The sea surface temperature anomalies of the NINO3.4 region of the equatorial Pacific are bordered by the coordinates of 5S-5N, 170W-120W. They are used by NOAA and other meteorological agencies to define when an El Niño event is taking place. According to the most-recent weekly satellite-enhanced data, the temperature anomalies for that region have dropped below the +0.5 deg C threshold between El Niño and ENSO-neutral phases, the latter of which is the “normal” phase of the tropical Pacific when neither El Niño or La Niña are taking place.
The weekly NINO3.4 region sea surface temperature anomaly data for Figures 1 and 2 are from the NOAA/CPC Monthly Atmospheric & SST Indices webpage, specifically the data here. The base years for anomalies for the NOAA/CPC data are referenced to the WMO-preferred 30-year period of 1981-2010.
Figure 1 includes the weekly sea surface temperature anomalies of the NINO3.4 region. They start in January 1991. The weekly NINO3.4 sea surface temperature anomalies for the week of May 18 were +0.2 Deg C.
Figure 1
And for Figure 2, the evolutions of the NINO3.4-region sea surface temperature anomalies in 2015/16 are compared to 1997/98, another very strong El Niño. Recall that 2015 started the year at or near El Niño conditions, where that was not the case in 1997.
Figure 2