At high latitudes, glaciers play an important – and in many regions, dominant – role in modulating river flow and delivering sediments and solutes to lowlands and coasts. As climate changes, increased run-off and the longer melt season are bringing widespread glacier retreat and augmented river flows, contributing to sea-level rise around the globe.

That's not all that's happening on the glacier front. Changes to the hydrological structure of the Greenland ice sheet can also speed up the flow of ice to the coast, where it forms icebergs and is lost. As warming progresses, areas where surface meltwater penetrates to the base of the ice are propagating inland, facilitating seasonal, episodic speed-up of the ice flow. Ice masses with marine margins are expected to be one of the most important factors contributing to global sea-level rise in the 21st century, and the Greenland ice sheet is the key source of this type in the northern hemisphere (Church et al. 2013).

Hidden depths

But it's hard to predict how the Greenland ice sheet and other ice masses will respond to the changing climate, because our understanding of the marine-terminating, or tidewater, glaciers that dominate ice outflow in Greenland lags behind our knowledge of their terrestrial counterparts. There are good reasons for this: one inescapable problem is that the run-off from tidewater glaciers emerges underwater and so we cannot monitor it directly (Straneo et al. 2013). While measuring the run-off in Greenland's many large, unstable, terrestrial rivers is challenging, and is reflected in a notable paucity of river-flow data, at tidewater outlets it is to all intents and purposes impossible.

That's where sediment comes in. Because it is basically melted snow and ice, run-off is fresh – and so buoyant – and because glaciers are good at erosion, their run-off is sediment-laden, or turbid. As a result, tidewater glacier run-off frequently forms visible plumes in fjords (figure 1, Chu 2013; NASA 2014). These plumes are typically much wider than river channels, which means they are much more amenable to observation by satellite. Their spectral signature contrasts with non-turbid marine waters, so we can measure the characteristics of the plumes and track their variations; this could help us study both the large-scale hydrology of remote, otherwise unmeasured – or unmeasurable – regions, and the stability and dynamics of large tidewater outlet glaciers draining ice sheets.

Plumes not only provide a visible marker of meltwater release at the marine margins of ice masses, but they also play an important role in the system's dynamics. The outwards flow of cold meltwater can set up a circulation that drives warmer, more salty ocean water towards the ice mass, in a process known as forced convection. Fjord waters often become stratified, with an outflowing fresh meltwater layer, from run-off and perhaps the melting of calved icebergs, overlying a saline inflowing layer. This warmer ocean water brings heat to the glacier terminus and can enhance rates of submarine melting, in positive feedback that may be important for the retreat of tidewater glaciers (Straneo et al. 2013).

Remote monitoring

Remote estuarine and coastal-plume monitoring in non-glacial environments is already relatively well-established, driven by a need to understand the circulation patterns and ecology of nearby coastal areas, and by the need to manage eutrophication, turbidity, and the spread of harmful pollutants. The wide availability of satellite multi/hyperspectral imagery from sensors such as SeaWiFS and MODIS has enabled much progress to be made since the late 1990s/early 2000s.

But it is early days for the technique in glacial fjords. These latest studies, while demonstrating great potential for plume monitoring in the Arctic, have focused on river-fed plumes, which enter a fjord at its surface rather than its base, mainly in West Greenland. Plume monitoring is promising but needs validation in a range of environments to become more robust. In particular, there are currently almost no remote studies of tidewater-glacier terminus plumes.

And monitoring plumes is not always straightforward. At tidewater margins, or in any fjords fed by tidewater glaciers rather than rivers, icebergs can obstruct a clear view of the plume and contaminate its spectral signature. How a plume develops further down the fjord depends on the meltwater conditions both before its entry into the fjord – like patterns of meltwater storage and release controlled by surface melting and the development of glacier drainage systems (figure 2) – and afterwards, such as the combined effect of tides and winds. It is complicated. The ultimate hindrance to using plume state to assess meltwater output may be the variability in the amount of suspended sediment, which generally depends chiefly on sediment availability, rather than on the volume or energy of the water flow itself.

As detailed above, the task of consistently linking some readily measurable plume descriptor to meltwater output, in the face of the confounding influences of sediment-supply variability, iceberg and/or sea-ice presence, and the overprinting of tide and wind, is not easy, although few tasks in remote high-latitude environments ever are. Yet plumes, as the only visible expression of tidewater glacier run-off, "offer a link between ice-sheet hydrology and the ocean that can plausibly be observed using remote sensing" (Chu et al. 2012) even more than melt-fed rivers, so the benefit from addressing such challenges is certainly worthwhile.

The monitoring of plumes by satellite, with frequent-repeat imagery such as the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Terra and Aqua satellites (NASA 2014), is one of a limited number of ways to approach ice-sheet-scale hydrometry (measurement of water flow). So we need studies to validate plume monitoring in as wide a range of locations and fjord and glacial configurations as possible (figure 3). Also we must link meltwater supply and routing to the patterns of meltwater release revealed by plume variability; identify lag times and the persistence of plume metrics in space and time; evaluate the significance of interactions with tides and winds in fjord systems; and determine the proportion of subglacial water flow that actually contributes to the formation of buoyant plumes, and the extent to which this is consistent (Chu 2013). This last task links the interests of glaciology and oceanography, so that plumes offer us the prospect of interdisciplinary progress in the understanding of ice-sheet hydrology.

Related links

References

  • V W Chu 2013 Greenland ice sheet hydrology: A review. Prog. Phys. Geog. 38 19–54, doi:10.1177/0309133313507075
  • V W Chu et al. 2012 Hydrologic controls on coastal suspended sediment plumes around the Greenland Ice Sheet The Cryosphere 6 1–19 doi:10.5194/tc-6-1-2012
  • J A Church et al. 2013 Sea Level Change Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change T F Stocker et al. eds (Cambridge: Cambridge University Press)
  • V Klemas 2012 Remote sensing of coastal plumes and ocean fronts: overview and case study Journal of Coastal Research 28 1–7 doi: 10.2112/JCOASTRES-D-11-00025.1
  • NASA Earth Observatory 2014 Sediment plumes around Greenland http://earthobservatory.nasa.gov/IOTD/view.php?id=84464&src=iotdrss Accessed 26/11/2014
  • F Straneo et al. 2013 Challenges to understand the dynamic response of Greenland's marine terminating glaciers to oceanic and atmospheric forcing Bull. Am. Met. Soc. 94 doi:10.1175/BAMS-D-12-00100.1