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The role of forcing and internal dynamics in explaining the “Medieval Climate Anomaly”

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Abstract

Proxy reconstructions suggest that peak global temperature during the past warm interval known as the Medieval Climate Anomaly (MCA, roughly 950–1250 AD) has been exceeded only during the most recent decades. To better understand the origin of this warm period, we use model simulations constrained by data assimilation establishing the spatial pattern of temperature changes that is most consistent with forcing estimates, model physics and the empirical information contained in paleoclimate proxy records. These numerical experiments demonstrate that the reconstructed spatial temperature pattern of the MCA can be explained by a simple thermodynamical response of the climate system to relatively weak changes in radiative forcing combined with a modification of the atmospheric circulation, displaying some similarities with the positive phase of the so-called Arctic Oscillation, and with northward shifts in the position of the Gulf Stream and Kuroshio currents. The mechanisms underlying the MCA are thus quite different from anthropogenic mechanisms responsible for modern global warming.

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Acknowledgments

We thank E. Zorita and R. Wilson for comments and all the scientists that collected and analysed the proxy data used in this work. H.G. is Senior Research Associate with the Fonds National de la Recherche Scientifique (FRS-FNRS-Belgium). This work is supported by the FRS-FNRS and by the Belgian Federal Science Policy Office (Research Program on Science for a Sustainable Development) and by EU (project Past4future). M.E.M. acknowledges support from the NSF Paleoclimate program (grant number ATM-0902133). Aurélien Mairesse helped in the design of Fig. 1. Computational resources have been provided by the supercomputing facilities of the Université Catholique de Louvain (CISM/UCL) and the Consortium des Equipements de Calcul Intensif en Fédération Wallonie Bruxelles (CECI) funded by the Fond de la Recherche Scientifique de Belgique (FRS-FNRS).

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Correspondence to Hugues Goosse.

Appendix: Sensitivity to the uncertainty of the reconstruction

Appendix: Sensitivity to the uncertainty of the reconstruction

The data assimilation methodology applied here provides estimates of the uncertainties. In addition, to test the validity of our conclusions, we have launched one supplementary simulation with data assimilation (Fig. 16) in which we use a slightly higher value for the uncertainty associated with the proxy-based reconstruction (0.7°C instead of 0.5°C) (Experiment Uncertain0.7). For the mean temperature over the region 30°N–60°N, the results of this new experiment are remarkably similar to the one of the standard simulation. The obtained spatial patterns of the temperature as well as the changes in atmospheric circulation (Fig. 17) are also in very close agreement to the ones described for the standard experiment. Locally, some small differences can be noticed, but they are too small to challenge the interpretation deduced from the results of the standard simulation, illustrating that our results are robust at least when using the model and proxy based reconstruction applied here.

Fig. 16
figure 16

Temperature changes in an additional model simulation with data assimilation in which, compared to the standard experiment, the uncertainty of the proxy-based reconstruction is assumed to be 0.7°C instead of 0.5°C (experiment Uncertain0.7). a Anomaly of annual mean temperature (°C) averaged over the region 30°N–60°N in the standard simulation with data assimilation (green) and in Uncertain0.7 (light blue). The reference period is 1850–1980. As in Fig. 1, the time series have been filtered using an 11-year Butterworth filter. The grey lines represent the range of the standard simulation with data assimilation (best estimate plus and minus two standard deviations). b Annual mean surface temperature difference between MCA (950–1250) and LIA (1400–1700) in the experiment Uncertain0.7

Fig. 17
figure 17

Annual mean difference in geopotential height at 800 hPa (in m) between MCA (950–1250) and LIA (1400–1700) in the experiment Uncertain0.7

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Goosse, H., Crespin, E., Dubinkina, S. et al. The role of forcing and internal dynamics in explaining the “Medieval Climate Anomaly”. Clim Dyn 39, 2847–2866 (2012). https://doi.org/10.1007/s00382-012-1297-0

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