Meltwater accumulated on the Greenland Ice Sheet drains to glacier beds, discharging into fjords hundreds of meters below sea level. The injection of meltwater at depth generates an upwelling plume that entrains warm ocean water as it rises along the terminus, increasing submarine melt and driving a fjord-scale exchange flow. However, due to sparse ocean-glacier observations, we lack a process understanding of how plumes control fjord circulation and submarine melt. Combining numerical modeling, theory, and observations, this dissertation investigates near-glacier plume dynamics, the influence of glacier depth on plume structure and submarine melt, and the role of fjord-glacier geometry on circulation in tidewater glacier fjords. In Chapter II, I use buoyant plume theory and a nonhydrostatic, three-dimensional ocean–ice model to investigate the sensitivity of plume dynamics to subglacial discharge, turbulent diffusivity, and conduit geometry. Large discharges result in plumes with positive temperature and salinity anomalies in the upper water column. Fjord circulation is sensitive to conduit geometry; distributed subglacial discharge results in a stronger return flow of warm water toward the terminus. In Chapter III, I use buoyant plume theory, initialized with realistic ranges of subglacial discharge, glacier depth, and ocean stratification, to investigate how plume structure and submarine melt vary during summer months in 12 Greenland fjords. Grounding line depth is a primary control on plume-induced submarine melt: deep glaciers produce warm, salty subsurface plumes that undercut termini, and shallow glaciers produce cold, fresh surface-confined plumes that can overcut. Finally, in Chapter IV, I use regional-scale numerical ocean simulations to systematically evaluate how fjord circulation forced by subglacial plumes, tides, and wind stress depends on fjord width, glacier depth, and sill height. Glaciers grounded below sill depth can draw shelf waters over a shallow sill and into fjord basins with seasonal subglacial discharge; this process is independent of external shelf forcing. These results underscore the first-order effect that subglacial discharge and fjord-glacier geometry have in controlling fjord circulation and, thus, ocean heat flux to the ice. This dissertation includes previously published and co-authored material.

The recent thinning, acceleration and retreat of tidewater glaciers around Greenland suggests that these systems are highly sensitive to a change in climate. Tidewater glacier dynamics have already had a significant impact on global sea level, and, given projected future climate warming, will likely continue to do so over the coming century. Understanding of the processes connecting climatic change to tidewater glacier response is, however, at an early stage. Current leading thinking links tidewater glacier change to ocean warming by submarine melting of glacier calving fronts, yet the process of submarine melting remains poorly understood. This thesis combines modelling and field data to investigate submarine melting at tidewater glaciers, ultimately seeking to constrain the sensitivity of the Greenland Ice Sheet to climate change. Submarine melting is thought to be enhanced where subglacial runoff enters the ocean and drives energetic ice-marginal plumes. In this thesis, two contrasting models are used to examine the dynamics of these plumes; the Massachusetts Institute of Technology general circulation model (MITgcm) and the simpler buoyant plume theory (BPT). The first result of this thesis, obtained with the MITgcm, is that the spatial distribution of subglacial runoff at the grounding line of a tidewater glacier is a key control on the rate and spatial distribution of submarine melting. Focussed subglacial runoff induces rapid but localised melting, while diffuse runoff induces slower but spatially homogeneous melting. Furthermore, for the same subglacial runoff, total ablation by submarine melting from diffuse runoff exceeds that from focussed runoff by at least a factor of five. BPT is then used to examine the relationship between plume-induced submarine melting and key physical parameters, such as plume geometry, fjord stratification, and the magnitude of subglacial runoff. It is shown that submarine melt rate is proportional to the magnitude of subglacial runoff raised to the exponent of 1/3, regardless of plume geometry, provided runoff lies below a critical threshold and the fjord is weakly stratified. Above the runoff threshold and for strongly stratified fjords, the exponent respectively decreases and increases. The obtained relationships are combined into a single parameterisation thereby providing a useful first-order estimate of submarine melt rate with potential for incorporation into predictive ice flow models. Having investigated many of the factors affecting submarine melt rate, this thesis turns to the effect of melting on tidewater glacier dynamics and calving processes. Specifically, feedbacks between submarine melting and calving front shape are evaluated by coupling BPT to a dynamic ice-ocean boundary which evolves according to modelled submarine melt rates. In agreement with observations, the model shows calving fronts becoming undercut by submarine melting, but hints at a critical role for subglacial channels in this process. The total ablation by submarine melting increases with the degree of undercutting due to increased ice-ocean surface area. It is suggested that the relative pace of undercutting versus ice velocity may define the dominant calving style at a tidewater glacier. Finally, comparison of plumes modelled in both MITgcm and BPT with those observed at Kangiata Nunata Sermia (KNS), a large tidewater glacier in south-west Greenland, suggests that subglacial runoff at KNS is often diffuse in nature. In addition to the above implications for submarine melting, diffuse drainage may enhance basal sliding during warmer summers, thereby providing a potential link between increasing atmospheric temperature and tidewater glacier acceleration which does not invoke the role of the ocean. This thesis provides a comprehensive investigation and quantification of the factors affecting submarine melting at tidewater glaciers, a complex process that is believed to be one of the key influences on the current and future stability of the Greenland Ice Sheet. Based on the magnitude of modelled melt rates, and their effect on calving front shape, the process of submarine melting is a likely driver of retreat at slower-flowing tidewater glaciers in Greenland. For melting to influence the largest and fastest-flowing glaciers requires invoking a sensitive coupling between melting and calving which is as yet obscure. It should however be noted that modelled melt rates depend critically on parameters which are poorly constrained. The results and parameterisations developed in this thesis should now be taken forward through testing against field observations - which are currently rare - and, from a modelling perspective, coupling with ice flow models to provide a more complete picture of the interaction of the Greenland Ice Sheet with the ocean.

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