Discharge plume dynamics at a tidewater glacier: linking upwelling plumes to the downstream exchange flow

At tidewater glaciers, subglacial discharge drives vigorous upwelling plumes, and these plumes modulate both the ocean’s impact on the glacier via submarine melting and the glacier’s impact on the ocean via buoyancy forcing. However, a lack of near-glacier observations has left open basic questions about the dynamics of these plumes and how they set up an exchange flow that imports heat and exports meltwater. Here, we investigate the discharge plume dynamics at LeConte Glacier, Alaska, with near-glacier surveying (in Sept 2017 & Sept 2018) and with Large Eddy Simulations (LES). Autonomous kayaks sampled the water properties and velocity in an upwelling plume, and extensively traced the evolution of the plume as it flows away from the glacier. These kayak observations are complemented by: downstream shipboard surveying including turbulence profiling; multibeam sonar imaging of the terminus morphology; and near-glacier moorings. Combined, these measurements allow us to investigate the structure and distribution of discharge plumes, and their relationship to the terminus morphology and downstream circulation. We confirm that buoyant plume theory can generally predict the bulk properties of a plume when it hits the surface (after accounting for a distributed plume geometry, as suggested by previous studies), but this bulk picture is insufficient for understanding the structure of the outflowing plume or the compensating inflow at depth. In particular, we explore how the plume’s properties and structure evolve after the plume hits the surface and subducts to become a stratified subsurface outflow, with similar features found in both the observations and LES. The dynamics and mixing in both the upwelling and subduction phases combine to set the structure and strength of the downstream exchange flow, controlling which fjord waters are drawn towards the glacier. Ultimately, this analysis works towards a generalizable framework for connecting near-glacier plume dynamics with the downstream exchange flow that imports heat and exports meltwater.