At marine-terminating glaciers, the interplay between meltwater buoyancy and local currents control turbulent exchanges. Because of challenges in making centimeter-scale measurements at glaciers, turbulent dynamics at near-vertical ice-ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically-bolted to an underwater ice face, and use these to elucidate the tug-of-war between meltwater-derived buoyancy and externally-forced currents in controlling boundary-layer dynamics. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near-boundary stress. When external currents strengthened, the plume diffused far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in-situ observations to demonstrate how buoyant melt plumes energize near-boundary turbulence, and why their dynamics are critical in predicting ice melt.

Buoyancy fluxes and glacial melt rates at vertical ice-ocean interfaces are commonly parameterized using theories derived for unbounded free plumes. A Large Eddy Simulation is used to analyze the disparate dynamics of free plumes and wall-bounded plumes; the distinctions between the two are supported by recent theoretical and experimental advances and demonstrate that unbounded plume theory does not adequately represent plume/boundary layer dynamics at ice-ocean interfaces. Modifications to parameterizations consistent with these simulations are tested and compared to results from numerical and laboratory experiments of meltwater plumes. These modifications include 50% weaker entrainment and a distinct plume-driven friction velocity in the shear boundary layer up to 8 times greater than the externally-driven friction velocity. Using these modifications leads to 40 times the ambient melt rate predicted by commonly used parameterizations at vertical glaciers faces, which is consistent (and necessary for consistency) with observed melt rates at LeConte Glacier, Alaska.

Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: (1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation; (2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies are an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.

The connections between Greenland’s tidewater glaciers and the continental shelf waters are modulated by fjords that are geometrically complex with numerous modes of variability. These fjords transport heat towards the glaciers and meltwater away, with a renewal determined by a range of factors including buoyancy forcing at the surface (surface melt) and at depth (subglacial discharge), wind stresses, shelf currents, bathymetry, and mixing processes. This study aims to classify the various regimes of flow and fjord renewal over the diverse parameter space of Greenland fjords with an emphasis on investigating the role of shelf-driven variability and mixing. We conduct idealized numerical simulations using MITgcm to explore how fjord renewal is controlled by elements of the shelf and fjord geometry, and by thermodynamic and mechanical forcings, with domains consisting of idealized bays and shelves representative of Greenland. Specifically, we investigate the sensitivity of the renewal to the following dynamic/thermodynamic influences: (a) bathymetric sills and their influence on mixing, (b) horizontal constrictions (peninsulas, islands, turns), (c) ice cover, and (d) subglacial meltwater discharge. To explain our numerical model results, we develop dynamical theories for the exchange flow as a function of the geometric and forcing parameters.

Fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit both overturning and horizontal recirculations, which are driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the interaction between the 3D fjord circulation and glacial melt and how relevant fjord properties influence them. In this study, high-resolution numerical simulations of idealized glacial fjords demonstrate that recirculation strength controls melt, which feeds back on overturning and recirculation. The overturning circulation strength is well predicted by existed plume models for face-wide melt and subglacial discharge, while relationships between the overturning, recirculation, and melt rate are well predicted by vorticity balance, reduced-order melt parameterizations, and empirical scaling arguments. These theories allow improved predictions of fjord overturning, recirculation, and glacial melt by taking intrafjord dynamics into account.