EAPS

Speaker: Marcus Löfverström, NCAR

"The Mutual Interaction Between Atmosphere and Ice Sheets Over the Last Glacial Cycle"

The last glacial cycle (c. 115 – 12 kya) was the most recent in a series of recurring glaciations of the subpolar continents in Eurasia and North America. At the last glacial maximum (LGM), the Northern Hemisphere ice sheets were of continental scale and lowered the global sea-level by approximately 100 m, which is comparable to the combined volume of Greenland and Antarctica today. The work presented here investigates the mutual interaction between the time-mean atmospheric circulation and the spatio-temporal evolution of the Northern Hemisphere ice sheets over the last glacial cycle. We use a hierarchy of GCM experiments, ranging from coupled atmosphere–ice-sheet model experiments designed to only capture first-order local interactions between the atmospheric circulation and an evolving ice sheet, to more realistic snapshot simulations with geologically constrained boundary conditions. We find that the warm and dry summer climate in the interior of the North American continent—in part due to thermal and mechanical stationary wave forcing by the Cordilleran mountain range—is important for the ice sheet’s growth trajectory; the prevailing climate conditions are further amplified by the presence of the ice sheet. Only when accounting for the first order climate impact of the Cordilleran range is an ice sheet obtained that is spatially consistent with geological data. However, despite their massive size, the pre-LGM ice sheets had a relatively small influence on the planetary scale circulation, as they were located in areas where their interaction with the westerly mean flow is limited. In contrast, we find that the continent-wide LGM Laurentide Ice Sheet (LIS) reorganizes the stationary waves, which leads to a strengthening and a zonalisation of the Atlantic winter jet. Sensitivity experiments suggest that this transition is in part explained by planetary wave reflection; for high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. Circulation changes of this magnitude have profound implications for the downstream climate and may help us better understand the southwestward migration of the Eurasian ice sheet, as well as marine and terrestrial proxy-data records that show a transition to a wetter climate in southern Europe at around the LGM.