The Effect of Ocean Ventilation on the Transient Climate Response to Emissions

The surface warming response to carbon emissions is affected by how the ocean sequesters excess heat and carbon supplied to the climate system. This ocean uptake involves the ventilation mechanism, where heat and carbon are taken up by the mixed layer and transferred to the thermocline and deep ocean. The effect of ocean ventilation on the surface warming response to carbon emissions is explored using simplified conceptual models of the atmosphere and ocean with and without explicit representation of the meridional overturning. Sensitivity experiments are conducted to investigate the effects of (i) mixed layer thickness, (ii) rate of ventilation of the ocean interior, (iii) strength of the meridional overturning, and (iv) extent of subduction in the Southern Ocean. Our diagnostics focus on a climate metric, the transient climate response to carbon emissions (TCRE), defined by the ratio of surface warming to the cumulative carbon emissions, which may be expressed in terms of separate thermal and carbon contributions. The variability in the thermal contribution due to changes in ocean ventilation dominates the variability in the TCRE on time scales from years to centuries, while that of the carbon contribution dominates on time scales from centuries to millennia. These ventilated controls are primarily from changes in the mixed layer thickness on decadal time scales, and in the rate of ventilated transfer from the mixed layer to the thermocline and deep ocean on centennial and millennial time scales, which is itself affected by the strength of the meridional overturning and extent of subduction in the Southern Ocean.