Has the Problem of a Permanent El Niño been Resolved for the Mid-Pliocene?

Abstract

The mid-Pliocene, the most recent warm geo-logical period, is thought to be indicative of the fate of the Earth's climate under global warming. Earlier evidence has suggested that permanent El Niño-like conditions existed in the mid-Pliocene, though the concept of a permanent El Niño remains controversial. Here, the authors analyzed Niño3.4 SST in pre-industrial and mid-Pliocene simulations with the low-resolution version of the Norwegian Earth System Model (NorESM-L). The simulated mid-Pliocene Niño3.4 SST, with a smaller standard deviation, indicated that a weaker ENSO existed in the mid-Pliocene relative to the pre-industrial experiment. Compared with earlier modeling studies, our simulations show that the problem of ENSO's standard deviations in the mid-Pliocene remains unresolved, although the mean and the period of ENSO in the mid-Pliocene have been re-solved by earlier geological and modeling studies.