AMOC as the key driver of the spread in Mid-Holocene winter temperature patterns over Europe in PMIP3 models

The mid-Holocene (6,000 years before present) was a warmer period than today in summer in most places of the Northern Hemisphere. In winter, over Europe, reconstructions of temperature based on pollen data show a dipole of temperature anomalies as compared to present-day, with warmer conditions in the north and colder in the south. It has been proposed that this pattern of temperature anomaly could be explained by a persisting positive phase of the North Atlantic Oscillation during this period, which was, however, not reproduced in general by climate models. Indeed, PMIP3 models show a large spread in their response to the mid-Holocene insolation changes, the physical origins of which are not understood. To improve the understanding of the reconstructed temperature changes and of the PMIP3 model spread, we analyze the dynamical response of these model simulations in the North Atlantic for mid-Holocene conditions as compared to pre-industrial. We focus on the European pattern of temperature in winter, which allows comparing the simulations with a pollen-based reconstruction. We find that some of the model simulations yield a similar pattern to the reconstructed one, with lower amplitude, but which remains within the reconstruction uncertainty. We attribute the northern warm part of the latitudinal dipole of temperature anomaly in winter to a lower sea-ice cover in the Nordic Seas. The decrease of sea ice in winter indeed reduces the sea-ice insulation effect there, allowing the ocean heat released in winter to reach the continental northern Europe. This decrease in winter sea-ice cover is related to an increase in the Atlantic meridional overturning circulation (AMOC) and its associated ocean heat transport, as well as the effect of insolation changes on sea ice in summer, which persists until winter. Concerning the cooling of southern Europe, we only find a slight cooling signal mainly related to the insolation-induced cooling in winter over Africa. We show that the models that failed to reproduce any AMOC increase under mid-Holocene conditions are also the ones that do not reproduce the temperature pattern over Europe. The change in sea level pressure is not sufficient to explain the spread among the models. The ocean-sea ice mechanisms that we proposed constitute an alternative explanation to the pattern of changes in winter temperatures over Europe in the mid-Holocene, which is in better agreement with available model simulations of this period. Finally, we argue that this period can provide interesting emerging constraints on key changes in European climate, and indirectly of AMOC response to radiative changes