Abstract
Melting at the base of floating ice shelves is a dominant term in the overall Antarctic mass budget. This study applies a high-resolution regional ice shelf/ocean model, constrained by observations, to (i) quantify present basal mass loss at the Fimbul Ice Shelf (FIS); and (ii) investigate the oceanic mechanisms that govern the heat supply to ice shelves in the Eastern Weddell Sea. The simulations confirm the low melt rates suggested by observations and show that melting is primarily determined by the depth of the coastal thermocline, regulating deep ocean heat fluxes towards the ice. Furthermore, the uneven distribution of ice shelf area at different depths modulates the melting response to oceanic forcing, causing the existence of two distinct states of melting at the FIS. In the simulated present-day state, only small amounts of Modified Warm Deep Water enter the continental shelf, and ocean temperatures beneath the ice are close to the surface freezing point. The basal mass loss in this so-called state of “shallow melting” is mainly controlled by the seasonal inflow of solar-heated surface water affecting large areas of shallow ice in the upper part of the cavity. This is in contrast to a state of “deep melting”, in which the thermocline rises above the shelf break depth, establishing a continuous inflow of Warm Deep Water towards the deep ice. The transition between the two states is found to be determined by a complex response of the Antarctic Slope Front overturning circulation to varying climate forcings. A proper representation of these frontal dynamics in climate models will therefore be crucial when assessing the evolution of ice shelf basal melting along this sector of Antarctica.
Highlights
Understanding the interaction of ice shelves with the ocean is a major challenge when assessing the role of Antarctica in a future, almost certainly warmer, climate system (Mercer, 1978; Joughin et al, 2012)
Floating ice shelves are believed to buttress the flow of the grounded ice sheet (Rignot et al, 2004; Dupont and Alley, 2005), and recent examples of sudden ice shelf break-up events along the Antarctic Peninsula (Scambos et al, 2000), as well as the rapid mass loss in western Antarctica (Rignot et al, 2008), have raised concerns about the ice/ocean system being highly sensitive to climate change
The magnitude and the general horizontal pattern of the simulated melt rates in the annual cycle (ANN)-100 experiment compare well with the results presented by Humbert (2010), who constrained basal melting from inverse ice flow modeling assuming a steadystate equilibrium ice shelf geometry
Summary
Understanding the interaction of ice shelves with the ocean is a major challenge when assessing the role of Antarctica in a future, almost certainly warmer, climate system (Mercer, 1978; Joughin et al, 2012). In West Antarctica, these warm waters are observed directly inside the ice shelf cavities (Jenkins et al, 2010), and there is growing evidence that the observed increased glacial mass loss may have been triggered by increased access of warm water onto the continental shelf (Pritchard et al, 2012; Jacobs et al, 2011). In East Antarctica, such a deep ocean heat transport is believed to be much weaker at present (Nicholls et al, 2009), the continental-scale warming simulations of Hellmer et al (2012) and Kusahara and Hasumi (2013) suggest that future circulation changes may increase basal melting on decadal time scales in this region. Because the circulation and water mass exchange inside the ice shelf cavity directly relates to ice shelf draft and bedrock topography, we briefly introduce the geometrical configuration of the FIS. The ice draft and grounding line position of the RTopo-1 dataset were refined based on ice-penetrating radar data (Humbert, 2010), as well as by using new ground-based and satellite observations acquired during the Fimbul Ice Shelf (FIS)
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