The dynamics of subglacial discharge plumes and their relationship to terminus morphology

Tidewater glaciers are vulnerable to accelerated melting under warming ocean conditions, potentially triggering glacier acceleration and retreat. The resulting ice mass loss will contribute to sea level rise, export additional freshwater to the ocean, and potentially alter coastal circulation in places such as Alaska, Svalbard, and Greenland. The efficiency of heat transfer from the ocean to the glaciers can be strongly influenced by fjord circulation near the glacier terminus, and one of the dominant drivers of near-glacier circulation is subglacial discharge plumes (buoyant plumes that result from the release of freshwater at the base of the glacier). The upwelling phase of these plumes are small features (tens of meters) directly in front of the glacier terminus where the dangers associated with iceberg calving have long prevented in situ sampling, with few exceptions. Much of our understanding of subglacial discharge plumes has come from theories and models, which have been largely unvalidated. This dissertation contributes direct observations of subglacial discharge plumes, several of which are the first of their kind, that elucidate key features of plume dynamics and the important role of terminus morphology in shaping them. The second chapter of this dissertation presents the first observations of velocity in an upwelling subglacial discharge plume. The data were collected in 2018 by a remotely operated kayak deployed to the terminus of Xeitl Sít’ (LeConte Glacier), in southeast Alaska. Resolving the velocity in the upwelling plume, which is smaller than the distance between the acoustic beams of current profilers, required developing a new processing method. The observations reveal a mostly cone-shaped plume that remains attached to the ice face and entrains more ocean water than predicted by standard plume theory. Comparing the observations to a high resolution (LES) model reveals that the overcut slope of the ice face accounts for a significant portion of the excess entrainment and suggests that the remainder may be due to enhanced mixing at the grounding line depth. The third chapter of this dissertation presents observations of subglacial discharge plumes collected by remotely operated boat at Xeitl Sít’ in 2024, which capture the upwelling phase as well as its transition to horizontal flow at the surface, subduction to neutral buoyancy, and subsurface outflow downstream. This analysis reveals that, at the surface, both convective and shear instabilities contribute to turbulence in the plume, and that the plume undergoes a hydraulic jump immediately prior to its subduction to neutral buoyancy. An LES model shows that the observed bifurcation of the plume at the surface is caused by the overcut ice face. Observations of the plume’s increasing volume flux, changing stratification, and evolving cross-sectional area provide a framework for modelers to improve the parameterization of subglacial discharge plumes in regional models. The fourth chapter of this dissertation presents observations collected in 2024 at Kangerlussuup Sermia in western Greenland by the hybrid remotely operated vehicle Nereid Under Ice (NUI), which was deployed to the terminus and operated at 25-100 m off the seafloor. Operating at depth enabled high resolution observations of horizontal velocities near the grounding line depth, vertical velocities up the face of the ice, and unprecedented detail of the ice face at the site of the main discharge plume, where multiple subglacial discharge outlets are embedded in a cave-like feature. The horizontal velocity data provide direct evidence of enhanced grounding line entrainment, supporting the results presented in the second chapter. The observations in the third chapter culminate to confirm what some modeling studies have predicted, that multiple upwelling plumes originating from nearby outlets can coalesce into a single plume. Unlike the models, though, the observations show that the glacier morphology plays an important role in directing the plumes towards each other. Together, this body of research represents a significant step forward in our collective understanding of subglacial discharge plumes, their role in near-glacier circulation and influence on glacier melt, and the importance of terminus morphology in shaping plume dynamics.