We do not consider 2019-20 to be an El Nino because we consider many different indices and datasets.  ENSO is a coupled ocean-atmosphere phenomenon.  The ONI is one index, or one measure, and it is not the only one.  In this case, as we explained in our blog post above, it’s likely that temperature trends that are influencing recent values and we will likely see a revision in January 2021 when the 30-year climatology is updated to 1991-2020 (right now we’re using 1986-2015). 

Keep in mind that ONI values over the past decade will continue to change as the rolling climatology moves forward.  The ONI is a retrospective index, meaning it’s primary use is for historical comparisons and composites going back to 1950.  But ONI values near our threshold (-/+0.5C) are marginal and may not be sufficient to achieve declaration (we did not consider late 2014 an El Nino either).   In sum, we do not rely on the ONI, by itself, for real time monitoring and our Advisory system is explained here, which uses multiple datasets and indices.