Mechanics and dynamics of pinning points on the Shirase Coast, West Antarctica

Abstract. Ice rises and rumples, sites of localised ice shelf grounding, contribute to shelf-wide mechanics by generating lateral and basal shear stresses, upstream compression and downstream tension. Here, using a case study approach, we simulate Ross Ice Shelf (RIS) and tributary ice stream flow, with and without the Shirase Coast Ice Rumples (SCIR), to quantify the specific contribution of these features to flow dynamics. While longitudinal stretching, and thus ice velocity, increases in response to pinning point removal, flow resistance generated by other grounded features also increases, providing a control on the magnitude of the velocity difference. Spatial variation in two parameters inferred during model initialisation, basal friction and ice softness, further condition the system response to the SCIR. MacAyeal Ice Stream (MacIS), located directly upstream of the SCIR, is less responsive to the loss of the ice rumples than the obliquely oriented Bindschadler Ice Stream due to zones of locally higher basal drag acting on the main trunk of MacIS. In the model, the larger basal drag acting on MacIS is itself, via regional changes in driving stress, a consequence of the coupled ice shelf and ice stream response to the SCIR. We also find that inversion of present-day flow and thickness for basal friction and ice softness, without feature-specific tuning, leads to the incorrect representation of ice rumple morphology, and by extension, any parameter that is affected by the initialisation procedure. Where pinning point effects are important, model tuning that respects pinning point morphology is necessary to represent the ice sheet-ice shelf system as a whole. Viewed from the perspective of change detection, we find that changes to the ice shelf geometry following removal of the SCIR are transient, as mass flux reduces thickness gradients in some areas and increases them in others, while changes to the ice streams persist, even without sustained grounding line retreat.