A new El Niño index that brings a more climate-robust measure of the strength of El Niño signals has been released by the European Centre for Medium-Range Weather Forecasts (ECMWF). With the World Meteorological Organisation’s recent update indicating an 80% likelihood of an El Niño event during June-August 2026 and a 90 percent probability of this continuing until at least November, it is more important than ever to have accurate data to rely on.

While the traditional Niño indices previously relied upon, compare current sea-surface temperatures with a fixed historical baseline, the new measure, known as the Relative Niño index, compares warming in the central Pacific with temperatures across the wider tropics. This can give a clearer picture of how unusual the conditions really are in today’s warming climate:

“As the climate warms, interpreting anomalies becomes more challenging”, said Dr Tim Stockdale, Principal Scientist at ECMWF.

“Rising background temperatures can make recent El Niño events appear stronger than they are, and La Niña events seem weaker. The new Relative El Niño indices compare the Niño 3.4 and other regions with the overall temperature of the tropics at the same time, offering a perspective that is less sensitive to long-term warming. This will provide an additional tool for describing the likely strength of an upcoming El Niño event. Even with this adjustment, the strength of the forecasts suggests that El Niño may be unusually strong later in the year”.

The new index is also more suited for monitoring and comparing events over time. It is less sensitive to picking up the background signal of global warming trends, meaning El Niño can be described more accurately and with greater separation from climate change.

It joins a range of El Niño indices and products; each designed for a slightly different scientific purpose. At the core are the Niño sea-surface temperature indices — Niño 3.4, Niño 3, Niño 4, and Niño 1+2, which track sea-surface temperature anomalies in different parts of the tropical Pacific. These are widely used to monitor where warming or cooling is concentrated and to assess the evolution of El Niño and La Niña events.

The Relative Niño index is provided on approximately the same scale as the traditional Niño index meaning forecasters and other users can adopt it without having to relearn thresholds or interpretation.

Florian Pappenberger, Director-General of ECMWF, said:

“Providing the index in this new form, as recently recommended by WMO, should mean it can be adopted almost immediately. Producing it has been a huge collective endeavour from ECMWF and its partners. We are grateful to the WMO for the support they have shown throughout this process. It has been a long time in the making, but its availability now has become more crucial than ever with indicators pointing towards the potential for an El Niño event of at least moderate strength but possibly reaching strong intensity. We could be looking at an event as significant as any we have seen in the past 50 years”.