Impact of a realistic river routing in coupled ocean–atmosphere simulations of the Last Glacial Maximum climate |
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| Authors: | Ramdane Alkama M Kageyama G Ramstein O Marti P Ribstein D Swingedouw |
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| Institution: | (1) Laboratoire des Sciences du Climat et de l’Environnement, IPSL, UMR CEA-CNRS-UVSQ 1572, CE Saclay, L’Orme des Merisiers, Bat. 701, 91191 Gif-sur-Yvette Cedex, France;(2) Structure et fonctionnement des systèmes hydriques continentaux (Sisyphe), Université Pierre et Marie Curie, Paris, France |
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| Abstract: | The presence of large ice sheets over North America and North Europe at the Last Glacial Maximum (LGM) strongly impacted Northern
hemisphere river pathways. Despite the fact that such changes may significantly alter the freshwater input to the ocean, modified
surface hydrology has never been accounted for in coupled ocean–atmosphere general circulation model simulations of the LGM
climate. To reconstruct the LGM river routing, we use the ICE-5G LGM topography. Because of the uncertainties in the extent
of the Fennoscandian ice sheet in the Eastern part of the Kara Sea, we consider two more realistic river routing scenarios.
The first scenario is characterised by the presence of an ice dammed lake south of the Fennoscandian ice sheet, and corresponds
to the ICE-5G topography. This lake is fed by the Ob and Yenisei rivers. In the second scenario, both these rivers flow directly
into the Arctic Ocean, which is more consistent with the latest QUEEN ice sheet margin reconstructions. We study the impact
of these changes on the LGM climate as simulated by the IPSL_CM4 model and focus on the overturning thermohaline circulation.
A comparison with a classical LGM simulation performed using the same model and modern river basins as designed in the PMIP2
exercise leads to the following conclusions: (1) The discharge into the North Atlantic Ocean is increased by 2,000 m3/s between 38° and 54°N in both simulations that contain LGM river routing, compared to the classical LGM experiment. (2)
The ice dammed lake is shown to have a weak impact, relative to the classical simulation, both in terms of climate and ocean
circulation. (3) In contrast, the North Atlantic deep convection and meridional overturning are weaker than during the classical
LGM run if the Ob and Yenisei rivers flow directly into the Arctic Ocean. The total discharge into the Arctic Ocean is increased
by 31,000 m3/s, relative to the classical LGM simulation. Consequentially, northward ocean heat transport is weaker, and sea ice more
extensive, in better agreement with existing proxy data. |
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| Keywords: | Ice sheet LGM river routing Thermohaline circulation |
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