Abstract
Abstract. It has been suggested that a "permanent" El Niño climate state has existed in the warm Pliocene. One of the main pieces of evidence of such conditions is the small east-west sea surface temperature (SST) difference that is found in proxy temperature records of the equatorial Pacific. Using a coupled version of the Zebiak-Cane model of intermediate complexity for the tropical Pacific, we study the sensitivity of the time-mean Pacific background state and El Niño/Southern Oscillation (ENSO) variability to Pliocene climate changes. The parameters varied in this sensitivity study include changes in the trade wind strength due to a reduced equator-to-pole temperature gradient, higher global mean temperatures and an open Panama gateway. All these changes lead to a westward shift of the position of the cold tongue along the equator by up to 2000 km. This result is consistent with data from the PRISM3D Pliocene SST reconstruction. Our model further suggests that ENSO variability is present in the Pliocene climate with only slight changes as compared to today. A background climate that would resemble a "permanent" El Niño with weak to no east-west temperature difference along the equator is only found for very weak trade winds which seem unrealistic for the Pliocene climate.
Highlights
The El Nino/Southern Oscillation phenomenon (ENSO) is one of the strongest and most studied modes of climate variability on interannual time scales
In the first subsection we study the changes in the Pacific climatology and ENSO variability to variations of the radiation equilibrium temperature T0 and the external wind stress strength τ0
We have studied the behaviour of ENSO variability and the time-mean tropical Pacific climatology under conditions mimicking the Pliocene climate state within the fully coupled Zebiak-Cane model
Summary
Pliocene proxy climate reconstructions mostly have a too coarse temporal resolution to resolve ENSO events, but they are useful to determine the past climatology of the Pacific. Consistent with proxy data, increased SSTs in the eastern equatorial Pacific are found, even with a significantly reduced east-west temperature gradient (1.92 ◦C on average) as compared to today, the model shows clear ENSO variability These simulations do not support the existence of “permanent” El Nino-like conditions in the Pliocene. Jochum et al (2009) tested the hypothesis that the emergence of the Halmahera island in the Pliocene could have changed the Indonesian throughflow in such a way that the zonal SST gradient in the tropical Pacific would be reduced They used a coupled climate model with different configurations of the Indonesian islands and their simulations show interannual ENSO variability with a slightly shifted west Pacific warm pool.
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