Coupling of Surface Ocean Heat and Carbon Perturbations Over the Subtropical Cells under 21stCentury Climate Change

It is well established that the ocean plays an important role in absorbing anthropogenic carbon from the atmosphere. Under global warming, Earth system model simulations and theoretical arguments indicate that the capacity of the ocean to absorb anthropogenic carbon will be reduced relative to what would be absorbed by an unperturbed physical state of the ocean, with this constituting a positive carbon-climate feedback. Recent studies emphasize the importance of the subpolar North Atlantic and/or the Southern Ocean in sustaining such feedbacks, although for these regions a consensus view has not emerged regarding the mechanistic controls on marine carbon-climate feedbacks. Here we apply a suite of sensitivity simulations with an individual Earth system model to demonstrate that the surface waters of the shallow overturning structures (spanning 45°S-45°N) sustain nearly half of the global ocean carbon-climate feedback. Mechanism denial is applied in the Earth system model by disallowing the secular trend in the physical state of the ocean under climate change, while simultaneously allowing the transient effects of heating/freshening on the solubility of CO2 in surface seawater. The main results indicate that heat and carbon coupling is the dominant driver of carbon-climate feedbacks over 45°S-45°N, and that this occurs through perturbations to the buffering capacity of seawater rather than through changes in ocean stratification and subduction rates.