Titre/Title : Topographic barriers preventing flow of warm ocean water towards the Antarctic ice sheet
Contact : Joel Someria (MEIGE)
Résumé/Abstract : The potential collapse of the Antarctic Ice Sheet in a future, warmer climate and the consequent dramatic rise in sea level would cause flooding of large, densely populated areas. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change identified the Antarctic Ice Sheet as the largest source of uncertainty in predictions of future sea level rise over their 50-200 year time horizon. The observed accelerating thinning of many of the ice-shelves surrounding and buttressing the Antarctic Ice Sheet is linked to increased oceanic heat fluxes, but the dynamics governing the flow of warm water towards and into the ice shelf cavities are poorly known. By improving the general understanding of these dynamics and the factors controlling the heat flux towards the ice shelf cavities, the proposed project will contribute to reduced uncertainties in sea level rise projections. This will improve risk analysis and social planning that will reduce the social and economic impact of sea level rise.
The heat reservoir threatening the ice shelves is located off the continental shelf, in the deep Southern Ocean, where relatively warm water resides below a shallow, cold and fresh surface layer. To reach the ice shelf cavities and induce basal melt, the warm water must get past two topographical barriers : the continental shelf break and the ice shelf front. Conservation of potential vorticity causes large scale, geostrophically balanced flows to follow f/H contours (f – Coriolis parameter, H – water column thickness), or, on a regional scale, to follow contours of constant depth and the flow of warm water towards the ice shelf cavities is hence topographically controlled.
In order to reach the ice shelf cavity and induce basal melt, the warm water must pass a second topographic barrier : the ice shelf front. The ice shelf front – a vertical wall with a draft of several hundreds of meter – represents a major discontinuity in the water column thickness and conservation of potential vorticity is expected to restrict barotropic flow across the front and into the ice-shelf cavity. The currents transporting warm water towards the cavities have been observed to be mainly barotropic, both along the troughs and directly in front of the cavity openings. It is not clear to what extent these barotropic currents can continue below the ice shelf, and, if they are blocked, whether a baroclinic flow can instead penetrate into the cavity and, if so, what role the barotropic currents then play.
We propose a set of experiments to explore the effect of these topographical barriers on the flow of warm water from the deep ocean into the ice shelf cavity. The experiments are to be carried out in the rotating platform at the Coriolis laboratory in Grenoble, France, where topography representing the Antarctic continental shelf – ice shelf system is to be inserted. Barotropic and baroclinic currents will be set up using pumps and the evolution of stratification and circulation studied.