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From June 2026 ECMWF has introduced an additional measure of El Niño strength, alongside the more traditional Niño 3.4 SST anomalies, the relative Niño indices. These indices compare each one of the classical monitoring Niño regions (e.g. Niño3.4) with the rest of the tropics at the same time, offering a perspective that is less sensitive to long-term warming. This relative index was first proposed in the paper "Defining El Niño indices in a warming climate" (Geert Jan van Oldenborgh et al., 2021, Environ. Res. Lett. 16 044003; DOI 10.1088/1748-9326/abe9ed) and further explored in the follow-up paper "A Relative Sea Surface Temperature Index for Classifying ENSO Events in a Changing Climate" (Michelle L. L’Heureux et al., 2024, J. Climate, 37; DOI 10.1175/JCLI-D-23-0406.1.). Its implementation at ECMWF has been described in an ECMWF Science blog post by Tim Stockdale "Measuring the strength of El Niño – introducing Relative Niño indices" (DOI 10.21957/d05ccfe150).
The details on of the scaling factors (s in the following equation) used at ECMWF to produce these relative indices are shown below, and some additional details of how they were computed are available in the blog post linked above.
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