La Nina is a naturally occurring climate pattern that cools surface temperatures in the central and eastern equatorial Pacific Ocean. Because of changes in vertical wind shear -- the variation in wind speed and direction high in the atmosphere -- La Nina tends to bring more hurricanes in the Atlantic basin and fewer in the Pacific.

Conditions oscillate between La Nina and its opposite, El Nino, with neutral phases in between.

Following a brief spell of weak La Nina conditions between December 2024 to March this year, neutral conditions had persisted, but the US National Weather Service declared Thursday La Nina was back.

"La Nina conditions emerged in September 2025, as indicated by the expansion of below-average sea surface temperatures (SSTs) across the central and eastern equatorial Pacific Ocean," the agency said.

La Nina is expected to persist through the winter, with a 55 percent chance of transitioning back to neutral conditions between January and March 2026.

The NWS added that this La Nina's predicted weakness "would be less likely to result in conventional winter impacts."

The unusually long-lasting 2020-2023 La Nina was the first so-called "triple-dip" event of the 21st century -- and only the third since 1950. It intensified both drought and flooding in different parts of the world.

However, despite La Nina's typical cooling influence, it did nothing to halt the streak of exceptionally hot years. The past decade comprises the ten hottest individual years ever recorded.

Temperatures have remained at record or near-record levels even after El Nino conditions faded last year -- with 2024 the hottest year on record.

The Atlantic hurricane season runs from June 1 to November 30. The National Oceanic and Atmospheric Administration continues to forecast an above-normal season, although so far it has been slightly below average and no storms have yet made US landfall.