Rather than define a somewhat arbitrary threshold for a La Niña / El Niño year (i.e. based on the
size of the index and number of months exceeding a certain threshold) or limiting the analysis to one ENSO index, I first took the
average of the three indices
mentioned above (ONI, MEI, and SOI, accounting for the fact that positive SOI indicates La Niña conditions while the opposite is true for ONI and MEI).