Exploring the relationship between submarine melt and subglacial discharge with observations from LeConte Glacier, Alaska

Abstract:

At the terminus of tidewater glaciers, small-scale plumes modulate submarine melt rates and vigorously mix glacial freshwater as it enters the ocean. Recent studies indicate that the prevalent parameterization for plume-melt dynamics significantly underpredicts melt rates, suggesting a critical limitation in our ability to model ocean-glacier interactions. Here, we present a suite of observations collected at LeConte Glacier, Alaska, over a 3-year field campaign. We focus on near-glacier ocean surveys by autonomous kayaks and ships – complemented by moorings, radar, multibeam sonar, and modeling with Large Eddy Simulations (LES) – as we try to reconcile two seemingly contradictory findings. First, kayak surveys have shown ubiquitous meltwater intrusions from ambient melting (melt occurring away from subglacial discharge plumes), indicating that ambient melt is a substantial part of the total terminus melt and is significantly underestimated by standard plume-melt theory. Second, fjord budgets have been evaluated from repeat shipboard surveys to derive the net fluxes of submarine meltwater and subglacial discharge across seven field seasons. These flux estimates confirm that the magnitude of total meltwater is significantly higher than plume-melt theory predicts, but, intriguingly, the relationship between net submarine melt and subglacial discharge follows the power law relationship from standard plume-melt theory. Thus, on the one hand, meltwater intrusions have revealed the importance of ambient melt and, on its own, this result might suggest that net ablation is less sensitive to subglacial discharge than standard theory predicts. On the other hand, the budget results suggest that the relationship between discharge and melt is well represented by standard theory, although significantly underpredicting melt in magnitude. We try to reconcile these findings using surveys of the subglacial discharge plume and near-glacier circulation, along with LES modeling of the near-glacier region. We investigate the linked dynamics of subglacial discharge and submarine melting, including the indirect role of discharge in enhancing ambient melt, and explore modifications to the standard plume-melt parameterizations.