Reconstructing the Late Miocene climate and oceanic heat flux using the AGCM ECHAM4 coupled to a mixed-layer ocean model with adjusted flux correction

In terms of climate, the Late Miocene represents a transitional phase between the Eocene greenhouse climate and the Quaternary icehouse situation; it is characterised by an extensive Antarctic ice sheet and incipient glaciation in the Northern Hemisphere. In order to better understand the Late Miocene climate, we performed a simulation study for the Tortonian using the complex atmospheric general circulation model ECHAM4 coupled to a mixed-layer ocean model. The boundary conditions such as the orography and the continental ice distribution were adapted to the Tortonian conditions; considering the coarse geographic resolution of the model, the land–sea distribution was assumed to be similar to the present situation and the atmospheric CO2 concentration was set to 353 ppm. Because the mixed-layer ocean model uses a flux correction tuned to the modern oceanic heat transport a method was developed to estimate the Tortonian palaeo-flux correction based on the zonally averaged SST gradients. As compared to the present-day world, the Tortonian model run shows a significant reduction in the oceanic heat transport, a considerable warming and reduction of sea ice in high latitudes, a weakening of the Asian monsoon, a cooling in the mid-latitudes–caused by the reduced oceanic heat transport–and an increase in precipitation in Southern Europe. Except for the cooling of the mid-latitudes these results are largely consistent with proxy-data and other modelling studies although the proxy data indicate much warmer conditions in high latitudes than calculated in the model. The reasons for the discrepancies between model-based and proxy data reconstructions are discussed.