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Alternative title: Topografiske barrierar og smelting av isbremmar i Antarktis: Korleis kontrollerer topografien innstrøyming av varmt vatn inn under isen?

Awarded: NOK 8.2 mill.

The floating ice shelves fringing Antarctica are melting at an accelerating rate, mainly due to relatively warm (above freezing) water carried to the ice base by ocean currents. The "warm" water is found in the deep ocean, north of the relatively shallow continental shelves surrounding the continent. To reach the cavities beneath the ice shelves, the "warm" water has to pass two topographic barriers: the continental shelf break and the ice front. Both features represent a significant step in water depth, which oceanic currents (which are prone to flow along depth contours) cannot easily traverse. TOBACO study processes by which "warm" water moves onto the continental shelf and into the ice shelf cavity. The focus is on flow into and along a trough cross-cutting the continental shelf, from the shelf break to the ice front. Laboratory experiments using idealized topography representing (i) a trough cross-cutting the shelf break and (ii) an ice front were conducted at the rotating Coriolis platform. The interaction between barotropic and baroclinic currents and the topography was investigated. Shelf break: While the shallow baroclinic coastal current entered the continental shelf break along the coast, the barotropic currents were steered onto the continental shelf along the trough. The results were included in L. Vignes's (LOCEAN) PhD-thesis (June 2021). Moorings were deployed in the Filchner Trough (FT) region in the Weddell Sea in 2017 to study the interaction of the Antarctic Slope Current (ASC) with the FT. The moorings were recovered (and redeployed) in Feb 2021. In November 2021, we arranged a workshop inviting the partners with data from the same region and period. Analysis of these and existing data from the area have shown: a prolonged warm inflow in 2017 (GRL, 2020), that the warm inflow is enhanced by the cold outflow down the slope (in review, Nat. Comms), that temperatures of the warmest water above the slope increased in 2020 (in prep) and the records have revealed new insight to the structure and variability of the Antarctic Slope Front in the region (in prep). During the 2021-cruise, we contributed to the deployment of 10 seal tags (Pangaea, 2022). A regional ocean model (ROMS) representing the southern Weddell Sea is used to explore mechanisms controlling the on-shelf transport of warm oceanic water. The results suggest that you need a combination of thermocline heaving and freshening of the dense shelf water for significant amounts of warm water to reach the Filchner-Ronne cavity (GRL, 2020). Analysis of mooring data from the Amundsen Sea has revealed a coupling between warm inflow and wind stress over the Amundsen polynya (OS, 2022). Ice front: In the lab, the barotropic current was to a large portion deflected at the ice front while the baroclinic current entered the ice shelf cavity unhindered. Mooring data from the Getz ice front in the Amundsen Sea, Antarctica, show similar results: The mooring closest to the ice front shows a flow of reduced velocity compared to moorings further away, suggesting that the barotropic component of the along-trough flow has been deflected (Nature, 2020). A real-scale, idealized model setup with MITgcm is set up to study the processes governing the flow at an ice front. The results suggest that about 30% of the volume flux in an along-trough forced barotropic current enters the cavity, but the value depends on, e.g., the ice shelf draft and the stratification. The externally forced, barotropic flow causes increased basal melt, especially near the ice front (JPO, 2022). The mooring data further reveal that polynya activity at the western front of the Getz ice shelf during periods of strong easterly winds in winter reduces the warm inflow by up to 25% during the winter. The deepening of the cold surface layer is initiated north of Siple Island and travels towards the ice front as a coastally trapped wave causing a significant reduction in the thickness of the warm layer entering the cavity (GRL, 2021). In the Filchner Sea, there is a seasonal outflow of Ice shelf water across the ice front (GRL, 2018). Ph.D. Thesis: N. Steiger successfully defended her thesis "The ice front as a topographic barrier for ocean heat transport" based on results from the project in June 2021. Outreach: We reported "live" from the experiments in Grenoble on our blog (, posting 71 posts. In addition, we ran an "Ask Me Anything" event across several social media platforms. We have written an article on about the Nature paper and a "junior" version of the same paper (Frontiers of young minds, 2021). Results from the project were included in "Nansen's minnesførelesning" to the Norwegian Academy of Science in 2019. The fieldwork onboard Polarstern in 2021 was documented in the photo novel "Ninja goes south," published on Instagram, Twitter, and (now available at

- The project has contributed to the education (PhD-degree) of a young, female scientist who through the international network build through the project now has continued her scientific career on a Post Doc together with a project partner. The PhD of a second young, female scientist (on-going) builds on results and data collected within the project. - Increased international cooperation regarding the collection of and the use of observational data in the southern Weddell Sea. - Increased visibility of Norwegian Antarctic Research internationally through (at least) XXX peer-reviewed publications (one in Nature) and numerous presentations at international meeting and workshops (albeit somewhat hampered by COVID-19) - A RCN-funded outreach project is partly based on material produced within the project.

TOBACO aims to increase our understanding of the oceanic heat flux towards the Antarctic ice shelf cavities; specifically, the flow of warm water past i) the shelf break and ii) the ice shelf front, two features that represents a major change in water depth and thus a barrier to ocean currents. The focus is on flow into and through a trough crosscutting the continental shelf from the shelf break to the ice front and beyond. Results from TOBACO will reduce the uncertainties in future predictions of ice shelf melting and sea level rise. The physics of the flow will first be investigated in large-scale laboratory experiments using advanced flow visualization techniques and highly idealized topographies (access to laboratory facilities is assured through an EU Hydralab+ grant), and in idealized, process-oriented numerical simulations. The knowledge obtained from the idealized activities will be brought on to the real world; as the results will be combined with and compared to field observations and regional modelling. TOBACO will synthesize the data legacy from ongoing NFR-funded projects with concurrent records from international collaborators - France, Germany and Sweden - in two climatically important locations where the processes studied in the laboratory are central: the opening of the Filchner Trough in the Weddell Sea and the Getz ice shelf front in the Amundsen Sea. TOBACO will guarantee that data records complementing international monitoring efforts are continued. TOBACO will strengthen Norway as a leading polar nation and integrate the Norwegian research in larger international collaborations; it will assure that results from the EU-financed experiments are processed, analyzed and published and it will advertise Norwegian Antarctic Research to the public through extended outreach activities. TOBACO will promote the career of a young, female scientist and allow a recruited PhD-student to actively be part of a large, international network.

Publications from Cristin

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