Parameterizing the impact of fjord circulation on the ocean forcing of melting ice sheets

• Main Author: Stanway, A
• Other Authors: Wells, A, Johnson, H, Ridley, J, Marshall, D, Inall, M
• Format: Thesis
• Language: English
• Published: 2023
• Subjects: Earth sciences; Cryosphere; Physics; Greenland; Ice Sheet
• Online Access: https://ora.ox.ac.uk/objects/uuid:6c2b97a5-d8df-4683-ab60-12b401aa53c8

Freshwater flux from the melting of the Greenland Ice Sheet (GrIS) accounts for 15% of the observed rise in global-mean sea level since 1993, with the rate of mass loss expected to increase as ocean temperatures rise. Exchanges of heat and freshwater between the GrIS and the ocean are mediated by circulation in proglacial fjords, ubiquitous around the coast of Greenland. Fjord circulation occurs on length scales which are too small to be resolved in contemporary Earth system models, representing a fundamental limitation in the ability of these models to form accurate long-term predictions of global climate. To include ice-ocean interaction mediated by fjord circulation, a sub-grid scale parameterization scheme is required. Here we take the first steps towards developing such a scheme, exploring and characterising several first-order controls on circulation within fjords. This thesis develops theoretical understanding of fjord circulation in a regime dominated by the injection of subglacial discharge at the grounding line. We employ computational modelling of idealised fjords and develop scaling laws for the vertical structure of the flow when averaged across the width of the fjord. We produce a dynamically motivated prediction of cross-fjord velocities at the ice face, which is used to predict submarine melt rates across the entire glacial terminus. These theoretical developments are used as the basis for a simple multi-layered box-model parameterization of fjord circulation in an estuarine regime, which is tested under a range of forcing conditions. Finally, we investigate the impact of bathymetric sills on plume-driven circulation. Within fjords with shallow sills, the stratification differs significantly from that of the neighbouring ocean outside of the sill, and submarine melt rates are reduced relative to sill-less fjords given a realistic ocean stratification. The work in this thesis could be used as the basis for a sub-grid scale parameterization scheme, with the potential to be extended to include ...