Stormy seas ahead for sea ice longevity

Stormy seas whipped up at the edge of the Antarctic sea ice zone can send large ice-breaking waves hundreds of kilometres into the pack ice.

The discovery, published in the journal Nature in May, suggests that large ocean waves play a bigger role in sea ice breakup and retreat than previously thought.

Hydrodynamics scientist and lead author of the paper, Dr Alison Kohout, of the National Institute of Water and Atmospheric Research in New Zealand, said increasing storminess in the Southern Ocean could accelerate sea ice retreat in the future, with implications for sea ice processes and marine creatures.

‘Waves propagating through the Marginal Ice Zone (MIZ) leave behind a wake of broken ice floes, which are then more easily broken up and deformed by winds and currents,’ she said.

‘This mixing and churning eliminates the sea ice barrier between the air and the ocean and increases heat exchange between the atmosphere and the ocean.’

The MIZ is a region of broken ice floes, hundreds of kilometres wide, that forms at the boundary of the open ocean and the pack ice. Predicting the scale of ice breakup requires an understanding of how far waves move into it.

To find out, Dr Kohout braved sea sickness during the Australian-led Sea Ice Physics and Ecosystem Experiment-II (SIPEX-II) in September 2012, to deploy five wave sensors on sea ice floes — from the edge of the MIZ to some 200km inside it.

Along this deployment transect the average floe diameter increased from 2–3m at the ice edge to 10–20m, while the ice thickness was estimated at between 0.5 and 1m thick.

The wave sensors transmitted information about their vertical acceleration to a satellite for up to 40 days. During this time they measured wave heights up to seven metres at the ice edge and three metres some 240km inside the ice edge.

Dr Kohout found that in calm conditions, wave heights dropped exponentially* as they moved deeper into the MIZ. However during large ‘wave events’ — over three metres high — the wave height did not decay exponentially, allowing the wave to persist deep into the pack ice.

To test her theory that an increase in large wave events would cause the sea ice edge to retreat, Dr Kohout compared model estimates of wave heights with satellite sea ice observations between 1997 and 2009.

‘We found that the retreat and expansion of the sea ice edge correlated with significant wave height increases and decreases respectively,’ she said.

‘A two metre increase in significant wave height over a decade led to a two degree latitudinal retreat in ice extent.’

Dr Kohout found that the largest increases in wave height occurred in the Amundsen-Bellingshausen Sea, where regional sea ice retreat is well documented. Similarly, the largest wave height decreases were in the Western Ross Sea, where sea ice has expanded.

‘Our results suggest that sea ice is vulnerable to changes in storminess. The southward shift in storm tracks over recent decades has resulted in fewer cyclones at mid latitudes and more cyclones at higher latitudes,’ Dr Kohout said.

‘In the future, wave heights are predicted to increase everywhere at the sea-ice edge in the Arctic and Antarctica. So it is conceivable that this will accelerate sea-ice retreat.’

Wendy Pyper
Corporate Communications, Australian Antarctic Division

*Exponential growth can be pictured as a cell dividing into two and then those two cells also dividing into two and so on, so that the number of cells doubles each time. Exponential decay is the same, but in the negative direction. (Any number, other than two, can be used as the base number). A similar concept applies to the reduction in wave energy as it passes through sea ice.