Modeling the impact of glacial runoff on fjord circulation and submarine melt rate using a new subgrid‐scale parameterization for glacial plumes

Abstract

The injection at depth of ice sheet runoff into fjords may be an important control on the frontal melt rate of tidewater glaciers. Here we develop a new parameterization for ice marginal plumes within the Massachusetts Institute of Technology General Circulation Model (MITgcm), allowing three-dimensional simulation of large (500 km 2 ) glacial fjords on annual (or longer) time scales. We find that for an idealized fjord (without shelf-driven circulation), subglacial runoff produces a thin, strong, and warm down-fjord current in the upper part of the water column, balanced by a thick and slow up-fjord current at greater depth. Although submarine melt rates increase with runoff due to higher melt rates where the plume is in contact with the ice front, we find that annual submarine melt rate across the ice front is relatively insensitive to variability in annual runoff. Better knowledge of the spatial distribution of runoff, controls on melt rate in those areas not directly in contact with plumes, and feedback mechanisms linking submarine melting and iceberg calving are necessary to more fully understand the sensitivity of glacier mass balance to runoff-driven fjord circulation.

Glacier Calving in Greenland

“…Modified from Cowton et al [20] These findings indicate that melt-undercutting is sensitive to ocean temperature, glacier runoff, and the morphology of the subglacial hydrological system. Runoff input from a single channel produces a small zone of highly concentrated melting, whereas distributing runoff more evenly across the width of the glacier decreases the maximum local melt rates but can increase the total submarine melting of the calving front by a factor of 5 [51,52]. This is likely a significant consideration with respect to the effect of melt-undercutting on calving: small melt rates distributed across the full terminus width may have a different impact on terminus stability than one or a few zones of focused melting (see below).…”

Estimating Spring Terminus Submarine Melt Rates at a Greenlandic Tidewater Glacier Using Satellite Imagery

“…Classical two-layered buoyancy-driven circulation operates in the fjord primarily during the spring and summer, where circulation is driven by subglacial meltwater plumes (Figure 3). Subglacial discharge exits the glacier at the grounding line, rises buoyantly along the ice front due to its lower density relative to the ambient fjord water, and flows down-fjord once neutral buoyancy is reached (Motyka et al, 2003;Jenkins, 2011;Cowton et al, 2015). In fjords with shallow glacier grounding line depths (<500 m) like KNS, summer discharge meltwater plumes often reach neutral buoyancy and horizontally enter the fjord within the upper 100 m of the water column (Carroll et al, 2016).…”

Non-linear retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

“…The submarine melt rate may vary dependent on glacier discharge and the slope of the submarine face. However, the relationships are still poorly 5 known and width-averaged submarine melt rates are even found to be relatively insensitive to glacier discharge (Cowton et al, 2015). In the model, a distance-dependent submarine melt rate profile can be implemented.…”

“…Holland et al, 2008a;Motyka et al, 2011;Hansen et al, 2012) and more than a doubling of melt rates when additionally considering the steepening of the calving front as consequence of large calving events. Submarine melt may 20 also be enhanced by an increase in subglacial discharge due to larger surface runoff (Jenkins, 2011;Xu et al, 2012;Sciascia et al, 2013;Xu et al, 2013), although this may only be a local effect in front of cavities and negligible when width-averaged (Cowton et al, 2015).…”