Characterising the Ice Shelf/Ocean Boundary Layer

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Global average sea level is rising by approximately 3 millimetres per year. Given the huge economic and societal impacts of this change, accurate forecasts of sea level are urgently needed to inform policymakers considering mitigation and adaptation strategies. Melting of the ice sheets of Antarctica and Greenland currently contributes about one third of sea level rise. The future of this melting is highly uncertain, and the worst-case scenario involves a substantial ice-sheet contribution to dangerous sea-level rise. The largest contribution to sea level rise from ice sheets occurs when the ocean melts the base of ice shelves (floating extensions of the grounded ice sheet). The melt rate of ice in seawater is determined by the transfer of heat and salt from the ocean towards the ice. Observations reveal a turbulent boundary layer in the ocean beneath ice shelves, where vigorous mixing is driven by the flow of rising meltwater, large-scale circulation in the ocean, and tides. Mixing of heat and salt in the boundary layer influences the ice melt rate, but the physical processes involved are poorly understood and will not be resolved in climate models for the foreseeable future. The proposed project will improve our understanding of the ice shelf/ocean boundary layer and develop improved representations of ice-shelf melting for use in climate models.To achieve these aims we will use a suite of numerical models and the latest observations. We will start with direct numerical simulations (DNS) to model a small box of ocean next to an ice shelf (~1 cubic metre) at ultra-high resolution (~1 millimetre). This will provide insight into the turbulence near the ice and its interaction with melting. We will then use large-eddy simulations (LES) to study a larger volume (~1 square kilometre in area by 100 metres height) at high resolution (~10 centimetres - 1 metre). This will resolve the largest turbulent motions in the whole boundary layer. Both models will be validated using recent observations obtained from mooring sites at the George VI and Larsen C ice shelves (Nicholls, NE/H009205/1). The model results will in turn help interpret and understand the observations.We will use these numerical models to devise and calibrate parameterisations for ice melting and vertical mixing for use in ocean climate models. We will add candidate parameterisations to a one-dimensional (vertical) model that incorporates many popular ocean mixing schemes, and test them directly against the DNS and LES results. We will begin with existing parameterisations and modify them as needed to match the high resolution models. The successful parameterisations will be implemented in the UK ocean model (NEMO) and shared with climate modelling groups (including the Met Office) to improve predictions of sea-level rise.