Borderline ENSO Conditions Over Equatorial Pacific, May Strengthen By May End

As of the end of April 2025, some atmospheric indicators continue to show feeble La Niña conditions, while oceanic conditions are closer to ENSO-neutral. For the period of the Indian Summer Monsoon, ENSO-neutral conditions remain favored at a probability of over 50% throughout the season. Thereafter, the probability components of El Niño and La Niña are both rising, but without any conclusive pattern. This indicates that after the Northern Hemisphere summer, the odds may tilt either towards El Niño or La Niña depending on the conditions that develop in the upcoming months. This in itself defies the normal evolution process of La Niña and El Niño. These two events typically tend to develop during the period April-June and reach their maximum strength in October-February, quite often lasting for 9-12 months.

ENSO: ENSO is a coupled ocean-atmosphere phenomenon. El Niño and La Niña are the oceanic components, while the Southern Oscillation is the atmospheric counterpart, thus giving rise to the term El Niño / Southern Oscillation. Monitoring and outlook of ENSO conditions are primarily based on SST anomalies, computed against a base period of 30 years, exceeding predefined thresholds in four geographical regions of the equatorial Pacific.

The SST anomaly in the Niño 3.4 region during the Jan-Mar 2025 season was -0.36°C. The average temperature anomaly in the Niño 3.4 region during April has remained -0.1°C. The most recent weekly average of the Niño 3.4 index on 28th April was 0.0°C. The SST anomaly for the quarter Feb-Apr 2025 is expected to be -0.17°C. Over the last four weeks, negative subsurface temperature anomalies have also weakened in the central and east-central Pacific Ocean. It may be construed that, ENSO-neutral conditions are favored during the upcoming monsoon season.

IOD: The Indian Ocean Dipole is neutral. The latest value of the IOD index for the week ending 27th April 2025 was 0.41°C. The index has suddenly risen very close to the threshold mark of 0.4°C. It is unlikely to sustain such warm anomalies. Models are predicting that IOD is likely to remain neutral until at least August. Skill for IOD forecast at this time of the year has historically been low for accuracy.

MJO: The Madden-Julian Oscillation pulse is likely to meander within the unit circle for the next two weeks. Its amplitude is too weak to enhance any convective activity. The month of April did not find any storm over the Indian Seas. Both the Bay of Bengal and the Arabian Sea are likely to remain dormant for the first two weeks of May 2025. Currently, there are no active storms anywhere around the globe. The typhoon season in the Western Pacific starts in May and lasts till October. The official hurricane season for the Atlantic basin is from 01st June to 30th November. But there are no strict bounds. The storm activity can occur before or after the official dates.

Since El Niño and La Niña can be predicted months in advance, their correlation with the global climate allows us an early picture of upcoming seasonal patterns. ENSO-neutral lowers the predictability. The risk of variability of tropical weather grows manifold during such episodes.