Vulnerability of the Antarctic Ice Sheet to a changing thermocline

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The Antarctic Ice Sheet is the largest potential source of future sea-level rise. Even small changes in its mass balance have enormous global impacts, which makes the question of its future evolution highly relevant for society. Over the past decades, the Antarctic Ice Sheet has been losing mass at an accelerating pace. The imbalance of the ice sheet is caused by - partially abrupt - changes in parts of West Antarctica where speed-up, thinning, retreat and discharge of the inland ice is driven by enhanced basal melting of the ice shelves. The amount of warm water which can access the ice shelf cavities, fueling basal ice-shelf melt, is regulated by the depth of the thermocline. Observations and numerical atmosphere-ocean modeling suggest that the thermocline is strongly driven by winds over the Southern Ocean, which connects it to global-scale climate variability. There is evidence that wind patterns that cause an elevated thermocline and thus facilitate warm-water penetration of the ice-shelf cavities will continue to strengthen in the future. However, the future evolution of the dynamics of the Antarctic Ice Sheet and its sea-level contribution in response to the expected wind-driven shifts in the thermocline remain unexplored.To overcome this knowledge gap, we propose a thorough investigation of the vulnerability of the Antarctic Ice Sheet to thermocline variations, guiding a first-order assessment of the ice-sheet’s mass changes in response to large-scale climate variability and change. For this purpose, we first plan to carry out an in-depth analysis of how the circum-antarctic thermocline depth depends on changes in wind speed and direction, putting their relation into functional form. Based on that, we aim to find critical wind-stress thresholds, indicating the regional potential of future warm-water intrusion into the different ice-shelf cavities around Antarctica. Further, we plan to implement this process of wind-driven thermocline changes into the sophisticated, yet computationally efficient ice-sheet model PISM. For the first time this will allow for simulations of the flow dynamics of the entire Antarctic Ice Sheet in response to basal ice-shelf melting that is sensitive to wind-driven thermocline changes. Conducting an ensemble of simulations, we plan to provide projections of the future Antarctic sea-level contribution, associated with projected regional changes in the atmosphere. In further sets of experiments, we intend to systematically vary the thermocline depth to conceptually explore the ice-sheet’s sensitivity to changes in larger-scale atmospheric variability, e.g., to changes in frequency, amplitude and trend of recurring El-Niño events. In a final step, we envisage to generate an Antarctic risk map that indicates the regional impact of potential warm-water entrainment with respect to the long-term ice-flow response and ice-sheet stability.