Simulation of Holocene cooling events in a coupled climate model |
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| Affiliation: | 1. Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, The Netherlands;2. Institut d’Astronomie et de Géophysique Georges Lemaître, Université catholique de Louvain, Belgium;1. Utrecht University, Department of Physical Geography, Faculty of Geosciences, P.O. Box 80115, NL-3508 TC Utrecht, The Netherlands. E-mail: w.hoek@geo.uu.nl. Tel.: +31(0)30 2532416; Fax.: +31 (0)30 2531145.;2. Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences, De Boelelaan 1085, HV Amsterdam. The Netherlands. E-mail: hanneke.bos@falw.vu.nl; Tel.:+31 20 525 7666; Fax.: +31 20 525 7832.;1. Earth and Climate Cluster, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands;2. Laboratoire des Sciences du Climat et de l''Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France;3. INSTAAR and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA;4. Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA;1. Dept. of Geography, University of California, Berkeley, CA, USA;2. Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan;3. Dept. of Earth and Planetary Sciences, University of California, Berkeley CA, USA;4. The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi''an, China;5. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA;1. State Key Lab of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi''an 710061, China;2. Institute of Global Environmental Change, Xi''an Jiaotong University, Xi''an 710049, China;3. Department of Geography and Berkeley Atmospheric Science Center, University of California, Berkeley, CA 94720, USA;4. Department of Sediment- and Isotope Geology, Ruhr-Universität Bochum, Germany;5. Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA;1. Institute of Geography RAS, Staromonetny-29, 119017, Staromonetny, Moscow, Russia;2. Tomsk State University, Tomsk, Russia;3. Department of Geosciences, University of Massachusetts, Amherst, MA 012003, USA;4. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK;5. Institute of Particle Physics, ETH Zurich, 8093 Zurich, Switzerland;6. Institute of Geography, University of Zurich, 8057 Zurich, Switzerland;7. Université Paris 1 Panthéon-Sorbonne, CNRS Laboratoire de Géographie Physique, 92195 Meudon, France;8. Antarctic Research Centre, Victoria University Wellington, New Zealand;9. Department of Earth Science, University of Bergen, N-5020 Bergen, Norway;10. Uni Research Klima, Bjerknes Centre for Climate Research, N-5020 Bergen Norway;11. Department of Geology, University of Cincinnati, Cincinnati, OH 45225, USA;12. Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, Switzerland;13. Department of Geology, The College of Wooster, Wooster, OH 44691, USA;14. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA;1. INSTAAR, and Dept. of Geological Sciences, Univ. of Colorado, USA;2. Centre for Past Climate Studies, Department of Geoscience and Arctic Research Centre, Aarhus University, Aarhus, Denmark;3. Akvaplan-niva AS, Fram Centre, N-9296 Tromsø, Norway;4. Department of Earth Science, University of Bergen, Bergen, Norway |
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| Abstract: | Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt–CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 °C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm−2. Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events. |
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