Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 5 1 2009 10.5194/cpd-5-1-2009 http://www.clim-past-discuss.net/5/1/2009/ http://www.clim-past-discuss.net/5/1/2009/cpd-5-1-2009.html http://www.clim-past-discuss.net/5/1/2009/cpd-5-1-2009.pdf 1 27 2009-01-07 Potential causes of 15th century Arctic warming using coupled model simulations with data assimilation E. Crespin elisabeth.crespin@uclouvain.be H. Goosse T. Fichefet M. E. Mann Université catholique de Louvain, Institut d'Astronomie et de Géophysique Georges Lemaître, Chemin du Cyclotron, 2, 1348 Louvain la Neuve, Belgium Department of Meteorology, Department of Geosciences, and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, USA An ensemble of simulations of the climate of the past millennium using a three-dimensional climate model of intermediate complexity are constrained to follow temperature histories obtained from a recent compilation of well-calibrated surface temperature proxies using a simple data assimilation technique. Those simulations provide a reconstruction of the climate of the Arctic that is compatible with model physics, the forcing applied and the proxy records. Available observational data, proxy-based reconstructions and our model results suggest that the Arctic climate is characterized by substantial variations in surface temperature over the past millennium. Though the most recent decades are likely to be the warmest of the past millennium, we find evidence for substantial past warming episodes in the Arctic. In particular, our model reconstructions show a particularly warm period at the end of the 15th century. This warm event is likely related to the internal variability of the climate system. We examine the roles of competing mechanisms that could potentially produce this anomaly. These examinations lead us to conclude that changes in atmospheric circulation, through enhanced southwesterly winds towards northern Europe, Siberia and Canada, are likely the main cause of the Arctic warming during the late 15th century. Bengtsson, L., Semenov, V. A., and Johannessen, O. M.: The early twentieth-century warming in the Arctic – a~possible mechanism, J. Climate, 17, 4045–4057, 2004. Berger, A.: Long-term variations of daily insolation and Quaternary climatic changes, J. Atmos. Sci., 35, 2363–2367, 1978. Black, D. E., Peterson, L. 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