Coupled ice/ocean interactions during the future retreat of West Antarctic ice streams

Abstract

Abstract. The Amundsen Sea sector has some of the fastest-thinning ice shelves in Antarctica, caused by high, ocean-driven basal melt rates, which can lead to increased ice stream flow, causing increased sea level rise (SLR) contributions. In this study, we present the results of a new synchronously coupled ice-sheet/ocean model of the Amundsen Sea sector. We use the WAVI ice sheet model to solve for ice velocities and the MITgcm to solve for ice thickness and three-dimensional ocean properties, allowing for full mass conservation in the coupled ice/ocean system. The coupled model is initialised in the present day and run forward under idealised warm and cold ocean conditions. We find that Thwaites Glacier dominates the future SLR from the Amundsen Sea sector, with a SLR that is approximately quadratic in time. The future evolution of Thwaites Glacier depends on the life-span of small pinning points that form during the retreat. The rate of melting around these pinning points provides the link between future ocean conditions and the SLR from this sector and will be difficult to capture without a coupled ice/ocean model. Grounding-line retreat leads to a progressively larger Thwaites ice-shelf cavity, leading to a positive trend in total melting, resulting from the increased ice basal surface area. Despite these important sensitivities, Thwaites Glacier retreats even in a scenario with zero ocean-driven melting. This demonstrates that a tipping point may have been passed and some SLR from this sector is now committed.

Calculating exposure to extreme sea level risk will require high resolution ice sheet models

“…As a result, the bed was 'dug' in order to better match the fine resolution data and to stop unrealistic advance of the PIG grounding line in a coupled ice-ocean model simulation. Here, we do not alter the bed data set provided from Bedmachinev3, and as a result we see less mass loss from PIG than Bett et al [21]. Wernecke et al [59] also quantify the effect of bed topography uncertainties on SLC projections, in their case under PIG, by using different bed data sets alongside statistically modelled beds that take into account bed topography uncertainties.…”

“…In a recent study on the same domain with the same initial state, issues with the bathymetry near the grounding line of Pine Island Glacier were found when compared with data [21]. As a result, the bed was 'dug' in order to better match the fine resolution data and to stop unrealistic advance of the PIG grounding line in a coupled ice-ocean model simulation.…”

“…In order to use this method for making realistic probabilistic projections, a more physically realistic parameterisation or, ideally, a coupled ice-ocean model is required [e.g. 21,51,61]. The simulations we have present are a limited set to explore the methodology and the specific effects of resolution on the upper tails, and do not span the range of uncertainties needed to make realistic projections.…”

“…Furthermore, to incorporate key processes and feedbacks, coupling of different elements of the Earth system such as ice-ocean modelling [e.g. 20,21] and ice-atmosphere models [e.g. 22] are required, and physics-based models of processes such as ice fracture need to be incorporated [e.g.…”

Hysteresis of idealized, instability-prone outlet glaciers under variation of pinning-point buttressing

“…The ice shelf of Thwaites Glacier continues to gradually lose contact to its last remaining pinning points (MacGregor et al, 2012;Rignot et al, 2014;Alley et al, 2021;Benn et al, 2022) and could unpin completely in less than a decade (Wild et al, 2022). This might further decrease pinning-point buttressing and contribute to the imbalance of the glacier also in the future (Bett et al, 2023), although to a possibly limited extent .…”