Interesting paper in the sense of I don't understand what a permanent El Nino even means. Since ENSO is conceptually a dipole and always reverts to a mean value of zero, how can it ever balance out to being predominately El Nino over La Nina?
Unless what they are saying is that warmer El Nino peaks are the quiescent state of the system and that the cold La Nina valleys were thus less severe. That is, the sloshing gradient was lower, resulting in an overall greater distribution of warmer surface water along the equator and thus less cold water brought up from the depths from a steeper thermocline gradient.
From the paper:
> "Previous work suggested a low zonal sea-surface temperature (SST) gradient in the tropical Pacific during the Pliocene, the so-called “permanent El Ni˜no.”