Please note: A thesis defence has no formal time limit. The defence may end earlier or later than the time posted.

Time lapse imagery of the calving front of Ryder Glacier, Northern Greenland, August 2019. Image: Felicity Holmes, Nina Kirchner, Abhay Prakash/Stockholm University

Name

Jonathan Wiskandt, PhD candidate
Department of Meteorology, Stockholm University, Sweden

Title

Modelling ice shelf-ocean interactions in Greenlandic fjords

Abstract

In this thesis, I investigate the complex interactions between the circulation and ice shelf melting in Greenlandic fjords, focusing particularly on ice shelves in North Greenland. The melting of ice shelves, driven by fjord circulation, is a major factor in the mass loss of ice sheets, which in turn contributes to global sea-level rise. Despite its importance, the processes that control marine ice shelf melt remain one of the largest contributors to uncertainty in climate and ice sheet models. This study aims to better understand the dynamics of these interactions by using high-resolution simulations from the Massachusetts Institute of Technology general circulation model (MITgcm) to explore the impact of oceanic forcing on marine melt and ocean circulation beneath the floating tongues of marine glaciers.

The research uses data from the 2019 Ryder Expedition, the first comprehensive survey of the region, to set up idealised model configurations for the Sherard Osborn Fjord and Ryder Glacier system. In this study, I examine how the warming of Atlantic Water, the injection of subglacial discharge, and the presence of fjord sills influence the fjord circulation and the resulting melt rates. One of the key findings is that melt rates exhibit a nonlinear response to oceanic temperature forcing for low ambient temperatures. This response becomes linear at higher temperatures. This behaviour combines results from previous studies, showing that glaciers experience a nonlinear dependence of melt on temperature forcing in colder conditions (such as at Antarctic ice shelves) and a linear dependence in warmer conditions, such as in Greenlandic fjords.

Additionally, in this study, I investigate the role of subglacial discharge, finding a sub-linear relationship between discharge volume and melt rates. The research also explores the spatial variability of melt rates under the ice shelf, driven by the ambient density stratification, which affects the ocean velocities at the ice shelf base. The presence of bathymetric features, such as fjord sills, can restrict the inflow of warm Atlantic Water and reduce overall melt rates by decreasing the temperature of water reaching the ice-ocean interface. I proceed to show the complex combined effect of variations in sill depth and variations in subglacial discharge volume.

During this work I assess different modelling approaches. I evaluate different approximations of the heat conduction into the ice in the parametrization commonly used to represent melting in ocean modelling. It further includes a comparison of results from MITgcm simulations with results from a novel finite element model. The latter offers advantages in representing complex seabed and ice shelf geometries and has the advantage of being able to locally refine the grid best represent regions of strong gradients, like the plume underneath ice shelves. The thesis also addresses the limitations of two-dimensional models. It shows that neglecting the effects of Earth’s rotation and the resulting across-fjord variability, even in narrow fjords, leads to an overestimation in shelf integrated melt by a factor of up to five. Three-dimensional models, although computationally more expensive, provide a more accurate representation of these processes, leading to more detailed representation of the melt rate underneath ice shelves.

The findings shown in this study have important implications for the modelling of ice shelves and the overall mass loss of the Greenland Ice Sheet and its contribution to global sea-level rise. By improving the understanding of the key processes driving marine melt in Greenlandic fjords, this research enhances the ability of climate models to include the effects of melting ice sheets. The insights gained from this work also provide a framework for further studies on ice-ocean interactions, particularly in the rapidly changing polar environments of fjords in Greenland.

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