What the mean overturning circulation tells us about transient ocean heat uptake: Introducing Ocean Heat Uptake Potential

Heat transport between the surface and deep-ocean strongly influences transient climate change and sea-level rise. Mechanisms setting this transport are investigated by partitioning the circulation into thermally direct and thermally indirect components. Thermally direct components such as Antarctic Bottom Water (AABW) cool the deep ocean. Global warming has the potential to weaken such thermally direct circulations, leading to less deep cooling and hence an effective deep warming. The pre-industrial heat flux by such thermally direct circulations can be seen as having a ‘heat uptake potential’ which bounds the impact of their collapse. Nine ocean models from the COREII project and 6 coupled climate models show a diversity of AABW vertical heat fluxes (heat uptake potentials) between 0.05 and 0.2 Wm² at 1000m depth in their modern day/pre-industrial states. When CO₂ is quadrupled in the coupled models the deep ocean warms due to an AABW slowdown. The consequent warming in each model is both bound and predicted by their heat uptake potentials derived from their pre-industrial states. Moreover the total ocean heat uptake of each model can be understood in terms of a weakening of thermally direct influences and by maintenance of thermally indirect effects (e.g. wind driven circulations and vertical mixing). Promisingly, AABW heat uptake potential is strongly correlated with observable quantities such as the deep meridional overturning.