Seminar by Dr. Marcus Loefverstroem

National Center for Atmospheric Research, USA

On the mutual interaction between atmosphere and ice over the last glacial cycle

Thelast glacial cycle (ca. 115-12 ka) was the most recent in a series of recurringglaciations of the subpolar continents in Eurasia and North America. At thelast glacial maximum (LGM), the Northern Hemisphere ice sheets were ofcontinental 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 thetime-mean atmospheric circulation and the spatio-temporal evolution of theNorthern Hemisphere ice sheets over the last glacial cycle. We use a hierarchyof GCM experiments, ranging from coupled atmosphere--ice-sheet modelexperiments designed to only capture first-order local interactions between theatmospheric circulation and an evolving ice sheet, to more realistic snapshotsimulations with geologically constrained boundary conditions. We find that thewarm and dry summer climate in the interior of the North Americancontinent---in part due to thermal and mechanical stationary wave forcing by theCordilleran mountain range---is important for the ice sheet's growthtrajectory; the prevailing climate conditions are further amplified by thepresence of the ice sheet. Only when accounting for the first order climateimpact of the Cordilleran range is an ice sheet obtained that is spatiallyconsistent with geological data. However, despite their massive size, the pre-and post LGM ice sheets had a relatively small influence on the planetary scalecirculation, as they were located in areas where their interaction with thewesterly mean flow is limited. In contrast, we find that the continent-wide LGMLaurentide Ice Sheet (LIS) reorganizes the stationary waves, which leads to astrengthening and a zonalisation of the Atlantic winter jet. Sensitivity experimentssuggest that this transition is in part explained by planetary wave reflection;for high LIS elevations, planetary wave reflection becomes sufficientlyprevalent that a poleward-directed flux of wave activity appears in theclimatology over the midlatitude North Atlantic. Circulation changes of thismagnitude have profound implications for the downstream climate and may help usbetter understand the southwestward migration of the Eurasian ice sheet, aswell as marine and terrestrial proxy-data records that show a transition to awetter climate in southern Europe at around the LGM.

Jasmine - Seminar Room (1010), 10th floor, M Building