Bistability of ice-shelf cavity circulation and melt under coastal polynya forcing

Antarctic ice shelves play a key role in controlling the mass loss of the Antarctic Ice Sheet. Increasing evidence from regional studies suggests the possibility of abrupt transitions from low to high basal melt regimes under future climate forcing. The computational cost and complexity of these realistic simulations yet limit their ability to systematically investigate the steady-state response of ice shelves to incremental forcing, the reversibility of abrupt transitions and the underlying positive feedback loops. Here, we report a detailed analysis of abrupt regime changes in an ice-shelf cavity with idealized geometry, subject to prescribed temperature and salinity stratifications in the far field and near-ice-front surface salt fluxes representing the impact of sea-ice production in coastal polynyas. We show that a cold regime emerges when the buoyancy loss in the polynya is sufficient to trigger deep convection, while a warm regime develops otherwise. The steady response of the system exhibits hysteresis, characteristic of irreversible transitions, and scaling laws are proposed for both melt rates and transition thresholds. We demonstrate that bistability emerges when a gyre recirculates the meltwater within the ice-shelf front area. The bistable forcing range increases with far-field ocean temperature but decreases with background stratification. We also show that increasing horizontal viscosity limits bistability, which may explain the range of behaviors reported in previous studies.