El Niño (“the little boy”) is the warm phase of an ocean/atmosphere oscillation; it helps heat go from the ocean to the atmosphere and warm up our weather. During its counterpart, la Niña (“the little girl”), it does the opposite, moving heat from the atmosphere into the oceans. The whole phenomenon is called the El Niño southern oscillation, or ENSO, but it often happens that we just say El Niño for the whole thing.
It’s one of the things that affects Earth’s global temperature — temporarily — and there are lots of ways to quantify it, i.e. to “put a number on it.” One of the best is MEI, the Multivariate El Niño Index. It’s the way I describe El Niño when I adjust global temperature for temporary factors (volcanic eruptions, solar variations, and yes, El Niño).
There’s a new kid in town, or at least, a new way to quantify El Niño. The scientists who constructed MEI have come up with a new, improved version (version 2), so of course I’ve re-computed the adjustments based on the new version of MEI data. Since the new version covers the year 1979 to the present, that’s the time span for which I’ve computed adjusted data.