Abstract. We present the Utrecht contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), using the Community Earth System Model version 1.0.5 (CCSM4-Utr). Using a standard pre-industrial configuration and the enhanced PlioMIP2 set of boundary conditions, we perform a set of simulations at various levels of atmospheric pCO2 (280, 400, and 560 ppm). This allows us to make an assessment of the mid-Pliocene reference (Eoi400) climate versus available proxy records and a pre-industrial control (E280), as well as determine the sensitivity to different external forcing mechanisms. We find that our simulated Pliocene climate is considerably warmer than the pre-industrial reference, even under the same levels of atmospheric pCO2. Compared to the E280 case, our simulated Eoi400 climate is on average almost 5 ∘C warmer at the surface. Our Eoi400 case is among the warmest within the PlioMIP2 ensemble and only comparable to the results of models with a much higher climate sensitivity (i.e. CESM2, EC-Earth3.3, and HadGEM3). This is accompanied by a considerable polar amplification factor, increased globally averaged precipitation, and greatly reduced sea ice cover with respect to the pre-industrial reference. In addition to radiative feedbacks (mainly surface albedo, CO2, and water vapour), a major contribution to the enhanced Pliocene warmth in these simulations is the warm model initialisation followed by a long spin-up, as opposed to starting from pre-industrial or present-day conditions. Added warmth in the deep ocean is partly the result of using an altered vertical mixing parameterisation in the Pliocene simulations, but this has a negligible effect at the surface. We find a stronger and deeper Atlantic meridional overturning circulation (AMOC) in the Eoi400 case, but the associated meridional heat transport is mostly unaffected. In addition to the mean state, we find significant shifts in the behaviour of the dominant modes of variability at annual to decadal timescales. The Eoi400 El Niño–Southern Oscillation (ENSO) amplitude is greatly reduced (−68 %) versus the E280 one, while the AMOC becomes more variable. There is also a strong coupling between AMOC strength and North Atlantic sea surface temperature (SST) variability in the Eoi400, while North Pacific SST anomalies seem to have a reduced global influence with respect to the E280 through the weakened ENSO.

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.

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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 as 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 both with time-mean proxies indicating a reduced zonal SST gradient, as well as reconstructions indicating a reduced, or similar, ENSO variability.

Abstract. We present the Utrecht contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), using the Community Earth System Model version 1.0.5 (CCSM4-Utr). Using a standard pre-industrial configuration and the enhanced PlioMIP2 set of boundary conditions, we perform a set of simulations at various levels of atmospheric pCO2. This allows us to make an assessment of the mid-Pliocene reference (Eoi400) climate versus available proxy records and a pre-industrial control (E280), as well as to determine the sensitivity to different external forcing mechanisms. We find that our simulated Pliocene climate is considerably warmer than the pre-industrial reference, even under the same levels of atmospheric pCO2. Compared to the E280 case, the simulated climate of our Eoi400 is on average almost 5 °C warmer at the surface. Our Eoi400 case is among the warmest within the PlioMIP2 ensemble and only comparable to the results of models with a much higher climate sensitivity (i.e. CESM2, EC-Earth3.3, and HadGEM3). This is accompanied by a considerable polar amplification factor, increased precipitation and greatly reduced sea ice cover. A primary contribution to this enhanced Pliocene warmth is likely our warm model initialisation followed by a long spin-up, as opposed to starting from pre-industrial or present-day conditions. Added warmth in the deep ocean is partly the result of using an altered vertical mixing parametrisation in the Pliocene simulations, but this has a negligible effect at the surface. We find a stronger and deeper Atlantic Meridional Overturning Circulation (AMOC) in the Eoi400 case, but the associated meridional heat transport is mostly unaffected. In addition to the mean state, we find significant shifts in the behaviour of the dominant modes of variability at annual to decadal timescales. The Eoi400 ENSO amplitude is greatly reduced (−68 %) versus the E280 one, while the AMOC becomes more variable. There is also a strong coupling between AMOC strength and North Atlantic SST variability in the Eoi400, while North Pacific SST anomalies seem to have a reduced global influence with respect to the E280 through the weakened ENSO