Parameterizing tidal-water intrusions in long-term Antarctic ice-sheet projections

The Antarctic Ice Sheet (AIS) is expected to be the dominant contributor to sea-level rise in the coming centuries. However, this contribution is deeply uncertain due to the lack of understanding of some fundamental processes influencing ice-sheet dynamics. A key question is the extent to which submarine melting takes place at the transition between grounded and floating ice. Traditionally, in continental-scale ice-sheet modelling, this area has been treated as an abrupt transition or grounding line with suppressed or strongly limited submarine melting upstream. However, several lines of evidence challenge this view. In many places, changes in ocean tides lead to back and forth migrations of the grounding line over a broad grounding zone and can cause intrusion of warm ocean waters several kilometres upstream of the grounding line, allowing for submarine melting there. Here, we propose a simple parameterization to represent the effect of tidally-controlled migrations of the grounding line and tidal-water intrusion in submarine melting in continental-scale ice-sheet models. We calibrate the magnitude of the parameter controlling the extent of oceanic water intrusions against the observational evidence as inferred from differential interferometry synthetic aperture radar. We use a three-dimensional ice-sheet model to investigate the impact of this parameterization on Antarctic ice-sheet projections under two high-emission climate scenarios extending to the year 3000. Our results show that increasing the extent of tidal intrusion, reinforced by dynamic feedbacks, leads to stronger and more widespread grounding-zone retreat under warming scenarios, and consequently ice-stream acceleration, ice-shelf thinning and debuttressing. This implies larger sea-level contributions compared to the usual treatment of melt at the grounding line and should be accounted for in future projections.