On the role of ice-ocean interaction on tidewater glacier and ice shelf evolutions.
Abstract
Ice-ocean interactions play a fundamental role in the evolution of tidewater glaciers in Greenland by undercutting the glacier front, which directly affects grounding line positions. An increase in ocean thermal forcing and/or subglacial discharge may push the glacier past a regime of natural variability into a newer, unstable position, which forces a retreat. In Greenland, we document that about half of the dominant 226 glaciers are directly controlled by this undercutting process, while others are calving on stabilizing sills and forming small floating sections, stand in shallow, cold water or are protected by an ice shelf. In comparison, surface thinning of the glaciers by surface melt and accelerated flow plays a minor role in the retreat. In response to this forcing, glaciers speed up, calving rates increase, which is conducive to further retreat. The observations match this interpretation quite well. Over ice shelves, the situation is different as the locus of high ice shelf melt is away now from the grounding line (hence almost not influencing basal resistance), ice shelf draft slopes are shallow, and the impact of ice shelf melt on buttressing is small. In fact, the fastest rate of grounding line retreat in the world, about 3 km/yr in West Antarctica vs 0.5 km/yr for Jakobshavn Isbrae, are observed on glaciers with almost unchanged ice shelf area over the past 40 years. Why? Because ice shelf melt near grounding lines is far more complex than anticipated. Grounding lines migrate by km, ten times more than expected from the interaction of ice on a hard bed, enabling seawater to infiltrate over considerable distances at tidal frequencies. On retreating grounding lines, we observe more drastic migration as seawater infiltrates in newly formed cavities. In this context, ice melt is not zero at the mean position of the grounding line, hence affecting basal friction directly, with considerable impact on ice dynamics. To make matters more complex, ice is not in hydrostatic equilibrium in the grounding zone. Until we understand and model these ice-ocean interactions in more detail, we caution that ice sheet/ocean models may not be able to reproduce changes in ice dynamics realistically and with the correct sensitivity level.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.C14A..08R
- Keywords:
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- 0720 Glaciers;
- CRYOSPHERE;
- 0726 Ice sheets;
- CRYOSPHERE;
- 0728 Ice shelves;
- CRYOSPHERE;
- 4207 Arctic and Antarctic oceanography;
- OCEANOGRAPHY: GENERAL