The US National Oceanic and Atmospheric Administration (NOAA) is updating how to classify El Nino and La Nina phenomena using a new index, taking into account the impacts of climate change.
According to this new scale, La Nina may be announced more frequently, while El Nino may be less confirmed than before. The reason is that the new index has taken into account the long-term warming trend of the oceans.
El Nino and La Nina are periods of strong ocean temperature fluctuations in the eastern and central equatorial areas of the Pacific Ocean.
When the sea surface temperature is higher than the average by at least 0.5 degrees Celsius for 5 consecutive months, El Nino will be confirmed.
Meanwhile, La Nina is defined when sea temperatures are lower than average.
Previously, the "normal" level was determined based on the 30-year average temperature of the central equatorial sea - eastern Pacific Ocean.
Currently, the temperature deviation threshold is calculated in the context of global ocean temperatures. The new index will take the temperature deviation of the tropical Pacific Ocean, minus the average temperature deviation of the entire ocean within a range of 20 degrees latitude around the equator.
Scientists say that this index, called the Relative Oceanic Nino Index, reflects more accurately tropical storm activity and atmospheric circulation patterns, which greatly affect the weather in California and many other areas of the United States.
According to Ms. Michelle L'Heureux - climate scientist at NOAA, the 2024-2025 winter is not classified as La Nina according to the old calculation method, but in fact, global climate changes are clearly characterized by La Nina. The 2025-2026 winter is currently also developing in a similar scenario.
The Australian Meteorological Agency has switched to using relative indicators since last year to provide a clearer view of El Nino and La Nina. However, Australia's deviation threshold is 0.8 degrees Celsius, higher than the 0.5 degrees Celsius level used by the NOAA.
If not considering the trend of heating up, El Nino may be similar to occurring more frequently, while La Nina is less," the Australian Meteorological Agency said.
NOAA once stated in a study published in 2024 that the old index was "problematic" because it did not accurately reflect El Nino - La Nina models in the context of the ocean heating up due to climate change.
NOAA will officially use the new index from February 1st for climate warning and forecast bulletins.
In general, the relative index is still interpreted as before, with a deviation of 0.5 degrees Celsius being El Nino or La Nina weak, 1 degree Celsius being average, 1.5 degrees Celsius or higher being strong.
Moderate and strong phenomena often have a clearer impact on the weather in California, USA.
The application of the new index also changes historical data. The 2024-2025 winter is currently classified as La Nina but only "nearly reached the threshold" according to the old calculation method. The 2023-2024 winter was also lowered from strong El Nino to average El Nino.
