated for merely two months: Dec 2024 and Jan 2025. Early withdrawal of La Niña presages a quick transition to ENSO-neutral anytime soon.

Equatorial SSTs are near-to-below average in the Central Pacific Ocean and above-average in the Eastern Pacific Ocean. The Nino 3.4 region has been the most volatile, wherein the temperature anomaly is swinging between marginal +ve and -ve scores. The marker index for the ONI is implying borderline La Niña conditions, more distinctly favouring a rapid transition to ENSO-neutral.

IOD: The Indian Ocean Dipole is a climatic phenomenon that involves variations in the sea surface temperature and atmospheric conditions across the Indian Ocean. Popularly, it is also called the Indian Niño. Primarily, the index is defined by the difference in SSTs between the Arabian Sea and the eastern Indian Ocean. IOD can be positive, negative, or neutral. Out of 10 episodes, about two each are positive or negative, and the rest are neutral. Consecutive positive or negative IODs are not very common; more so, the repeated positive occurrences are rare.

The IOD is neutral. The latest value of the IOD index for the week ending 30 Mar 2025 was +0.74°C. There has been significant cooling in the eastern Indian Ocean Dipole region over the past week, which has contributed to this positive index value. As per the Bureau of Meteorology, this is expected to be temporary, with the index returning to neutral value during April itself. The Bureau’s model predicts a neutral IOD until at least August.

MJO: The Madden-Julian Oscillation rapidly propagated eastward from the Western Indian Ocean into the Western Pacific during the second half of March. Quite unusually, the pulse had retrograded for some time and also retreated inside the unit circle. The weakened MJO with marginal amplitude will shift from the Western Pacific to the Western Hemisphere & Africa, in Phases 7 and 8, during the first half of April 2025. It may remain stagnant and decrepit too, for a brief period. The MJO may not contribute effectively to the generation of tropical cyclones in the Pacific and Indian Oceans during the next two weeks. There will be an uptick in the pre-monsoon activity over South India, but the Indian Seas will remain devoid of any major disturbance during the first half of April.

Borderline La Niña conditions will soon turn ENSO-neutral. This is likely to be the most dominant category during the Indian Summer Monsoon. ENSO and IOD may pool together to fetch a worthy monsoon in 2025.