IAP2-22-320

Investigating the impact of glacial meltwater plumes in Greenland’s fjords

The connection between oceans and ice sheets is a key coupling in the climate system, with major implications for ice loss, sea level rise, and ocean circulation (e.g. Straneo & Heimbach, 2013; Golledge et al., 2019). Oceanic heat drives submarine melting at Greenland’s ~280 marine-terminating glaciers, with increased mass loss from these glaciers responsible for around two-thirds of the ice sheet’s mass deficit since the 1970s. The increased freshwater discharged from the ice sheet has in turn been associated with regional climatic impacts, and may be capable of modifying ocean circulation on a global scale. There remains however significant limitations in our understanding of this interaction, hindering our ability to study and simulate these systems and their evolution in the face of ongoing climate change.

One of the biggest challenges is created by the long, deep and narrow fjords that link the Greenland Ice Sheet to the wider ocean (Straneo & Cenedese, 2015). By modulating the exchange of heat and freshwater between the ocean and ice sheet, fjord processes play a key yet poorly understood role in governing the interaction between these systems. If the impact of increasing ocean temperatures on Greenland’s glaciers is to be understood, then we need to understand how and why water temperature in glacial fjords differs from that on the adjacent continental shelf. Similarly, the impact of the ice sheet on the ocean is dependent on how freshwater entering fjords is transformed as it is transported to the shelf.

One of the key processes driving the modification and exchange of waters in Greenland’s fjords is thought to be the formation of powerful plumes adjacent to glacier termini. These form where meltwater runoff from the ice sheet, draining at the bed of outlet glaciers, enters fjords at depth. This freshwater runoff rises buoyantly, forming turbulent plumes which entrain and upwell warm subsurface waters, modifying fjord water properties and generating an overturning circulation which enhances exchange between the fjord and continental shelf. Numerical modelling studies suggest plumes may be the key driver of fjord circulation and modification (e.g. Cowton et al., 2016; Slater et al., 2022), but this remains poorly constrained. This project will seek to address this gap by using hydrographic observations in conjunction with numerical modelling methods to evaluate the role of plumes in modifying water properties in Greenland’s fjords, though the following research questions:

– How do water properties in Greenland’s fjords differ from those on the continental shelf?
– What role do plumes play in driving this modification of fjord waters?
– How and why does this plume modification very spatially (within and between fjords) and temporally (over seasonal and inter-annual timescales)?

Click on an image to expand

Image Captions

The surface expression of a meltwater plume at Kangiata Nunata Sermia, SW Greenland (Credit: Pete Nienow)

Methodology

Objective 1. Assess the extent and nature of the modification of waters in Greenland’s fjords relative to those on the continental shelf
The modification of fjord waters relative to shelf water properties may vary significantly between Greenland’s fjords due to the differing physical setting of the fjords and their connection to the ice sheet and ocean. To evaluate this, conductivity-temperature-depth (CTD) observations from NASA’s Oceans Melting Greenland (OMG) project will be used to systematically assess the difference between fjord and adjacent shelf water properties on a Greenland-wide scale. To examine the controls on fjord water modification, these differences will be evaluated against key variables such as fjord geometry (length, width, depth and the presence of sills), glacial inputs (ice and meltwater discharge, as well as glacier grounding line depths), iceberg density, terrestrial freshwater inputs, shelf hydrography, tidal amplitude and regional climatology. Data required for this assessment (e.g. bathymetry, climate, ice and meltwater flux) will be obtained through extraction and processing of information from key databases. This will provide the first ice sheet wide evaluation of the extent to which fjord water properties differ from those on the shelf, and valuable insight into the factors governing the magnitude and nature of this modification.

Objective 2. Evaluate the role of plumes as drivers of fjord modification
The second stage of the project will be to examine the role that glacial plumes play in driving the fjord water modification identified in Objective 1. To achieve this, a numerical plume model will be used to model the properties of water output by glacial plumes in each fjord as a function of meltwater runoff, grounding line depth and shelf and fjord stratification. These plume properties will then be used in conjunction with a simple mixing model to assess the extent to which plume upwelling can explain the observed differences between shelf and fjord water properties. The findings of this study will be analysed in conjunction with those from Objective 1 to provide an assessment of the importance of plumes as a driver of fjord water modification, and how this is influenced by key properties such as glacial discharge and grounding line depth.

Objective 3. Examine the spatial and temporal variability of plume impacts within a fjord system
The final stage of the project will examine how the impact of plume waters varies spatially and temporally within a single fjord system. Identifying a fjord with a good availability of observations, the methods developed with respect to Objective 2 will be used to assess how the impact of plumes varies over time (e.g. in response to changing meltwater inputs) and throughout the fjord (e.g. due to dispersal during transport to the shelf). To support this, there is the opportunity to use the numerical circulation model MITgcm on the state-of-the-art ARCHER2 high-performance computing cluster to simulate the three dimensional circulation of the fjord in response to glacial runoff and plume upwelling. This will provide valuable context for the interpretation of hydrographic observations and permit the experimental testing of theories relating to the key processes occurring in the fjord.

Project Timeline

Year 1

Literature review and research design. Begin work on Objective 1.

Year 2

Complete Objective 1 and commence Objective 2.

Year 3

Complete Objective 2 and 3.

Year 3.5

Write up thesis

Training
& Skills

The student will develop the skills necessary to undertake the data analysis and modelling outlined in the project description. This will be delivered primarily through in-house expertise in plume modelling and MITgcm, with the opportunity of attending external training courses (e.g. in Matlab/Python). No specific experience in numerical modelling is required. The project will run alongside a larger NERC funded project on a related theme, and the student will benefit from the experience of working as part of a larger team spanning several institutions. Although no fieldwork is included as part of the PhD project, there may be the opportunity to join other groups undertaking fieldwork at this time.

Further training in transferable skills, including project management, oral and written presentation and media and outreach engagement is available through the Centre for Educational Enhancement and Development (CEED) at the University of St Andrews. The student will be expected to present their work at appropriate national and international conferences throughout their PhD research.

References & further reading

Cowton, T., et al (2016). Controls on the transport of oceanic heat to Kangerdlugssuaq Glacier, east Greenland. Journal of Glaciology, 62(236), 1167-1180.

Golledge, N. R., et al (2019). Global environmental consequences of twenty-first-century ice-sheet melt. Nature, 566(7742), 65-72

Slater, D., et al (2022). Characteristic depths, fluxes and timescales for Greenland’s tidewater glacier fjords from subglacial discharge‐driven upwelling during summer. Geophysical Research Letters, e2021GL097081.

Straneo, F., & Heimbach, P. (2013). North Atlantic warming and the retreat of Greenland’s outlet glaciers. Nature, 504(7478), 36-43. https://doi.org/10.1038/nature12854

Straneo, F., & Cenedese, C. (2015). The dynamics of Greenland’s glacial fjords and their role in climate. Annual Review of Marine Science, 7, 1-24.

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