Warm mid-Pliocene conditions without high climate sensitivity: the CCSM4-Utrecht (CESM 1.0.5) contribution to the PlioMIP2.

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. [ABSTRACT FROM AUTHOR]