Yesterday, a new weekly report on the El Niño conditions was released by NOAA. El Niño is a little boy in general or Baby Jesus in particular (in Spanish). It indicates that the Pacific Ocean has something extra (positive temperature anomaly) in between the leggy hemispheres (equator), unlike La Niña (a little girl) who is missing something (some heat) in the middle.
For the first time, the conditions were labeled as an El Niño episode again; see the 22nd page of the PDF file above. An episode requires at least five consecutive overlapping 3-month periods with the anomaly at least 0.5. Because the January-February-March 2015 anomaly had been reduced to 0.4 some months ago, the first contributing 3-month period is the February-March-April period which seems to have had 0.5 now. The fifth one is June-July-August.
The anomaly to define El Niño and La Niña episodes is measured in long rectangular regions located in the Pacific equatorial area. The regions are 1+2 (some distance South from the equator, near the Westernmost Southern American beaches), 3, 3.4, and 4, where 3.4 overlaps both with 3 and 4. The region 3.4 is the most important one because it's used to define the ONI index which decides on the existence of the episodes.
The table on the 22nd page makes it clear that the ongoing El Niño episode is already matching the 2009-2010 episode – with the latest 3-month average at 1.2 (all these numbers are de facto temperature anomalies of the region in °C) – but the weekly report makes it clear that the conditions are much stronger now. The 1+2, 3, 3.4, 4 anomalies are 2.2, 2.4, 2.1, 1.0, respectively.
It's probably not hard for the models to predict – and they do – that the anomaly will "probably" remain above 1.5 for the rest of the year 2015 which is enough for a "strong El Niño".
If you look at the ENSO anomalies since 1950, you will see that the warmest 3-month anomalies were seen during the 1997-1998 El Niño – up to 2.3 °C – followed by the 1982-1983 El Niño – up to 2.1 °C. The ongoing El Niño seems to be highly comparable to those two. So the odds are about 1/3, 1/3, and 1/3 that the 2015-2016 El Niño will beat 1997-1998, remain in the middle, or lose to 1982-1983, respectively.
Will the strong El Niño mean that 2016 will be the hottest year on the record, even according to the satellites? (The current graphs make it pretty unlikely for 2015.)
I think it's likely but far from guaranteed. Look at the RSS AMSU satellite graph of the lower troposphere above. While the spike in the middle that made 1998 a warm year and that we associate with the 1997-1998 "El Niño of the century" seems obvious, the similar increase – if any – in 1983 seems inconclusive. This fact suggests that "only" the 1997-1998 El Niño – as we normally define it by the anomalies – just couldn't have been enough for producing the very warm year 1998. Just to be sure, the correlation is clearly there and El Niño is the strongest single meteorological factor influencing the global mean temperatures. But the correlation is far from perfect.
Relatively to the adjacent years, the year 2016 may turn out to be similar to 1997 – but it may also turn out to be similar to 1983.