This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system. Paleo-evidence suggests that this circulation has experienced rapid reorganizations, particularly during the last glacial period. Understanding the driving mechanisms of this climate engine and its sensitivity to past and future perturbations requires an in-depth understanding of the dynamical processes that maintain the AMOC. Here, we focus on the processes that establish its sinking branch. This project will conduct a comprehensive study of the dynamics of thermally-driven circulation in the subpolar North Atlantic. The overall goal is to understand the fundamental, 3-dimensional dynamics of the AMOC?s large-scale descending branch, including its dependence on horizontal and vertical mixing, buoyancy fluxes, winds, bottom topography and basin geometry, and southern-hemisphere forcing. To address this goal, we will develop a hierarchy of solutions to an ocean general circulation model (OGCM) and, when useful, obtain corresponding solutions to layer models. At the base of our hierarchy is a thermally forced, inviscid, OGCM solution that develops no diapycnal overturning. A first task in our study, then, is to understand in detail the processes (horizontal vs. vertical mixing, diffusion vs. viscosity, strength of surface heating) that breakdown this null state. Further tasks will sequentially consider the impacts of the other processes listed in our goal. The expected outcome is an improved understanding of the processes that determine the strength and location of deep subpolar downwelling and its relationship to the large-scale AMOC. Through our focus on the physics of the AMOC sinking branch, this research will provide important dynamical insights that will help to assess the stability of the AMOC with respect to density perturbations. It will also help in the interpretation of paleo-proxy data that show evidence of rapid AMOC transitions in the North Atlantic region and in the evaluation of non-eddy resolving future climate change model simulations.
