New research reveals how the speed of ocean currents and the shape of the seabed influence the amount of heat flowing underneath Antarctic ice shelves, contributing to melting.
Scientists at the University of East Anglia (UEA) used an autonomous underwater vehicle to survey beneath the Dotson Ice Shelf in the Amundsen Sea, an area of rapid glacial ice loss largely due to increasing ocean heat around and below ice shelves.
The circulation of warm water and the heat transport within ice shelf cavities - significant areas beneath ice shelves - remains mostly unknown. To address this the team collected data from over 100 kilometres of dive tracks the underwater robot made along the seabed in the Dotson cavity.
The findings are published today in the journal Ocean Sciences.
Lead author Dr Maren Richter, from UEA's Centre for Ocean and Atmospheric Sciences, said: "Upward transport of deep warm water to the shallower ice-ocean boundary in ice shelf cavities is what drives melting at the underside of the ice shelf. This melting makes the ice shelf thinner, and therefore less strong.
"We found that while there is mixing of warm water with other, cooler, water, under the Dotson Ice Shelf most of the warm water is not mixed upward. Instead, it flows horizontally to the grounding line, the point where the glacier loses contact with the seabed and starts to float.
"This means that the water stays warm all the way to the grounding line, where it can melt the glacier directly. This can cause the glacier to retreat, speed up and lose more ice into the ocean. Together, the retreat, increased speed, and increased melt contribute to sea level rise globally."
During the mission, the first of its kind under the Dotson Ice Shelf, the researchers found warm, salty water below colder, fresher water. It is already known that warm water is transported upward by mixing, however this study shows that the mixing and upward transport of warm water are strongest in the inflow areas to the east of the ice shelf, where the currents are faster and the seabed is steep, with the gradient of the bedrock being particularly significant.
Current speeds recorded in this area by the Autosub Long Range (ALR) autonomous underwater vehicle - named Boaty McBoatface and operated by the National Oceanography Centre - were around five centimetres per second up to 10 centimetres per second. The gradient was about 45 degrees in the steepest areas.
Dr Richter added: "We were expecting the influence of current speed on the mixing to be much higher than what we found. Instead, the shape of the seabed seems to be really important.
"We also found water in the deepest part of the cavity that was surprisingly warm, and we are now working to explain how and when it got there."
The data was collected over four missions in 2022 when Boaty, equipped with sensors to measure properties of the water including temperature, current, turbulence (mixing) and oxygen, travelled along the bottom of the ice shelf cavity, staying about 100 metres above the seabed. Boaty was in the cavity for approximately 74 hours.
Missions to send a robot into an ice shelf cavity and then get it back at the end are very difficult, and ones with an instrument that can measure mixing are especially rare.
"This mission was the first of its kind under the Dotson Ice Shelf," said Dr Richter. "We gained very valuable baseline measurements which can now be compared to assumptions about mixing in regional and global models of ice shelf-ocean interactions, and to measurements under other ice shelf cavities, helping us understand how these cavities are similar or different from each other."
Warm deep water that is mixed upward not only increases the temperature in the upper ocean, it can also transport nutrients and trace-metals upward, which is very important for local algae blooms and the creatures that depend on them for food.
While this study did not measure nutrient transports through mixing, the data can be used by other researchers who want to calculate the effects of mixing in the cavity.
The work was carried out as part of a project for the International Thwaites Glacier Collaboration , a major five-year research programme aiming to understand what is causing ice loss and better predict how this could contribute to sea level rise. It was funded by the UK's Natural Environment Research Council and the US National Science Foundation.
'Observations of turbulent mixing in the Dotson Ice Shelf cavity' , Maren Richter, Karen Heywood, Rob Hall and Peter Davis, is published in Ocean Sciences on December 10.
