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Abstract At tidewater glaciers, the ocean supplies heat for submarine ice melt and the glacier supplies freshwater that impacts ocean circulation. Models that employ buoyant plume theory are widely used to represent the effects of subglacial discharge on both glacier melt and freshwater export, but a scarcity of observations means that these models are largely unvalidated. The challenges and inherent risks of working near actively calving glaciers make it difficult to collect in situ observations. This study, conducted at Xeitl Sít’ (LeConte Glacier) in southeast Alaska, reports the first observations of velocity and geometry of the upwelling core of a subglacial discharge plume. This subglacial discharge plume rises along an overcut ice face, with vertical velocities in excess of 1 m s⁻¹, and a plume shape consistent with subglacial discharge emerging from a narrow outlet. Buoyant plume theory, as commonly applied, fails to replicate the observed entrainment, underestimating the plume's volume flux by more than 50%. Large eddy simulations reveal that over half of this mismatch can be attributed to the overcut slope of the ice, which enhances entrainment. Enhanced mixing near the grounding line may account for the additional entrainment. Accurate representation of plume geometry and entrainment is critical for understanding plume‐driven melt of the terminus and the initial mixing of glacial meltwater as it is exported into the ocean. 

Abstract The energy and momentum balance of an abyssal overflow across a major sill in the Samoan Passage is estimated from two highly resolved towed sections, set 16 months apart, and results from a two-dimensional numerical simulation. Driven by the density anomaly across the sill, the flow is relatively steady. The system gains energy from divergence of horizontal pressure work and flux of available potential energy . Approximately half of these gains are transferred into kinetic energy while the other half is lost to turbulent dissipation, bottom drag, and divergence in vertical pressure work. Small-scale internal waves emanating downstream of the sill within the overflow layer radiate upward but dissipate most of their energy within the dense overflow layer and at its upper interface. The strongly sheared and highly stratified upper interface acts as a critical layer inhibiting any appreciable upward radiation of energy via topographically generated lee waves. Form drag of , estimated from the pressure drop across the sill, is consistent with energy lost to dissipation and internal wave fluxes. The topographic drag removes momentum from the mean flow, slowing it down and feeding a countercurrent aloft. The processes discussed in this study combine to convert about one-third of the energy released from the cross-sill density difference into turbulent mixing within the overflow and at its upper interface. The observed and modeled vertical momentum flux divergence sustains gradients in shear and stratification, thereby maintaining an efficient route for abyssal water mass transformation downstream of this Samoan Passage sill. 

Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.