Stratification and overturning circulation are intertwined controls on ocean heat uptake efficiency in climate models

Abstract. The global ocean takes up over 90 % of the excess heat added to the climate system due to anthropogenic emissions, thereby buffering climate change at the Earth's surface. A key metric to quantify the role of the oceanic processes removing this heat from the atmosphere and storing it in the ocean is the ocean heat uptake efficiency (OHUE), defined as the amount of ocean heat uptake per degree of global surface warming. Despite the importance of OHUE, there remain substantial uncertainties concerning the physical mechanisms controlling its magnitude in global climate model simulations: ocean mixed layer depth, Atlantic meridional overturning circulation (AMOC) strength, and upper ocean stratification strength have all been previously proposed as controlling factors.

In this study, we analyze model output from an ensemble of 28 climate models from the Coupled Model Intercomparison Project, phase 6 (CMIP6), in order to resolve these apparently divergent explanations. We find that stratification in the mid-latitude Southern Ocean is a key model property setting the value of OHUE due to its influence on Southern Ocean overturning. The previously proposed role of the AMOC for OHUE is explained by a linkage of stratification model biases between the subpolar North Atlantic and the Southern Ocean. Our analysis thus reconciles previous attempts at explaining controls on OHUE, and highlights the importance of interlinked model biases across variables and geographical regions.