Simulation of atmospheric radiocarbon during abrupt oceanic circulation changes: trying to reconcile models and reconstructions

Gilles Delaygue¹, Thomas F. Stocker, Fortunat Joos, and Gian-Kasper Plattner², Climate and Environmental Physics, Physics Insitute, University of Bern, Bern, Switzerland
¹Now at CEREGE, Europole de l'Arbois BP80, 13545 Aix-en-Provence, Cedex 4, France
²Now at Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California, USA

An abrupt increase in the atmospheric 14C content at the beginning of the Younger Dryas, about 13,000 years ago, has been related to a slow-down of the global oceanic circulation. This has been well simulated by box models, but the amplitude and timing of such a 14C change could not be represented with dynamical models. We have forced a climate model of intermediate complexity with a meltwater discharge to test three model parameters that have the potential to strongly influence the simulated 14C change andmay help us to reconcile the model results with reconstructions. We find that a decrease of the tropical wind speed accompanying a slow-down of the oceanic circulation largely amplifies the circulation-induced 14C change and brings it closer to the reconstructions. Second, we find that vertical mixing in a dynamical ocean model leads to a substantial damping in the 14C response of simple models. Third, we demonstrate that neglecting radiocarbon 'reservoir age' variations during abrupt climate change may lead to a substantial overestimate of atmospheric 14C fluctuations reconstructed from paleoceanographic archives.