Measurements from 'lost' Seaglider offer new insights into Antarctic ice melting

Scientists from the University of East Anglia (UEA) say the study -- the result of their autonomous Seaglider getting accidentally stuck underneath the Ross Ice Shelf -- suggests this will likely only increase further as climate change drives continued ocean warming.

The glider, named Marlin, was deployed in December 2022 into the Ross Sea from the edge of the sea ice. Carrying a range of sensors to collect data on ocean processes that are important for climate, it was programmed to travel northward into open water.

However, Marlin was caught in a southward-flowing current and pulled into the ice shelf cavity where it remained, with its sensors on, for four days before re-emerging. During this time the 'lost' glider completed 79 dives, taking measurements of the water within the cavity to a depth of 200 metres, right up to base of the overlying ice shelf.

Researchers from UEA's School of Environmental Sciences recorded a 50 metre-thick 'intrusion' of -- relatively -- warm water that had entered the cavity from the nearby open water. Water temperatures ranged from -1.9°C to a warmer -1.7°C under the ice.

Subsequent re-analysis of all available measurements shows that heat transported into cavity has increased over the last 45 years, most likely due to warming of the Ross Sea because of climate change. The findings are published in the journal Science Advances.

"While the temperature increase -- four thousandths of a degree a year -- might not seem all that much, it could lead to around 20 to 80 cm of additional ice loss per year over the 45 years we look at," explained lead author Dr Peter Sheehan.

"We found the waters of the intrusion were warm enough to melt the underside of the ice shelf, unlike the freezing-point waters they likely displaced. What's new here is that we can track the warm water pretty much from the open water of the Ross Sea at the ice front, back into the cavity. We have not seen one of these intrusions happening directly before."

Dr Sheehan added: "A trip into the cavity underneath the Ross Ice Shelf was not planned, and it's not normally possible to measure this region of an ice shelf: you can't send instruments this close to the underside of an ice shelf deliberately, it's too risky."

The ice shelves that surround Antarctica are exposed to the warmth of the ocean across the expanse of their undersides that float out over the continent's shelf seas, and the ocean-driven melting that occurs at the ice base is the largest cause of Antarctic ice-mass loss.

While the melting of floating ice does not itself substantially raise sea level, ice shelves slow the seaward flow of land ice and so stabilize the Antarctic ice sheet; their thinning and disintegration would hasten the delivery of land ice to the ocean and accelerate global sea-level rise.

One of the processes that can drive warm surface water under the Ross Ice Shelf is wind. Certain wind patterns lead to southward flow in the surface ocean and into the ice shelf cavity.

These wind-driven ocean-surface flows are called Ekman currents, and as with any ocean current, these have an associated heat transport. Because this is an ocean-surface process, this heat is instantly available to melt the overlying ice: it doesn't have to wait to be mixed upward to the ice base.

Ekman heat transport is particularly relevant for climate scientists because oceans absorb and redistribute much of the Earth's heat. Changes in this system can have profound effects on weather, sea levels, and global temperature trends.

Dr Sheehan and co-author Prof Karen Heywood used long-term measurements of wind and ocean temperature -- blended with a model to fill in spatial and temporal gaps in the record -- to calculate the strength of southward Ekman heat transport over the last 45 years. They found that the heat transported into the cavity by Ekman currents has increased.

Year-to-year variability is driven by the wind. However, the trend towards greater heat transport into the cavity is likely linked to warming of the Ross Sea -- because the water has warmed, winds today will transport more heat energy into the cavity than winds of comparable strength in the past.

Prof Heywood said: "It appears reasonable to expect that the magnitude of the Ekman heat flux, and of the melting that it drives, will increase yet further as climate change drives continued ocean warming. This trend is a concern in itself.

"The influence of surface-water intrusions, alongside the trends and variability in the Ekman dynamics that can drive these, must be incorporated into climate models, not least given continued uncertainty in the response of Antarctic land-based ice to climate change."

This is the first time that this process has been looked at using a long-term, multi-decadal data set. Previous understanding of surface-water intrusions has come mainly from comparisons of hydrography in open water, for example from ships, observations from tagged seals, and ice moorings deployed within a cavity.

The study was funded by the UK Natural Environment Research Council, the US National Science Foundation and European Research Council Horizon 2020 programme.