Boundary rider

Dr Bishakhdatta Gayen standing beside the Derwent River in Hobart.
Australian National University geophysicist and RJL Hawke Post-Doctoral Fellow, Dr Bishakhdatta Gayen, aims to define the boundary layer between the ocean and Antarctic ice sheet, to improve ocean model estimates of ice melt and sea level changes. (Photo: Wendy Pyper)
A schematic of the ocean circulation on the Antarctic continental shelf.

Dr Bishakhdatta Gayen from the Australian National University (ANU) has been awarded the third RJL Hawke Post-Doctoral Fellowship* in Antarctic Environmental Science. Dr Gayen will use numerical simulations to investigate the small-scale physical processes occurring at the boundary between the Antarctic ice sheet and the ocean. His work will enable global ocean models to better represent these processes, leading to more accurate projections of current and future ice melt and sea level changes.

“The ‘boundary layer’ is everything.”

New RJL Hawke Post-Doctoral Fellow, Dr Bishakhdatta Gayen, is referring to the millimetre-thick boundary where salt and heat from the ocean meets the base of the ice shelf around the Antarctic continent.

Salt, in direct contact with the basal ice surface, triggers ice melt by lowering the freezing point of water – just as salt on the road on a cold winter’s day melts ice.

The fresh meltwater is often unstable and turbulent, producing tiny eddies (whirlpools of water) within the boundary layer, which are thought to boost melting by transferring heat to the ice surface from the surrounding ocean (see diagram).

These complex boundary layer dynamics and heat transfer processes are the focus of Dr Gayen’s research over the next two years, with the aim of developing a simple mathematical relationship between melt rate and turbulence, which can be scaled up to represent ocean-wide processes in models.

“In ocean models the boundary layer is poorly represented and errors in this representation propagate all the way up to the measurement of ice melting and sea level change,” Dr Gayen said.

“If you really want to know how the ocean affects ice sheet melting you have to understand it at the millimetre scale of the boundary layer, because this is the layer that affects melting first.

“If you can resolve this smallest scale, then the larger ones will automatically be correct.”

To resolve this small scale Dr Gayen will attempt to define the boundary layer – the physics that create it and its physical characteristics, including thickness. To do this he will use a supercomputer to solve a series of mathematical algorithms that describe the movement of fluid, heat and salinity, building mathematical relationships between these characteristics that can then be used to predict ice melt under defined conditions.

Data generated by these ice melt simulations will be validated by experiments in the ANU’s Geophysical Fluid Dynamics laboratory. Within a tank in the laboratory, a large block of ice will be placed in contact with salty water. The temperature, salinity and movement of the water can then be modified to test its effect on the melt rate of the ice and compare these results to the supercomputer predictions.

From this Dr Gayen aims to develop a “paramaterisation” – a mathematical model representing the boundary layer.

“This parametarisation will be able to be scaled up in ocean models, to predict the rate of ice melt under different environmental conditions,” he said.

Dr Gayen will also test his paramaterisation under different conditions that further drive ice melt. These include the slope of the ice shelf, and the effect of tides, currents, upwelling water and internal ocean waves (waves that oscillate horizontally and vertically within water masses).

Finally, Dr Gayen will be able to validate his paramaterisation of the boundary layer, by assessing it against field observations obtained on the Amery Ice Shelf and elsewhere in Antarctica. On the Amery Ice Shelf, for example, scientists from the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre have run a multi-year drilling project, deploying instruments through boreholes in the ice to measure changes in ocean temperature, salinity, water movement and the melting of ice beneath the shelf (Australian Antarctic Magazine 31: 18-19, 2016).

“The Amery drilling program has measured the properties of the boundary layer next to the ice face. So we can feed those conditions and my paramaterisation into ocean models and evolve them,” Dr Gayen said.

“This work will provide a knowledge base for improvements in the representation of Antarctic processes in ocean models. This in turn will lead to more accurate projections of future Antarctic ice melt and sea level changes.”

Wendy Pyper
Australian Antarctic Division

*The RJL Hawke Postdoctoral Fellowship was named in honour of former Australian Prime Minister Bob Hawke, acknowledging his contribution to protecting the Antarctic environment. The fellowship is awarded on the basis of scientific excellence for early career doctoral graduates to pursue policy-relevant science aligned to the Australian Antarctic Science Plan.