OCE-PRF Track 2 (International): Towards a 3-Dimensional Understanding of the Meridional Overturning Circulation

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The "global meridional overturning circulation" is an ocean current that brings warm, often salty water from the tropics to the colder polar regions. This circulation acts to redistribute heat around the globe and has a profound influence of the Earth's climate, and for this reason is sometimes called the "global ocean conveyor belt." The Southern Ocean, which encircles the Antarctic continent, exerts a strong influence on the global overturning. However, in the Southern Ocean, the dynamics of this current are very poorly understood. In this project, the fellow (Christopher Chapman) together with sponsoring scientist Jean-Baptiste Sallée, at LOCEAN-IPSL, Paris, will work towards understanding the role that large undersea topography and much smaller scale turbulent flows, called "eddies" play in global overturning using a mixture of simple ocean models; observations from oceanographic instruments; and sophisticated global climate models. Our results will help reduce uncertainties in projections of future climate change and will aid policy makers to understand the potential impacts of their decisions. The Southern Ocean strongly influences the global overturning circulation. It is in the Southern Ocean that cold, deep waters upwell to the surface, that surface buoyancy forcing and diapycnal mixing processes cause water mass transformations and where energetic baroclinic eddies act to oppose the general northward overturning circulation driven by wind forcing. However, a lack of observations in the region and the prohibitive expense of running large-scale eddy-resolving simulations has meant that the Southern Ocean overturning is poorly understood. In particular, most studies to date exploit the fact that the Southern Ocean currents are primarily zonally oriented to treat the MOC as a two dimensional system, ignoring variations with longitude. However, recent observations have shown that the Southern Ocean is not 2-dimensional. Turbulence, tracer-mixing and water-mass subduction are strongly localized to regions downstream of large bathymetric features, such as the Kergulean Plateau. Recent work has indicated that turbulent processes near these bathymetric features can cause a rearrangement of the ocean's physical structure on large-scales. As such, these local-scale dynamics may have an influence of the large-scale structure of the global overturning. This project aims to blend descriptive and process based approaches to study the influence of "local" dynamics on the "global" overturning. The first stage of the project will use data from satellites, Argo floats, ships and instrumented elephant seals to determine how dynamical processes originating due to flow interaction with large bathymetric features influence the large-scale oceanic structure - in particular the potential vorticity structure and the meridional flux of heat and mass. The second stage of the project will use a high-resolution (eddy-resolving) numerical ocean model, run in an idealized configuration, meant to be representative of the Southern Ocean. The model will be run both with, and without, large-scale bathymetry and with, and without, turbulent eddies. Finally, the output of climate models contained in the CMIP5 database will be examined to determine what influence, if any, the accurate representation of flow/bathymetry interaction in the Southern Ocean has on the response of the global overturning.