New knowledge about the meltwater outflow from beneath the Antarctic ice shelf
An international team of scientists have recently published the paper Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf in the journal Nature; lead author Prof. Alberto C. Naveira Gatabato at University of Southampton. Senior scientist Dr Satoshi Kimura, currently affiliated with the Nansen Center, is among the authors. The research team have addressed the instability and accelerated melting of an Antarctic Ice Shelf. They have conducted detailed measurements of the hydrography, velocity and turbulence properties near the calving front of Pine Island Ice Shelf, West Antarctica, one of the fastest melting ice shelves in Antarctica. The observations show that centrifugal instability plays a key role in mixing the glacial meltwater and the ambient ocean waters.
Melting of ice shelves (floating extensions of ice sheets) will reduce the basal drag on the ice sheets resting upstream, thereby increasing the ice sheets’ contribution to the freshwater budget of the ocean. The increased freshwater output from Antarctica to ocean is important in 1) determining sea-level rise, 2) changing the sea-ice growth and its effect on the Earth’s albedo, 3) ongoing changes in global deep-ocean ventilation and 4) the evolution of Southern Ocean ecosystem and carbon sequestration. Yet, the mechanism that determines the vertical distribution of the exported glacial meltwater is poorly understood. Climate models have often treated the glacial meltwater as a near-surface freshwater input to the ocean, even though the observations around Antarctica had revealed the meltwater below the surface. Gatabato et al. have identified the mechanism responsible for the depth of the meltwater by using the turbulence measurements of the meltwater outflow from beneath Pine Island Ice Shelf. They have shown that the initial ascent of the meltwater outflow from the ice-shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. These observations were supported by idealized ocean model experiments, which show that the centrifugal instability is relevant to a broad spectrum of Antarctic ice shelves. Their findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models.
Dr Kimura et al has also published the analysis of their measurements below the Antactic ice shelf in a resent article in Journal of Geophysical Research - Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica. Using CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV) they have obtained measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf. They found the highest turbulent kinetic energy dissipation rate near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ~0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice were used to estimate basal melting of the ice shelf. The dissipation-rate-based melt rate estimates was found to be sensitive to the stability correction parameter in the linear approximation of universal function of the Monin-Obukhov similarity theory for stratified boundary layers. In the paper they argued that their estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting of ice shelves.
Alberto C. Naveira Garabato, Alexander Forryan, Pierre Dutrieux, Liam Brannigan, Louise C. Biddle, Karen J. Heywood, Adrian Jenkins, Yvonne L. Firing and Satoshi Kimura (2017): Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf. Nature 542, 219–222 (09 February 2017) doi:10.1038/nature20825
Kimura, S., A. Jenkins, P. Dutrieux, A. Forryan, A. C. Naveira Garabato, and Y. Firing (2016), Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica, J. Geophys. Res. Oceans, 121, 8496–8510, doi:10.1002/ 2016JC012149.