A permanent El Niño–like state during the Pliocene?

The Pliocene may have been characterized by permanent El Niño–like conditions. Initial modeling studies suggest that this may have contributed to Pliocene warmth. The termination of this state may have influenced Northern Hemisphere glaciation (NHG). We use the Hadley Centre Coupled Model version 3 to examine the role of the oceans and ocean structure on Pliocene warmth. A permanent El Niño–like state is not predicted. Annual mean sea surface temperatures in the eastern equatorial Pacific at Ocean Drilling Program Sites 847 and 851 increase by 1.71°C and 1.15°C, respectively. However, El Niño Southern Oscillation events are clearly expressed by the model. Sensitivity tests indicate that a prescribed permanent El Niño–like condition increases annual global mean surface temperatures by a maximum of 0.6°C. If the Pliocene was characterized by such a condition, it is questionable that it provided a major contribution to global warmth and therefore unlikely that the termination of this state contributed significantly to the onset of NHG.

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