Geometric Controls of Tidewater Glacier Dynamics

Abstract

Abstract. Retreat of marine outlet glaciers often initiates depletion of inland ice through dynamic adjustments of the upstream glacier. The local topography of a fjord may promote or inhibit such retreat, and therefore fjord geometry constitutes a critical control on ice sheet mass balance. To quantify the processes of ice-topography interactions and enhance the understanding of the dynamics involved, we analyze a multitude of topographic fjord settings and scenarios using the Ice-sheet and Sea-level System Model (ISSM). We systematically study glacier retreat through a variety of artificial fjord geometries and quantify the modeled dynamics directly in relation to topographic features. We find that retreat in an upstream widening or deepening fjord does not necessarily promote retreat, but conversely, may stabilize a glacier because converging ice flow towards a constriction enhances lateral shear. An upstream narrowing or shoaling fjord, in turn, may promote retreat since fjord walls or bed provide little stability to the glacier where ice flow diverges. Furthermore, we identify distinct quantitative relationships directly linking grounding line discharge and retreat rate to fjord topography, and transfer these results to a long-term study of the retreat of Jakobshavn Isbræ. These findings offer new perspectives on ice-topography interactions, and give guidance to an ad-hoc assessment of future topographically induced ice loss based on knowledge of the upstream fjord geometry.

Helheim Glacier Poised for Dramatic Retreat

“…Within the central region of ice flow there is a bedrock high (Figure 1a) that may act as a pinning point should the glacier retreat this far inland. Similarly, strong lateral flow convergence from Helheim Glacier's three branches of ice flow may stabilize the terminus, even in regions with a reverse‐sloping bed (Frank et al., 2021; Gudmundsson et al., 2012). However, both the bedrock high and the convergence of Helheim Glacier's flow branches are located >6 km inland of the 2019 terminus, and thus the glacier would already have retreated significantly should it reach this point.…”