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Polar researchers discover impact of underwater tsunamis

When Antarctic glaciers break away, an event known as calving, they cause huge internal waves that contribute hugely to ocean mixing. Photo: Mike Meredith @BAS
When Antarctic glaciers break away, an event known as calving, they cause huge internal waves that contribute hugely to ocean mixing. Photo: Mike Meredith @BAS

SAMS scientists are co-authors on a new publication in the journal Science Advances, which examines a phenomenon known as an 'internal' underwater tsunami and how this has previously been overlooked in our understanding of ocean mixing.

Their findings were the result of a research cruise on board the RRS James Clark Ross research vessel in Antarctica, when a science team led by British Antarctic Survey (BAS) watched the front of a glacier disintegrate and their measurements ‘went off the scale’. As well as witnessing disruptions on the ocean surface, they recorded internal underwater tsunamis as tall as a house

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. The team on board the 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 temperature much more even across different depths.

Lead author of the study Professor Michael Meredith, who’s head of the Polar Oceans team at BAS, was there. He says: “This was remarkable to see, and we were lucky to be in the right place at the right time. Lots of glaciers end in the sea, and their ends regularly split off into icebergs. This can cause big waves at the surface but we know now it also creates waves inside the ocean. When they break, 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.

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.

SAMS oceanographer and co-author on the paper, Prof Mark Inall, said: "The greatest surprise in all this was that even around Antarctica, where storms can be ferocious, icebergs calving from ice sheets can stir up the ocean much more than the most severe winter gale."

The research cruise on RRS James Clark Ross was part of the ICEBERGS project and was funded by the Natural Environment Research Council.