During the warm early Pliocene (~4.5 to 3.0 million years ago), the most recent interval with a climate warmer than today, the eastern Pacific thermocline was deep and the average west-to-east sea surface temperature difference across the equatorial Pacific was only 1.5 ± 0.9°C, much like it is during a modern El Niño event. Thus, the modern strong sea surface temperature gradient across the equatorial Pacific is not a stable and permanent feature. Sustained El Niño-like conditions, including relatively weak zonal atmospheric (Walker) circulation, could be a consequence of, and play an important role in determining, global warmth.

Supplement to: Wara, Michael W; Ravelo, Ana Christina; Delaney, Margaret Lois (2005): Permanent El Niño-like conditions during the Pliocene warm period. Science, 309(5735), 758-761