‘Weak’ La Nina likely to emerge during Sept-Nov, says US weather agency

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The Climate Prediction Centre (CPC), an arm of the US weather agency National Oceanic and Atmospheric Administration (NOAA), has raised the chances of La Nina emerging during the September-November 2024 period, though it could turn out to be a “weak” one.

“La Niña is favoured to emerge in September-November (71 per cent chance) and is expected to persist through January-March 2025,” the CPC said in its latest update on Thursday. 

The continuation of negative subsurface temperatures and enhanced low-level easterly wind anomalies supports the formation of a weak La Niña. “A weaker La Niña implies that it would be less likely to result in conventional winter impacts, though predictable signals could still influence the forecast guidance,” said the US weather agency.

Up 5 percentage points

Last month, CPC said there was a 66 per cent chance of La Nina emerging during September-November. It said El Nino-Southern Oscillation (ENSO) neutral conditions were expected to continue for the next several months. It also said there was a 74 per cent chance for La Nina to persist through the Northern Hemisphere winter 2024-25 (November-January).

The US weather agency said ENSO-neutral conditions continued during August 2024, with near-average sea surface temperatures (SSTs) observed across most of the equatorial Pacific Ocean. 

“This month, the (CPC) team relies more on the latest North American Multi-Model Ensemble (NMME) guidance, which predicts La Niña to emerge in the next couple of months and continue through the Northern Hemisphere winter,” it said. 

Collectively, the coupled ocean-atmosphere system reflected ENSO-neutral. “The IRI (International Research Institute for Climate and Society) plume predicts a weak and short duration La Niña, as indicated by the Niño-3.4 index values less than -0.5 C,” CPC said. 

BoM predictions

Last week, Australia’s Bureau of Meteorology (BoM) said  ENSO and Indian Ocean Dipole (IOD) will remain neutral during the southern hemisphere spring (September 22-December 22). 

It said its model indicated a neutral but cooler-than-average ENSO state. Of the remaining 6 international models BoM surveyed by the Bureau, 3 indicate SSTs (Sea Surface Temperatures) in the central tropical Pacific remaining within historically ENSO-neutral values (between −0.8 °C and +0.8 °C), and 3 indicate SSTs exceeding the La Niña threshold (below −0.8 °C) from October. 

CPC, however, said the weekly Niño indices did not change substantially during August, with the latest weekly index values varying between +0.2C (Niño-4) and -0.4C. “Below-average subsurface temperatures were also similar to those in early August. Negative temperature anomalies continued to dominate across most of the subsurface equatorial Pacific Ocean,” it said.