Weather organisations divided over emergence of La Nina
Meteorological organisations across the world are divided over the emergence of La Nina weather phenomenon. While the US Climate Prediction Centre (CPC) says the event emerged in December 2024, Australia’s Bureau of Meteorology (BoM) and APEC Climate Center (APCC) have different views.
“La Nina conditions emerged in December 2024 and were reflected in below-average sea surface temperatures (SSTs) across the central and east-central equatorial Pacific Ocean. The latest weekly indices were -0.7 C in Niño-3.4 … Subsurface cooling in the equatorial Pacific Ocean strengthened significantly with below-average temperatures dominating the central and eastern equatorial Pacific Ocean,” CPC, an arm of the US’ National Oceanic and Atmospheric Administration (NOAA) said.
Unable to sustain
BoM said it considers that the El Nino Southern Oscillation (ENSO) in the tropical Pacific remains neutral. “While many indicators have recently met the threshold for La Nina they have not been sustained for levels or duration sufficient to warrant La Nina status,” it said.
APCC said its ENSO alert suggests “La Nina Watch”. Nino3.4 index is expected to be -0.8℃ for February 2025 and gradually increase to 0.1℃ for July 2025. “ENSO-neutral conditions are most probable for the whole forecast period,” it said in its climate service update on Wednesday.
CPC said low-level wind anomalies were easterly over the western and central Pacific, while upper-level wind anomalies were westerly over the central and eastern Pacific. Convection was suppressed over the Date Line and was enhanced over Indonesia.
“Collectively, the coupled ocean-atmosphere system indicated La Nina conditions. The dynamical models in the IRI plume continue to predict a weak La Nina during the winter seasons, as indicated by the Nino-3.4 index values less than -0.5C,” said the US weather agency.
Strong coupling signs
BoM said the ENSO in the tropical Pacific remained neutral over the past 6 months, despite changes in sea surface temperature patterns consistent with a developing La Nina over this period.
“In recent observations, both ocean and atmosphere indicators are now showing signs of stronger coupling, that is more consistent with a La Nina event,” said the Australian weather agency.
CPC said the North American Multi-Model Ensemble (NMME) predicted slightly cooler SST anomalies with La Nina persisting through February-April 2025. “The forecast team favours the NMME guidance, predicting weak La Nina conditions through the early spring before transitioning to ENSO neutral,” it said.
Weak La Nina conditions are less likely to result in conventional winter/spring impacts, though predictable signals can still influence the forecast guidance. “…La Nina conditions are present and are expected to persist through February-April 2025 (59 per cent chance), with a transition to ENSO-neutral likely during March-May 2025 (60 per cent chance), the CPC said.
Closer to threshold
BoM said trade winds have also been stronger than average across the equatorial Pacific, strengthening during December. “The most recent value of the Nino3.4 SST index in the central Pacific Ocean to 5 January is −0.83 °C, which meets the La Niña threshold of −0.8 °C,” it said.
The most recent 30-, 60- and 90-day Southern Oscillation Index (SOI) values are close to or above the La Nina thresholds. A continuation of SOI values above the threshold will likely see a La Nina event established for at least part of the summer of 2025.
“Historically, it is very late in the typical ENSO cycle for a La Nina event to become established. Typically, an event forms during winter and spring and declines over autumn. Most models have ENSO returning to neutral by March,” said the Australian weather agency.
APCC has predicted above-normal temperatures for most of the globe except for the central and eastern tropical Pacific during February-July 2025. There is a strongly enhanced probability for above normal temperatures for the Indian Ocean. It also forecast above-normal rainfall for India — synchronising with the India Meteorological Department (IMD) outlook.