The ONI values changed when the data set used to create the indices changed from ERSST version 3b to version 4.  Changes to the underlying data sets occur from time to time, as new data or techniques are used to improve the data set. For the current event, since the values for SON 2014 and JFM 2015 decreased to 0.4, all of these values were changed back to black, as the historical definition was no longer met.  However, the ONI is used as a simple way to classify historical ENSO episodes, which has its advantages and disadvantages.   Weaker/shorter episodes in the record will always fluctuate during ERSST version and climatology updates.  One can only recognize historical episodes (at least 5 consecutive overlapping season) retrospectively.  In month-to-month updates, CPC does not rely on the ONI.   We examine many indices to assess conditions.  CPC began issuing El Nino advisories in March 2015, so according to NOAA, that is when this episode began and changes to the ONI index will not impact this.