Atmospheric Δ¹⁴C reduction in simulations of Atlantic overturning circulation shutdown

A rapid reduction in the Atlantic meridional overturning circulation (AMOC) can significantly disrupt the global heat transport and likely triggered abrupt climate change during the last glacial cycle. A slowdown in AMOC has long been assumed to inhibit the exchange of carbon between the atmosphere and the deep ocean and thus cause radiocarbon (¹⁴C), which is produced in the atmosphere, to accumulate in the atmosphere. Indeed previous model studies have demonstrated that a reduction in AMOC leads to higher atmospheric ¹⁴C abundance (Δ¹⁴C). However, this seems inconsistent with the observed rise in atmospheric pCO₂ during Heinrich 1 and the Younger Dryas stadial events and the emerging view that this CO₂ rise resulted from the deep ocean venting "old" carbon. Using an Earth system model, we offer an alternative scenario that AMOC slowdown and an accompanying dynamical response in the south (i.e., the bipolar seesaw) can in fact lead to a decline in atmospheric Δ¹⁴C. This decline is realized in the model when the bipolar seesaw and thus the flux of old carbon from the Southern Ocean are sufficiently large so as to overcome the accumulation of ¹⁴C in the atmosphere as AMOC is reduced. The bipolar seesaw we describe invokes an oceanic teleconnection, whereby a freshwater perturbation in the North Atlantic drives a southern Δ¹⁴C response, but this does not necessarily preclude an atmospheric teleconnection.