Orbital and freshwater forcing during the last interglacial: analysis of climate and vegetation response patterns

Abstract. Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G which explicitly resolves the atmosphere – ocean – sea ice dynamics. It is shown that in contrast to boreal summer where the climate mainly follows the local radiative forcing, the boreal winter climate is strongly determined by modulation of the atmospheric circulation. We find that during a positive phase of the Arctic Oscillation/North Atlantic Oscillation the convection in the tropical Pacific is below normal. The atmospheric circulation patterns induce non-uniform temperature anomalies, much stronger in amplitude than by the direct solar insolation. Together with the direct solar insolation this provides for a temperature drop over the Northern Hemisphere continents for 115 000 years before present, large areas over northern Asia and Alaska become a desert, and the grass land expanded to the north. The spatial pattern of temperature and vegetation changes differs from a more hemisphere-wide cooling, i.e. induced by oceanic freshwater in the northern North Atlantic. The signatures of different forcing mechanisms are important for the interpretation of proxy data as well as for the understanding of underlying mechanisms at the end of the last interglacial.