La Nina Event Observes 'Break': ONI Drops Below Threshold

As of mid-March 2025, the equatorial Pacific Ocean was transitioning from weak La Niña conditions to an ENSO-neutral state. The average Niño 3.4 anomaly in Feb 2025 was very close to the threshold value of -0.5°C.  However, in March 2025, this anomaly was reduced to zero.  The shift is marked by sea surface temperatures in the Niño 3.4 region coming very close to average. The borderline value of -0.6°C of ONI for the quarter Dec-Jan-Feb has now risen to -0.4°C for the recent quarter of Jan-Feb-Mar, which is above the -0.5°C threshold required for a La Niña event.

ENSO: The ONI is based on SST departure from average in the Nino 3.4 region and is a principal measure for monitoring, assessing and predicting ENSO. By historic standards, to be classified as a full-fledged La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons. However, CPC considers La Niña conditions to occur when the monthly Niño 3.4 OISST departures meet or exceed -0.5°C along with consistent atmospheric features. These anomalies must also persist for three consecutive months.

The average Niño 3.4 anomaly for the months of Dec 2024, Jan 2025 and Feb 2025 was -0.6°C, -0.7°C and -0.5°C, respectively. These were below the threshold mark of -0.5°C. Now, for the month of Mar 2025, the average Niño 3.4 index works out to be +0.04°C. The ONI index of -0.4°C for the quarter Jan-Feb-Mar, as such, has exceeded the threshold mark of -0.5°C and therefore fails to qualify for continuation of the La Niña event.

The atmospheric component of ENSO is still aligned with La Niña conditions. The Southern Oscillation Index (SOI) for the month of March is 1.7. However, the ocean surface temperature is not synchronous with the atmospheric arm. The remnant of La Niña conditions is expected to ‘break down’ and turn ENSO-neutral anytime soon.

IOD: The Indian Ocean Dipole is neutral. The latest value of the index for the week ending 06 April 2025 was +0.35°C. Apparently, there was noteworthy cooling in the eastern Indian Ocean Dipole region over the last two weeks, which has contributed to this positive index value. As per BoM, this is expected to be temporary and the index is likely to return to a neutral value during this month. The Bureau also predicts a neutral IOD until at least August. This is in agreement with the other international models. However, the skill for IOD forecasts at this time of the year has historically been low for forecasts beyond a month ahead.

MJO: The Madden Julian Oscillation has propagated as a weak pulse from the Western Hemisphere to Phase 2 & 3 in the Indian Ocean. The MJO has retreated back into the unit circle with the least amplitude in the second week of April and incrementally growing in the third week. There is no broad agreement among the models on the likely course of MJO. The GEFS shows a strengthening MJO over the Western Pacific; the ECMWF continues with a stagnant signal in the unit circle. The MJO is likely to interact meekly with the flattening La Niña over the warm sea surface temperatures in the Eastern Equatorial Pacific Ocean.

This La Niña episode will go on record to be the shortest ever, lasting for two quarters, Nov-Dec-Jan and Dec-Jan-Feb, with respective anomalies of -0.5°C and -0.6°C. This will not be counted against a full-fledged event, as it falls short of the essential requirement of 5 consecutive overlapping 3-month seasons.