Diagnosing the Sensitivity of Grounding-Line Flux to Changes in Sub-Ice-Shelf Melting
Ice shelves provide resistance that limits the ice sheet flux across the grounding line and into the ocean. This “buttressing” is reduced by ice shelf thinning. We investigate how commonly used measures of buttressing correlate with changes in grounding line flux (GLF) when the ice shelf is subjected to localized increases in sub-ice shelf melting. While correlations between local buttressing and changes in GLF exist in some idealized configurations they are generally weak for realistic ice shelves and a physical understanding for their cause remains elusive. We thus argue against the use of local buttressing metrics for assessing the sensitivity of changes in GLF to in ice shelf melting. As a robust alternative, we propose and demonstrate an adjoint-based approach.
Grounding-line flux (GLF) sensitivity diagnosed from the adjoint-based sensitivity is identical to that from pointwise, diagnostic model perturbation experiments. The adjoint-based method provides an accurate assessment of GLF sensitivity and many-orders-of-magnitude computational cost savings.
Using a numerical ice flow model, we study changes in the resistance ice shelves provide to the flow of ice across the grounding line (commonly referred to as "buttressing") due to increases in sub-ice-shelf melting. We explore correlations between commonly used metrics for ice shelf buttressing and changes in the integrated grounding-line flux. While correlations exist, their origins remain elusive from the perspective of a theoretical or physically based understanding. This and the fact that the correlations are generally worse for realistic ice shelf domains motivates us to seek an alternative approach. We propose and demonstrate an adjoint-based method for calculating the sensitivity of the integrated grounding-line flux to local changes in ice-shelf thickness and show that the adjoint-based sensitivity is identical to that deduced from pointwise model perturbation experiments. Based on its much wider applicability and the significant computational cost savings, we propose that the adjoint-based method is ideally suited for assessing grounding-line flux sensitivity to changes in sub-ice-shelf melting.