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The Western Transition Zone as a Gatekeeper for the North Atlantic MOC Throughput

The Atlantic Meridional Overturning Circulation (AMOC) requires significant transport between  the North Atlantic subtropical and subpolar gyres. This transport contributes appreciably to the  Atlantic’s mean ocean heat transport and its variability has been linked to climate variations on  a wide range of time scales, including paleoclimate shifts and Atlantic multidecadal sea surface  temperature variability. Despite the importance to our climate system, no clear consensus on the  dynamical mechanisms controlling this throughput and its variability has emerged to date. Fur- thermore, recent Lagrangian studies have challenged the traditional understanding of the geometry  of the throughput in both the upper and lower AMOC limbs. The goal of our work is to build on  past Eulerian and Lagrangian studies in order to work toward a consensus on AMOC variability  mechanisms.
We believe that such a consensus is possible with a focus on the dynamics at the western margin of  the subtropical-subpolar gyre boundary, a region referred to as the western transition zone (WTZ).  Our working hypothesis is that the WTZ is a gatekeeper for the throughput, whereby buoyancy  anomalies in the WTZ establish the throughput variability and influence decadal AMOC variability  in both the subtropical and subpolar gyres. Importantly, buoyancy anomalies in the WTZ are not  forced solely by local processes; rather they are the result of a wide array of ocean processes.  Thus, we may consider the WTZ an integrator of various processes, a view that may reconcile  various proposed mechanisms of AMOC variability, such as the influences of deep convection and  Rossby waves. As such, a focus on the WTZ may considerably aid the interpretation of AMOC  measurements across the RAPID and OSNAP lines.
Using Eulerian and Lagrangian studies, conducted with ocean observations and two ocean mod- els, our proposed work will address the following questions: (1) Do temporal changes in WTZ  buoyancy anomalies align with throughput changes measured in the Lagrangian frame? (2) What  mechanism creates these buoyancy anomalies? (3) On interannual to decadal time scales, what is  the relationship between WTZ buoyancy anomalies and AMOC variability in the subtropical and  subpolar gyres? To answer these questions we will: quantify AMOC pathways through the WTZ  and develop a Lagrangian metric of the throughput; use Lagrangian experiments and statistical  analyses to investigate the relationship between throughput variability and WTZ buoyancy anoma- lies; use buoyancy budgets, ocean model experiments, and adjoint experiments to understand the  origin of buoyancy anomalies in the WTZ; and determine the extent to which WTZ buoyancy and  AMOC anomalies are related to buoyancy and AMOC anomalies at other latitudes.
Our proposal is targeted at the competition “AMOC-Climate Linkages in the North/South At- lantic”, and is directly relevant to its program objectives, as well as to these research priorities  highlighted in the US AMOC 2014 annual report: (1) Provide a more detailed understanding of  AMOC flow pathways and their impact on variability; (2) Investigate connections between surface  forcing and AMOC variability; (3) Continue investigation of AMOC “fingerprints”; (4) Synthe- size results from theoretical, idealized models, and complex GCM investigations into a common  conceptual framework regarding key AMOC variability mechanisms. More broadly, our proposal  advances the field of decadal prediction by developing an improved understanding of the dynamics  of important modes of climate variability, such as the AMOC, which must be accurately captured  in models used to make decadal predictions.

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