The evolution of deep‐ocean flow speeds and δ 13 C under large changes in the Atlantic overturning circulation: Toward a more direct model‐data comparison

To investigate the dynamics of the Atlantic meridional overturning circulation (AMOC) on timescales longer than the observational records, model‐data comparisons of past AMOC variability are imperative. However, this remains challenging because of dissimilarities between different proxy‐based AMOC tracers and the difficulty of comparing these to model output. We present an i LOVECLIM simulation with tuned AMOC evolution and focus on AMOC tracers that are directly comparable to reconstructions: flow speeds and δ 13 C. We deduce their driving factors and show that they yield different but complementary information about AMOC changes. Simulated flow speed changes are only linked to AMOC changes in regions bathed by North Atlantic Deep Water; however, in those regions they do provide details on vertical migration and thickness changes of the water masses. Simulated δ 13 C changes in the North Atlantic Deep Water region are again related to AMOC changes. Yet in regions bathed by Antarctic Bottom Water or Antarctic Intermediate Water, the δ 13 C evolution is driven by Southern Hemisphere source water δ 13 C changes, while in the Nordic Seas and the two major overflow regions it is driven by Northern Hemisphere source water δ 13 C changes. This shows that AMOC changes are not necessarily recorded by δ 13 C and stresses the need for combining both tracers in paleoclimate studies. A preliminary model‐data comparison for Last Interglacial flow speeds and δ 13 C changes in the Deep Western Boundary Current shows that this integrated approach is far from straightforward and currently inconclusive on the Last Interglacial AMOC evolution. Nonetheless, the approach yields potential for more direct and in‐depth model‐data comparisons of past AMOC changes.