The end of the last ice age, the “Last Termination”, is characterized by several rapid shifts between very cold (stadial) and warm (interstadial) periods. Contrasting the continuous increase in radiative forcing, these climate instabilities are (hypothetically) linked to large variations in the strength of the Atlantic Overturning Circulation (AMOC) in response to salinity disturbances by freshwater fluxes from melting ice sheets. The drivers and mechanisms behind these rapid shifts and different regional climatic feedbacks, and their global impacts via teleconnections, are however not well known.
As part of our project “Climate instabilities during the last interstadial period” funded by the Swedish Research Council (VR 2015-04418), we continue to perform high resolution (0.9°x1.25°, ~100 km) global climate simulations for several different (inter-)stadial periods during the late deglaciation (~16.000 to ~10.000 years before present). Our main focus is on climatic changes during warm seasons of the different oscillations with large impacts on ecosystems and ice sheet melting. These are characterized by different combinations of cold/warm ocean states under low/high radiative forcing. Specifically, we want to study the impact of large continental ice sheets on the large-scale atmospheric flow. First results show that persistent orographic and spatially extended but temporally more variable atmospheric blocking dominate quite extreme changes in summer temperatures and hydroclimate. We test the hypothesis that extreme changes in seasonality dominate the signals reconstructed by various proxy data rather than summer conditions alone.
In addition to a direct model-proxy comparison, we will use our climate model output to run a numerical lake model. This enables us to account for potentially diverging responses of lake water temperatures, which may be reflected by lake proxy data, relative to ambient air temperatures simulated by the climate model.
For our climate simulations, we use the NCAR Community Earth System Model version 1 (CESM 1.0.5). In addition to changes in orbital and greenhouse gas forcing, horizontal boundary conditions are adjusted to incorporate realistic changes in the paleo-topography of continental ice sheets, glacio-isostatic land uplift and exposed land masses due to lower sea-level stands.