Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Edited by Eric A. D’Asaro, University of Washington, Seattle, WA, and approved December 30, 2020 (received for review April 20, 2020)
Significance
Ocean-driven melting of ice shelves is a leading cause of mass loss from Antarctica. However, the small-scale ocean processes responsible for melting are poorly understood due to the difficulty of making direct measurements in these hard to reach environments. Here, we use a high-resolution ocean model to fill this knowledge gap. At low current speeds and relatively warm ocean temperatures, we show that a small-scale ocean process called double-diffusive convection controls ice shelf melt rates and turbulent mixing, forming a unique “staircase” structure beneath the ice. This process is currently missing from ocean–climate models, which consider only turbulent melting due to ocean currents.
Abstract
The Antarctic Ice Sheet loses about half its mass through ocean-driven melting of its fringing ice shelves. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica’s contribution to global sea level. We use high-resolution large-eddy simulation to examine ocean-driven melt, in a geophysical-scale model of the turbulent ice shelf–ocean boundary layer, focusing on the ocean conditions observed beneath the Ross Ice Shelf. We quantify the role of double-diffusive convection in determining ice shelf melt rates and oceanic mixed layer properties in relatively warm and low-velocity cavity environments. We demonstrate that double-diffusive convection is the first-order process controlling the melt rate and mixed layer evolution at these flow conditions, even more important than vertical shear due to a mean flow, and is responsible for the step-like temperature and salinity structure, or thermohaline staircase, observed beneath the ice. A robust feature of the multiday simulations is a growing saline diffusive sublayer that drives a time-dependent melt rate. This melt rate is lower than current ice–ocean parameterizations, which consider only shear-controlled turbulent melting, would predict. Our main finding is that double-diffusive convection is an important process beneath ice shelves, yet is currently neglected in ocean–climate models.
- ice
- ocean interactions
- double-diffusive convection
- basal melting of Antarctic ice shelves
- large-eddy simulation
- thermohaline staircases
Footnotes
- ↵1To whom correspondence may be addressed. Email: madi.rosevear{at}uwa.edu.au.
Author contributions: M.G.R., B.G., and B.K.G.-F. designed research; M.G.R. performed research; M.G.R. analyzed data; and M.G.R. and B.G. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2007541118/-/DCSupplemental.
Data Availability.
Model output data have been deposited in Github (10.5281/zenodo.4269193, ref. 54).
Published under the PNAS license.
References
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The role of double-diffusive convection in basal melting of Antarctic ice shelves
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- Physical Sciences
- Earth, Atmospheric, and Planetary Sciences
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