Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium

© 2017 Springer International Publishing AG. Open access funding provided by Max Planck society (Max Planck Institute for Meteorology). We are grateful to David Lund for providing the data of the Florida Current reconstruction. We also thank Jöel Hirschi for feedback relating the theory and Fig. 2. We acknowledge the PalMA group for technical support and for hosting the first author during a period of time when some of these ideas where developed. We thank the valuable comments from Johann H. Jungclaus and Davide Zanchettin as well. Dallas Murphy, Jochem Marotzke, and participants in the workshop Advanced Scientific Writing at the Max Planck Institute for Meteorology are thanked for comments on draft versions of the manuscript. We also thank the three anonymous reviewers for their constructive comments and reviews. Primary data and scripts used in the analysis and other supplementary information that may be useful in reproducing this work can be obtained from the authors upon request.
We assess the use of the meridional thermal-wind transport estimated from zonal density gradients to reconstruct the oceanic circulation variability during the last millennium in a forced simulation with the ECHO-G coupled climate model. Following a perfect-model approach, model-based pseudo-reconstructions of the Atlantic meridional overturning circulation (AMOC) and the Florida Current volume transport (FCT) are evaluated against their true simulated variability. The pseudo-FCT is additionally verified as proxy for AMOC strength and compared with the available proxy-based reconstruction. The thermal-wind component reproduces most of the simulated AMOC variability, which is mostly driven by internal climate dynamics during the preindustrial period and by increasing greenhouse gases afterwards. The pseudo-reconstructed FCT reproduces well the simulated FCT and reasonably well the variability of the AMOC strength, including the response to external forcing. The pseudo-reconstructed FCT, however, underestimates/overestimates the simulated variability at deep/shallow levels. Density changes responsible for the pseudo-reconstructed FCT are mainly driven by zonal temperature differences; salinity differences oppose but play a minor role. These results thus support the use of the thermal-wind relationship to reconstruct the oceanic circulation past variability, in particular at multidecadal timescales. Yet model-data comparison highlights important differences between the simulated and the proxy-based FCT variability. ECHO-G simulates a prominent weakening in the North Atlantic circulation that contrasts with the reconstructed enhancement. Our model results thus do not support the reconstructed FC minimum during the Little Ice Age. This points to a failure in the reconstruction, misrepresented processes in the model, or an important role of internal ocean dynamics.
Max Planck society (Max Planck Institute for Meteorology)
Depto. de Física de la Tierra y Astrofísica
Fac. de Ciencias Físicas
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