Surface emergence of glacial plumes determined by fjord stratification

Abstract

Abstract. Meltwater and sediment-laden plumes at tidewater glaciers, resulting from the localized subglacial discharge of surface melt, influence submarine melting of the glacier and the delivery of nutrients to the fjord's surface waters. It is usually assumed that increased subglacial discharge will promote the surfacing of these plumes. Here, at a western Greenland tidewater glacier, we investigate the counterintuitive observation of a non-surfacing plume in July 2012 (a year of record surface melting) compared to the surfacing of the plume in July 2013 (an average melt year). We combine oceanographic observations, subglacial discharge estimates and an idealized plume model to explain the observed plumes' behavior and evaluate the relative impact of fjord stratification and subglacial discharge on plume dynamics. We find that increased fjord stratification prevented the plume from surfacing in 2012, show that the fjord was more stratified in 2012 due to increased freshwater content and speculate that this arose from an accumulation of ice sheet surface meltwater in the fjord in this record melt year. By developing theoretical scalings, we show that fjord stratification in general exerts a dominant control on plume vertical extent (and thus surface expression), so that studies using plume surface expression as a means of diagnosing variability in glacial processes should account for possible changes in stratification. We introduce the idea that, despite projections of increased surface melting over Greenland, the appearance of plumes at the fjord surface could in the future become less common if the increased freshwater acts to stratify fjords around the Greenland ice sheet. We discuss the implications of our findings for nutrient fluxes, trapping of atmospheric CO2 and the properties of water exported from Greenland's fjords.

Calving Multiplier Effect Controlled by Melt Undercut Geometry

“…The highest rates of submarine melting are found in upwelling plumes, generated by the emergence of fresh subglacial discharge from the grounding line, that flush warm water along the terminus (e.g., Mankoff et al., 2016). The height reached by the plume then determines the vertical extent of the region of high submarine melting (De Andrés et al., 2020). At smaller glaciers with shallow grounding lines, the plume is likely to reach the surface, promoting uniform undercutting reaching the fjord surface (Figure 1b; Carroll et al., 2016) and serac failure.…”

Iceberg melting substantially modifies oceanic heat flux towards a major Greenlandic tidewater glacier

“…Several studies have linked either increase in oceanic heat availability 3 , 4 or increases in up-fjord oceanic heat flux 8 to tidewater glacier retreat. More recently, estimates of ocean thermal forcing during the 21st century have been used to drive parameterisations of glacier retreat as part of the ISMIP6 project 56 . Due to the ice-sheet wide nature and long timescale of this exercise, together with a lack of simple parameterisations for the modification of water masses during fjord transit, the ocean thermal forcing used was based on spatial averages of far-field ocean conditions 57 .…”

Glacier–plume or glacier–fjord circulation models? A 2-D comparison for Hansbreen–Hansbukta system, Svalbard

“…Although the global glacier volume is only ~0.6% of the ice-sheet volume (Greenland and Antarctica), current glacier contribution to SLR is close to that of the ice sheets (IPCC, 2019), primarily due to the high sensitivity of glaciers to atmospheric and oceanic forcing (Rignot and others, 2010; Motyka and others, 2013; Straneo and Heimbach, 2013; Luckman and others, 2015; Holmes and others, 2019). Beyond the SLR issue, the fresh water input from glacier wastage generates considerable changes in fjord stratification (De Andrés and others, 2020) and sediment distribution (Mugford and Dowdeswell, 2011; Overeem and others, 2017), affecting surrounding marine ecosystems (Meire and others, 2017; Hopwood and others, 2018; Oliver and others, 2018), atmospheric CO 2 intakes (Meire and others, 2015) and regional ocean circulation (Bamber and others, 2018; Oliver and others, 2018). Thus, studying processes occurring at the glacier–fjord interface is key to understand ongoing changes and generating future projections.…”

“…One of the key processes favouring submarine melting at tidewater glacier fronts and driving fresh water export from fjords is the presence of buoyant plumes. Buoyant plumes are primarily driven by subglacial discharge of surface meltwater (SMW) through localised channels at the grounding line, so this process is mostly limited to the melting period in the Arctic (Motyka and others, 2013; Schild and others, 2016; De Andrés and others, 2020). These plumes carry sediment from the glacier–fjord bottom towards the surface, and at times become visible at the fjord surface as patches of turbid water (Mankoff and others, 2016; How and others, 2019; De Andrés and others, 2020).…”

“…Buoyant plumes are primarily driven by subglacial discharge of surface meltwater (SMW) through localised channels at the grounding line, so this process is mostly limited to the melting period in the Arctic (Motyka and others, 2013; Schild and others, 2016; De Andrés and others, 2020). These plumes carry sediment from the glacier–fjord bottom towards the surface, and at times become visible at the fjord surface as patches of turbid water (Mankoff and others, 2016; How and others, 2019; De Andrés and others, 2020). As the plumes rise, they entrain large volumes of ambient fjord waters, increasing their initial volume by more than an order of magnitude (Mortensen and others, 2013; Mankoff and others, 2016) and acting as the engine of convection-driven circulation in the fjords (Motyka and others, 2013; De Andrés and others, 2018), thus favouring the input of warmer ocean waters (Straneo and others, 2011).…”