Transient simulations over the last interglacial period (126-115 kyr BP): feedback and forcing analysis

This study addresses the mechanisms of climatic change in the northern high latitudes during the last interglacial (126-115 kyr BP) using the earth system model of intermediate complexity "MoBidiC". Two series of sensitivity experiments have been performed to assess (a) the respective roles played by different feed-backs represented in the model and (b) the respective impacts of obliquity and precession. The model simulates important environmental changes at northern high latitudes prior the last glacial inception, i.e.: (a) an annual mean cooling of 5 degreesC, mainly taking place between 122 and 120 kyr BP; (b) a southward shift of the northern treeline by 14degrees in latitude; (c) accumulation of perennial snow starting at about 122 kyr BP and (d) gradual appearance of perennial sea ice in the Arctic. As summer sea ice, summer snow and vegetation are particularly sensitive to spring and summer insolation, their evolution is mainly controlled by precession. Furthermore, these environmental changes cause variations in surface albedo that nearly quadruple the direct effect of the astronomical forcing. We conclude that in MoBidiC, precession is the main contributor to climatic change during the last interglacial. The feedback analysis reveals that the synergy between snow and vegetation is crucial for the gradual settlement of perennial snow at northern high latitudes. The interactions between the ocean and the continent are less critical, but the gradual growth of summer sea ice throughout the Eemian hastens the transition between taiga and tundra by about 1000 years. The model also simulates a 5% weakening of the North Atlantic branch of the thermohaline circulation throughout the study interval. This is a consequence of a slight warming of sub-surface Atlantic water at mid-latitudes caused by a decrease in winter heat exchanges between the ocean and the atmosphere. It is partly compensated for by a salinity increase at polar latitudes caused by a decrease in the local freshwater balance. Such a slight change in the thermohaline circulation does not have any significant impact on the heat balance of the northern high latitudes between 126 and 115 kyr BP.