From flooding to ‘greening’ – how ocean waves contribute to the seasonal melting of Antarctic sea ice
A possible 'missing link' in what drives the rapid melt back of Southern Ocean sea ice each summer has been identified in a new study by an international team, led by Australian Antarctic Program* scientists.
Study leader Dr Rob Massom said ocean waves contribute to the seasonal melting of Antarctic sea ice in ways that have been overlooked, until now.
“Ocean waves promote melting at the base and sides of sea-ice floes, by breaking them up and exposing more of their edges to ocean waters that are warmed by sunlight each summer. But this is not the full story,” he said.
“Our new study shows how waves can also cause surface melting of sea-ice floes, by washing over and flooding them, removing their snow cover, and grinding them into slush.
“This ice floe–wave interaction also creates conditions for the rapid growth of algae in both the seawater ponds on the floe surface, and within the ice floe.
“This then enhances melting in a beautiful interplay of physical and biological processes.”
Detecting change in our planet’s icy pulse
Each year, the annual cycle of sea-ice growth and retreat around Antarctica fluctuates from 18–19 million square kilometres in winter, to 2–3 million square kilometres in summer – one of the largest seasonal changes on Earth.
This 'heart beat' of the planet’s climate system moderates global temperatures and drives ocean circulation, and is vital to the survival of biodiverse Southern Ocean marine ecosystems.
“Antarctic sea ice, and the snow that accumulates on its surface, help keep our planet cool by reflecting sunlight back into space,” Dr Massom said.
“Much of this reflectance is due to the snow cover, which forms one of the brightest natural surfaces on Earth.
“The snow also acts like an insulating blanket, shielding the ice from rising air temperatures in summer.
“Snow-covered ice floes reflect more than 85 per cent of the sunlight hitting them, compared to about 60 per cent for bare, thick sea ice. The darker ocean absorbs about 93 per cent of the sunlight, causing the open water around the floes to seasonally warm.”
Dr Massom said that wave-driven surface melting could help account for the large differences in the timing and magnitude of summer sea-ice retreat observed by satellites, compared to those simulated by climate models.
“Climate models largely underestimate the average rate of sea-ice retreat observed by satellites each summer,” he said.
“This suggests incomplete knowledge and representation of important interactions and processes involving the ice, ocean, atmosphere and biota.
“This study provides a possible solution towards addressing a major gap in our ability to accurately model Antarctic sea ice and its seasonal cycle, as we strive to understand the causes of recent dramatic sea-ice losses around Antarctica and reduce uncertainty in model projections of future sea-ice conditions and climate.”
A perfect storm of ‘wave melting’
Using observations, modeling and theoretical insights, the study shows that waves in the stormiest ocean on Earth can wash snow off sea-ice floes, and cause seawater to pool on their surface.
This substantially reduces the ability of the ice to reflect sunlight, causing it to absorb more heat from the sun and melt from the surface down (in addition to the bottom and sideways melting) as summer progresses.
Wave action also acts like a blender, grinding the ice floes together and creating a ‘wave slush’ that exposes more of the ice to sunlight and the warming ocean.
“The outer sea-ice zone is more than just a collection of snow-covered floes broken up by waves, as is generally accepted to be the case,” Dr Massom said.
“The presence of wave slush creates perfect conditions for sea-ice algae to grow, turning the ice green, further reducing its reflectiveness, and amplifying melting.”
The research team found that these three linked processes – called ‘wave flooding’, ‘wave pulverisation’ and ‘wave greening’ – could enhance the speed of summer melting by between 5.2 cm and 6.1 cm per day in summer.
“Our calculations suggest that wave melting alone could melt a one metre-thick slab of sea ice in just 20 days, and in about 16 days if amplified by greening ,” Dr Massom said.
“These enhanced melt rates are likely to be underestimates, due to potential acceleration and amplification of surface melting by a suite of previously unconsidered positive feedback mechanisms, driven by the same wave processes.”
Zoning in on sea-ice melt
These wave melting processes and feedbacks occur in the ‘marginal ice zone’ – the outer part of the sea-ice zone affected by incoming waves from the adjacent, stormy open ocean.
Wave melting may also be important within the interior sea-ice zone, due to the periodic penetration of ocean swells, and wind-driven waves in areas of open water within the sea ice – although more observations are needed to confirm this.
Predicted increases in storminess and waviness over the Southern Ocean are likely to intensify wave melting, greening and associated feedbacks over coming decades, to potentially disrupt the annual sea-ice cycle and cause further sea-ice loss.
Dr Massom said the research is also relevant to the changing Arctic, where declining sea-ice coverage is opening larger areas of the central Arctic Ocean to wind-generated waves.
“This underlines a need for modelling research, and observations of these previously neglected wave processes and feedbacks, to fully understand and quantify their overall contribution to seasonal sea-ice melting around Antarctica and in the Arctic.
“Sophisticated technologies such as autonomous camera systems on icebreakers are crucial to assisting with the new observations.
“Ultimately, we should encourage and enable the inclusion of wave-melting processes and feedbacks in next-generation climate and Earth-system models.
“This would help improve understanding of recent sea-ice change, and more accurate prediction of the likely fate of polar sea-ice systems and the wider Earth system as the planet continues to warm.”
Further research is needed to determine how wave processes affect the production of sea-ice algae, and their role in removing carbon dioxide from the atmosphere, supporting krill and polar-marine ecosystems, and driving biogeochemical processes that influence cloud formation and climate.
The study is published today in The Cryosphere, and also in The Conversation.
*Australian Antarctic Program researchers involved in this study are affiliated with the Australian Antarctic Division, Australian Antarctic Program Partnership, and the Australian Centre for Excellence in Antarctic Science.
This article is republished from the Australian Antarctic Program. View the original article here.
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