Pliocene Model Intercomparison Project (PlioMIP2) simulations using the Model for Interdisciplinary Research on Climate (MIROC4m)

The second phase of the Pliocene Model Intercomparison Project (PlioMIP2) has attracted many climate modelling groups in its continuing efforts to better understand the climate of the mid-Piacenzian warm period (mPWP) when atmospheric CO2 was last closest to present-day levels. Like the first phase, PlioMIP1, it is an internationally coordinated initiative that allows for a systematic comparison of various models in a similar manner to the Paleoclimate Modelling Intercomparison Project (PMIP). Model intercomparison and model-data comparison now focus specifically on the interglacial at marine isotope stage KM5c (3.205 Ma), and experimental design is not only based on new boundary conditions but includes various sensitivity experiments. In this study, we present results from long-term model integrations using the MIROC4m (Model for Interdisciplinary Research on Climate) atmosphere-ocean coupled general circulation model, developed at the institutes CCSR, NIES and FRCGC in Japan. The core experiment, with CO2 levels set to 400 ppm, shows a warming of 3.1 degrees C compared to the pre-industrial period, with two-thirds of the warming being attributed to the increase in CO2. Although this level of warming is less than that in the equivalent PlioMIP1 experiment, there is slightly better agreement with proxy sea surface temperature (SST) data at PRISM3 (PRISM - Pliocene Research Interpretation and Synoptic Mapping) locations, especially in the northern North Atlantic where there were large model-data discrepancies in PlioMIP1. Similar spatial changes in precipitation and sea ice are seen and the Arctic remains ice-free in the summer in the core experiments of both phases. Comparisons with both the proxy SST data and proxy surface air temperature data from paleobotanical sites indicate a weaker polar amplification in model results. Unlike PlioMIP1, the Atlantic Meridional Overturning Circulation (AMOC) is now stronger than that of the pre-industrial period, even though increasing CO2 tends to weaken it. This stronger AMOC is a consequence of a closed Bering Strait in the PlioMIP2 paleogeography. Also, when present-day boundary conditions are replaced by those of the Pliocene, the dependency of the AMOC strength on CO2 is significantly weakened. Sensitivity tests show that lower values of CO2 give a global SST which is overall more consistent with the PRISM3 SST field presented in PlioMIP1, while SSTs at many of the PRISM4 sites are still too high to be reconciled