Abstract
Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period during which atmospheric CO2 levels were similar to recent historical values (∼400 ppm). Several proxy reconstructions for the mid-Pliocene show highly reduced zonal sea surface temperature (SST) gradients in the tropical Pacific Ocean, indicating an El Niño-like mean state. However, past modelling studies do not show these highly reduced gradients. Efforts to understand mid-Pliocene climate dynamics have led to the Pliocene Model Intercomparison Project (PlioMIP). Results from the first phase (PlioMIP1) showed clear El Niño variability (albeit significantly reduced) and did not show the greatly reduced time-mean zonal SST gradient suggested by some of the proxies. In this work, we study El Niño–Southern Oscillation (ENSO) variability in the PlioMIP2 ensemble, which consists of additional global coupled climate models and updated boundary conditions compared to PlioMIP1. We quantify ENSO amplitude, period, spatial structure and “flavour”, as well as the tropical Pacific annual mean state in mid-Pliocene and pre-industrial simulations. Results show a reduced ENSO amplitude in the model-ensemble mean (−24 %) with respect to the pre-industrial, with 15 out of 17 individual models showing such a reduction. Furthermore, the spectral power of this variability considerably decreases in the 3–4-year band. The spatial structure of the dominant empirical orthogonal function shows no particular change in the patterns of tropical Pacific variability in the model-ensemble mean, compared to the pre-industrial. Although the time-mean zonal SST gradient in the equatorial Pacific decreases for 14 out of 17 models (0.2 ∘C reduction in the ensemble mean), there does not seem to be a correlation with the decrease in ENSO amplitude. The models showing the most “El Niño-like” mean state changes show a similar ENSO amplitude to that in the pre-industrial reference, while models showing more “La Niña-like” mean state changes generally show a large reduction in ENSO variability. The PlioMIP2 results show a reasonable agreement with both time-mean proxies indicating a reduced zonal SST gradient and reconstructions indicating a reduced, or similar, ENSO variability.
Highlights
The mid-Piacenzian or mid-Pliocene warm period was a recent geological interval of sustained warmth with global mean temperatures 2–5 ◦C higher than the pre-industrial (Haywood et al, 2010; Dowsett et al, 2010, 2016; Haywood et al, 2020)
We have studied the changes in El Niño–Southern Oscillation (ENSO) variability in the mid-Pliocene using the PlioMIP2 ensemble and related those to sea surface temperature (SST) changes in the tropical Pacific mean climate
Even with identical forcing and geographical boundaries, the PlioMIP2 ensemble includes a range of model resolutions, parametrizations and initializations, all which contribute to a considerable model spread in the PlioMIP2 ensemble
Summary
The mid-Piacenzian or mid-Pliocene warm period (mPWP, 3.264–3.025 Ma) was a recent geological interval of sustained warmth with global mean temperatures 2–5 ◦C higher than the pre-industrial (Haywood et al, 2010; Dowsett et al, 2010, 2016; Haywood et al, 2020). The PlioMIP phase 2 was initiated to further understand the mPWP climate and designed to reduce uncertainties in model boundary conditions and in proxy data reconstruction (Haywood et al, 2016a, 2020). It employs boundary conditions from the Pliocene Research, Interpretation and Synoptic Mapping (PRISM) version 4, including updated reconstructions of ocean bathymetry and land-ice surface topography, as well as Pliocene soils and lakes (Dowsett et al, 2016; Haywood et al, 2016a). The PlioMIP2 global average, annual mean surface air temperature (SAT) increase is 1.7–5.2 ◦C (3.3 ◦C in the ensemble mean) compared to the pre-industrial, when implementing PRISM4 boundary conditions in PlioMIP2 (Haywood et al, 2020; Williams et al, 2021)
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