As well as witnessing disruptions on the ocean surface, they recorded ‘internal’ underwater tsunamis as tall as a house, a phenomenon previously missed in the understanding of ocean mixing. The team which included Bangor University researchers, and led by the British Antarctic Survey, report their in the journal Science Advances.
Internal tsunamis are an important factor in ocean mixing, which affects life in the ocean, temperatures at different depths, and how much ice the ocean can melt.
Ice in Antarctica flows to the coast along glacier-filled valleys. While some ice melts into the ocean, a lot breaks off into icebergs, which range in size from small chunks up to the size of a country. A team on board the British Antarctic Survey (BAS) research ship RRS James Clark Ross were taking ocean measurements close to the William Glacier, situated on the Antarctic Peninsula, as the front of it dramatically disintegrated into thousands of small pieces.
The William Glacier typically has one or two large calving events per year, and the team estimated this one broke off around 78,000 square metres of ice – around the area of 10 football pitches – with the front of the glacier towering 40m above sea level. Before it broke away, the water temperature was cooler at around 50-100m in depth and warmer below this. After the calving, this changed dramatically, with the temperature becoming more even across different depths.
Katrien Van Landeghem and postgraduate student, Kate Retallick from Bangor University’s School of Ocean Sciences joined the surveys in the West Antarctic Peninsula to map the seafloor of the fjords directly in front of the retreating glaciers.
The vessel was often so close to the glaciers, that they could also map the shape of the ice front where it sits on the bed. This unique data reveals the role of geological processes in glacier retreat, and helps uncover the effects a disintegrating ice front has on the entire body of water and the ecosystems that thrive there.
Katrien said:
“After witnessing the sudden calving event up close, the team spotted that the large icebergs breaking off the glaciers caused an “internal” underwater tsunami and mixed the water column. With glaciers predicted to disintegrate even more dramatically in our warming oceans, the importance of understanding the impact such calving events has on the rest of the polar ecosystems should be considered on a larger scale.”
Important chance observation
Lead author of the study Professor Michael Meredith, who’s head of the Polar Oceans team at BAS, added:
“When glaciers calve, these internal waves cause the sea to mix and this affects life in the sea, how warm it is at different depths and how much ice it can melt.
“This is important for us to understand better. Ocean mixing influences where nutrients are in the water and that matters for ecosystems and biodiversity. We thought we knew what caused this mixing – in summer, we thought it was mainly wind and tides, but it never occurred to us that iceberg calving could cause internal tsunamis that would mix things up so substantially.”
As opposed to the waves caused by wind and tides, tsunamis are caused by geophysical events where water is suddenly shifted, for example by an earthquake or landslide. Internal tsunamis have been noticed in a handful of places, caused by landslides. Until now, no one had noticed that they are happening around Antarctica, probably all the time because of the thousands of calving glaciers there. Other places with glaciers are likely affected also, including Greenland and elsewhere in the Arctic.
This chance observation and understanding is important, as glaciers are set to retreat and calve more as global warming continues. This could likely increase the number of internal tsunamis created and the mixing they cause.
Underwater tsunami
This process is not factored into current computer models enabling us to predict what might happen around Antarctica. This discovery changes our understanding of how the ocean around Antarctica is mixed and will improve knowledge about what it means for climate, the ecosystem and sea level rise.
Michael Meredith concludes: “Our fortuitous timing shows how much more we need to learn about these remote environments and how they matter for our planet.”
The research cruise on RRS James Clark Ross was part of the ICEBERGS project and was funded by the Natural Environment Research Council.
Internal tsunamigenesis and ocean mixing driven by glacier calving in Antarctica by Michael P. Meredith, Mark E. Inall, J. Alexander Brearley, Tobias Ehmen, Katy Sheen, David Munday, Alison Cook, Katherine Retallick, Katrien Van Landeghem, Laura Gerrish, Amber Annett, Filipa Carvalho, Rhiannon Jones, Alberto C. Naveira Garabato, Christopher Y. S. Bull, Benjamin J. Wallis, Anna E. Hogg, James Scourse is published in the journal Science Advances