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Servonnat J.,French Climate and Environment Sciences Laboratory | Yiou P.,French Climate and Environment Sciences Laboratory | Khodri M.,Laboratoire dOceanographie et du Climat Experimentation et Approches Numeriques LOCEAN | Swingedouw D.,French Climate and Environment Sciences Laboratory | And 2 more authors.
Climate of the Past | Year: 2010

Studying the climate of the last millennium gives the possibility to deal with a relatively well-documented climate essentially driven by natural forcings. We have performed two simulations with the IPSLCM4 climate model to evaluate the impact of Total Solar Irradiance (TSI), CO 2 and orbital forcing on secular temperature variability during the preindustrial part of the last millennium. The Northern Hemisphere (NH) temperature of the simulation reproduces the amplitude of the NH temperature reconstructions over the last millennium. Using a linear statistical decomposition we evaluated that TSI and CO 2 have similar contributions to secular temperature variability between 1425 and 1850 AD. They generate a temperature minimum comparable to the Little Ice Age shown by the temperature reconstructions. Solar forcing explains ̃80% of the NH temperature variability during the first part of the millennium (1000-1425 AD) including the Medieval Climate Anomaly (MCA). It is responsible for a warm period which occurs two centuries later than in the reconstructions. This mismatch implies that the secular variability during the MCA is not fully explained by the response of the model to the TSI reconstruction. With a signal-noise ratio (SNR) estimate we found that the temperature signal of the forced simulation is significantly different from internal variability over area wider than ̃5.106 km2, i.e. approximately the extent of Europe. Orbital forcing plays a significant role in latitudes higher than 65° N in summer and supports the conclusions of a recent studyon an Arctic temperature reconstruction over past two millennia. The forced variability represents at least half of the temperature signal on only ̃30% of the surface of the globe. This study suggests that regional reconstructions of the temperature between 1000 and 1850 AD are likely to show weak signatures of solar, CO 2 and orbital forcings compared to internal variability. © Author(s) 2010.

Bouillon S.,Nansen Environmental and Remote Sensing Center | Fichefet T.,Catholic University of Louvain | Legat V.,Catholic University of Louvain | Madec G.,Laboratoire dOceanographie et du Climat Experimentation et Approches Numeriques LOCEAN | Madec G.,UK National Oceanography Center
Ocean Modelling | Year: 2013

In this paper, we show that one of the most widely used methods to solve the non-linear viscous-plastic (VP) sea ice rheology, the elastic-viscous-plastic (EVP) method, generates artificial linear bands of high deformation that may be confounded with real linear kinematic features observed in the Arctic ice pack. These numerical artefacts are easily filtered out by using a slightly different regularization of the internal stress. In addition, the EVP method is reinterpreted as an iterative solver and a clear distinction appears between the numerical and physical parameters. Two numerical parameters determine the stability and accuracy of the method and are adjusted to avoid the noisy ice deformation fields frequently observed with the EVP method in nearly rigid ice areas. This study also confirms the unsatisfactory numerical convergence of the EVP method and investigates the effects of the numerical parameters on sea ice deformation, internal stress and velocity fields obtained with unconverged solutions. © 2013 Elsevier Ltd.

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