US weather agency says La Nina threshold yet to be reached, though conditions persist

The Climate Prediction Centre (CPC), an arm of the US National Oceanic and Atmospheric Administration (NOAA) says “La Nina conditions are present” currently but its data show that Ocean Nino Index (ONI) value has not reached the threshold. 

CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5ºC along with consistent atmospheric features. These anomalies must also be forecast to persist for 3 consecutive months.

However, the most recent ONI value (October-December 2024) is -0.4ºC. The value for November-January, which is yet to be made available, should confirm if La Nina has emerged. 

The ONI is a rolling 3-month average temperature anomaly—difference from average—in surface waters of east-central tropical Pacific Ocean.

ENSO neutral for past 6 months

CPC said negative subsurface temperature (SST) anomalies strengthened in December 2024, reaching a minimum late in the month. During January 2025, negative anomalies weakened. 

It is probably an indication that sustained atmospheric and oceanic responses have not been observed—a phenomenon that Australia’s Bureau of Meteorology (BoM) has reported.

BoM said the El Niño–Southern Oscillation (ENSO) has remained neutral for the past 6 months, despite changes in sea surface temperature patterns consistent with a developing La Niña.

Since late December, conditions across the tropical Pacific have been more La Niña like, with both oceanic and atmospheric indicators beginning to align. “However, until a sustained atmospheric and oceanic response is observed, the Bureau’s ENSO status remains neutral,” it said.

59% chance of La Nina

All surveyed international models have a neutral ENSO outlook from March until at least June, the Australian weather agency said.

CPC said the upper ocean heat anomalies, below average, and oceanic thermocline slope index (above average) reflect La Nina conditions. 

“Over the last couple of months, negative subsurface temperature anomalies have persisted in the central and eastern equatorial Pacific Ocean,” the US weather agency said.

It said there was a 59 per cent chance of La Nina conditions persisting through February-April 2025, while there was a 60 per cent chance of a transition to ENSO-neutral during March-May 2025.

Too late?

During the Northern Hemisphere spring and summer, ENSO-neutral is favoured by the dynamical model average and La Niña is favoured to persist by the statistical model average, CPC said. 

BoM said it was “very late” historically in the typical ENSO cycle for a La Niña event to develop.  La Niña tends to have a weaker association with Australian temperature and rainfall patterns during summer compared to winter and spring, it said.

The Indian Ocean Dipole (IOD) is neutral, it said, adding that global SSTs were substantially above average.