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In order to create a meaningful multi-system combination of the SST indices, all their anomalies (both for the traditional and the relative indices) from each individual forecast system have been adjusted so their variance resembles that of the real world.
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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 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 has been was first proposed in the paper titled "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.) and its implementation at ECMWF has been described at ECMWF Science blog post by Tim Stockdale "Measuring the strength of El Niño – introducing Relative Niño indices" (DOI 10.21957/d05ccfe150).
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