High latitude controls on ocean heat uptake efficiency

Emily Rose Newsom, University of Oxford, Department of Physics, Oxford, United Kingdom, Laure Zanna, University of Oxford, Dept. of Physics, Oxford, United Kingdom and Samar Khatiwala, University of Oxford, Department of Earth Sciences, Oxford, United Kingdom
The climate’s response to changes in radiative forcing depends intimately on the ocean. Specifically, for decades to centuries after an increase in atmospheric carbon dioxide, surface climate changes are substantially mitigated by the absorption and storage of heat by the ocean. However, how efficiently ocean warming mitigates surface warming --- specifically, the ratio of ocean heat uptake per degree global mean temperature change --- varies significantly across climate models. Notably, this “ocean heat uptake efficiency” is traditionally considered a global property. Here, we argue that uncertainties in this global property stem primarily from several key high latitude regions of the Southern and North Atlantic Oceans.

We demonstrate this using a Green’s Function representation of the ocean circulation in the UVIC ESCM. Through its linearity, we can deconstruct the global ocean circulation into independent regional components, quantifying the individual contributions of different ocean regions in the global uptake of surface properties. Our analysis highlights two unique aspects of the high latitudes: first, these regions primarily control the timescales over which the ocean “remembers” surface perturbations. Secondly, these regions are co-located with strong radiative feedbacks at the ocean’s surface. We show that, given these unique characteristics, the global ocean’s response to surface forcing scenarios depends largely on ocean-atmosphere coupling, and ocean dynamics, in the high latitudes.