High-resolution tracer study of AMOC pathways and timescales

Redistribution of heat, freshwater and carbon anomalies by the Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating climate variability. The knowledge of pathways and timescales associated with AMOC is required for understanding linkages different parts of the global ocean and for interpretation of the observed variability. The progress in this direction is challenging because of the important modulations of these linkages by mesoscale eddy- induced mixing. The challenge comes in large part from the enormous computing costs of running extended numerical simulations and high spatial resolutions. Instead, vast majority of numerical studies of AMOC rely on coarse-resolution simulations, which parameterize all important small- scale processes. Despite significant advances in these parameterization schemes, their fidelity is challenging to establish, which results in biases and uncertainties in studies on the role of AMOC in climate and its variability. This is particularly important in light of the fact that the oceanic uptake of anomalous heat and anthropogenic carbon by AMOC in climate models represent one of the major sources of uncertainty in future climate projections.
The main goal of the proposed study is to examine pathways and timescales associated with AMOC and its interactions with the Southern Ocean, and to establish the relative importance of the mean advection and the material transport induced by mesoscale currents (“eddy mixing”). This goal will be achieved by using a highly efficient offline technique for tracer simulation, which allows multiple extended simulations at high spatial resolution and targeted sensitivity studies that can isolate and quantify the effects of mesoscale advection. The “boundary impulse response” will be used in high-resolution, high-fidelity numerical simulations of AMOC to obtain objective, non tracer-specific characterization of AMOC pathways and timescales. The importance of eddies and inhomogeneity of their transports will be analyzed using arguably the most accurate and straightforward technique of contrasting simulations with and without eddies, but with the same mean stratification. Finally, the results will be used for interpretation of the observed variability in the North and South Atlantic.
This proposal is in response to the “CVP – AMOC-Climate Linkages in the North and/or South Atlantic” Competition. This study is relevant to this competition because it aims to improve the understanding of linkages between various branches of AMOC and the rest of the climate system. Proposed techniques are perfectly suited for studies of the AMOC flow pathways in the presence of explicit effects of mesoscale eddies, and will help to identify important fingerprints of AMOC and to develop useful metrics for model evaluation. Progress in this direction is critical for reducing uncertainty in multi-decadal prediction of the Earth system and for interpretation of observations. The proposed work is relevant to NOAA’s long-term goal of “Climate Adaptation and Mitigation” and its objective of “Improved scientific understanding of the changing climate system and its impacts”, by offering a comprehensive, novel study of the pathways and timescales of AMOC that will lead to the improved understanding and modeling of the global ocean circulation and its effects on the Earth System, including sea level rise and changes in the biogeochemical cycles.