Icebergs as Depth Sounders: Improving Bathymetry Maps in Uncharted Regions
Abstract
Heat transported at depth in the ocean strongly influences mass loss from the Greenland ice sheet by controlling the rate of melting at the terminus of marine terminating outlet glaciers, which in turn influences terminus stability and ice flow. The ability of warm subsurface waters to reach glacier termini is strongly influenced by coastal bathymetry, with deep troughs channeling warm waters into fjords and sills effectively blocking the warmest waters from reaching glacier termini. Despite its importance, measurements of fjord bathymetry exist for a relatively small proportion of Greenland's glacial fjords, limiting our ability to fully understand the importance of subsurface ocean warming on the asynchronous changes in dynamic mass loss between neighboring glaciers.
Here we present on two companion, remote sensing-based methods to estimate bathymetry using icebergs as depth sounders. Both methods capitalize on the ideas that stranded icebergs indicate the presence of bathymetric highs and that the draft (i.e. keel depth) of stranded icebergs can be used to place bounds on water depth. First, we identify regions of iceberg stranding and drifting by manual inspection of imagery from multiple sensors. Then, to estimate draft, we extract iceberg freeboard from digital elevation models (DEMs) of stranded icebergs constructed using very high-resolution WorldView stereo satellite images. Based on the assumption of hydrostatic equilibrium, we infer median and maximum iceberg drafts, placing bounds on the water depth at the location of the stranded iceberg. To extend the spatial coverage of inferred bathymetry to include regions where stranded icebergs are identified in Landsat and Sentinel imagery but where there are no stereo image pairs to construct DEMs, we compute fjord-specific iceberg depth-width ratios using DEMs of all available icebergs and apply these to the stranded icebergs. We demonstrate the successes and limitations of our approach by comparing iceberg-inferred water depths with sonar-derived bathymetric maps in two west Greenland fjords. Then, we demonstrate the utility of the remote sensing iceberg-derived bathymetry method in a previously unmapped fjord suspected to have a sill.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.C21B1316S
- Keywords:
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- 0720 Glaciers;
- CRYOSPHEREDE: 0720 Glaciers;
- CRYOSPHEREDE: 0728 Ice shelves;
- CRYOSPHEREDE: 0728 Ice shelves;
- CRYOSPHEREDE: 0732 Icebergs;
- CRYOSPHEREDE: 0732 Icebergs;
- CRYOSPHEREDE: 0758 Remote sensing;
- CRYOSPHEREDE: 0758 Remote sensing;
- CRYOSPHEREDE: 0774 Dynamics;
- CRYOSPHEREDE: 0774 Dynamics;
- CRYOSPHEREDE: 0776 Glaciology;
- CRYOSPHEREDE: 0776 Glaciology;
- CRYOSPHEREDE: 0794 Instruments and techniques;
- CRYOSPHEREDE: 0794 Instruments and techniques;
- CRYOSPHERE