Soutenance de Louis Saddier

The response of the Antarctic Ice Sheet to climate change remains a major source of uncertainty in sea-level projections. Ice shelves, which buttress grounded glaciers, regulate ice-sheet mass loss, but their stability depends primarily on ocean-driven basal melting caused by warm waters entering ice-shelf cavities. Recent numerical studies suggest that these cavities may undergo abrupt transitions between cold and warm circulation regimes, dramatically increasing melt rates. However, the mechanisms governing these transitions and their reversibility remain poorly understood because realistic simulations are computationally expensive.

This thesis investigates the physical processes underlying these regime shifts using two complementary idealized approaches: a low-dimensional ocean box model and an idealized three-dimensional configuration of the NEMO ocean model. Together, they systematically explore the steady-state response of ice-shelf cavities to changes in oceanic and atmospheric forcing, enabling identification of transition thresholds and assessment of reversibility.

The box model represents two competing circulation states: a warm regime driven by Circumpolar Deep Water (CDW) intrusion and a cold regime sustained by Dense Shelf Water (DSW) formed through sea-ice production in coastal polynyas. The model reveals widespread bistability across realistic ranges of sea-ice formation and CDW temperatures. Applied to nine Antarctic ice-shelf cavities, it suggests that most operate within a bistable regime and may therefore be vulnerable to irreversible transitions. It also identifies vertical mixing as a key control on transition reversibility.

Building on these findings, idealized NEMO simulations reproduce the same hysteresis and bistability while showing that CDW properties, large-scale ocean circulation, and model viscosity strongly influence whether transitions are reversible. These results help reconcile contrasting behaviors reported in previous studies.

Finally, the thesis examines time-dependent forcing, including seasonal variability. It demonstrates that the relative time scales of cavity overturning and thermocline adjustment determine whether one or several unusually mild winters can trigger irreversible transitions. Overall, this work provides a mechanistic framework for understanding abrupt ice-shelf cavity transitions and improves confidence in projections of Antarctic ice-sheet evolution under climate change.