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

Abstract

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 CO 2 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.