The floating ice shelves at the terminus of Antarctic glaciers act as a buttress, slowing the rate of ice loss into the ocean and thus limiting sea level rise¹. Over the course of the last two decades of Antarctic research, it’s become clear that melt and retreat of these coastal ice masses is a serious concern for the stability of the Antarctic ice sheet². Localized melt and refreezing can amplify variations in local ice shelf thickness, which has been linked to increased stresses within the ice and eventually fracturing, such as the 2016 calving of the Nansen Ice Shelf (NIS), where about 10% of the surface area of the ice shelf was lost in a single event³. Our group been working to determine the forcing mechanisms behind the transport of heat into ice shelf cavities with the long-term goal of assisting in the prediction of the state of Antarctic ice shelves under future climate scenarios.

At East Antarctic ice shelves like the NIS, the warmest water is generally surface water, which is several degrees warmer than the deep water that is usually found beneath the ice shelf. Ocean eddies have the potential to mix the warm water downward hundreds of meters to depths where it may access the ice cavity. On the other hand, eddies may also manipulate the frigid ice shelf meltwater outflow, which, when transported upwards to shallower depths, may refreeze onto the underside of the ice. In early 2019, our group used underwater robotics to capture detailed measurements of such an eddy in the bay directly in front of the NIS.

Terra Nova Bay (TNB) is a polynya, or a sea ice-free region, located in the western Ross Sea. The bay is known for its strong katabatic wind events that persistently blow down the nearby Reeves and Priestley glaciers and out over Inexpressible Island (named due to the inexpressible misery it brought members of Robert Scott’s crew that wintered there in 1912). In the wintertime, these winds help TNB rapidly produce and export sea ice, which, through brine rejection, results in the formation of some of the densest, saltiest water on the planet. In the summertime, the winds help support the existence of a pair of submesoscale ocean eddies (defined as having a diameter smaller than 50 km). These eddies have been previously identified through satellite imagery that uses sea ice as a tracer⁴, but until our research, their impact on the coastal ocean, namely heat transport near the NIS, has remained unstudied. 

The icebreaker R/V Araon of the Korea Polar Research Institute is a frequent visitor of Terra Nova Bay, due to its proximity to Jang Bogo Station. In the summer of 2018/19, researchers aboard the R/V Araon collected oceanographic profiles (salinity, temperature, depth, and current velocity) at 65 stations in TNB to examine the water masses present near the NIS. A team from the University of California – Davis also deployed an autonomous underwater glider from the icebreaker’s zodiac, which completed 208 profiles across 160 km. Glider transects were completed both parallel and perpendicular to the NIS front, including a nerve-wracking out-and-back stretch of 18 profiles that reached 6.5 km into the ice cavity. The glider was equipped with a turbulence package from Rockland Scientific, allowing us to measure tiny fluctuations in temperature over millimeter scale distances along the entire deployment. From this combined data set, we were able to not only resolve the shape of the southern TNB eddy, but also quantify the elevated turbulence that was occurring within the eddy.

Our data revealed that the eddy in front of the Nansen Ice Shelf was driving a downward mixing of warm surface water to depths greater than the minimum nearby ice draft, while at the same time upwelling cold meltwater to depths shallower than the maximum ice draft. The eddy was thus a mechanism that could support concurrent melting beneath the thinnest ice and refreezing beneath the thickest ice. This could lead to an exacerbation of the preexisting variations in ice shelf thickness, which is concerning because such variations were linked to the 2016 calving event. Ocean density and velocity measurements confirmed that the eddy was in geostrophic balance, suggesting an that the rotation is able to exist for days to weeks – long enough to allow the eddy to travel along the ice shelf front, or maybe spin off into the ice shelf cavity. We believe that similar eddies form repeatedly in TNB each summer, driven by the current that spirals around the Drygalski Ice Tongue and prevailing winds off of Inexpressible Island. Our findings were published this year in Communications Earth and Environment⁵.

The major takeaway from our research is that eddies in front of ice shelves in East Antarctica should be studied due to their ability to 1) mix warm surface water downward into the typically cold deep water, and 2) lift cold meltwater up to the ice-ocean interface. An investigation of the Antarctic coastline suggests that this eddy behavior could be a widespread phenomenon, as several other polynyas with similar forcing mechanisms dot the East Antarctica coastline⁶. However, there are presently very few in situ measurements in comparable polynyas and beneath similar ice shelves that are of the scale required to resolve these rotational features. Our work suggests that oceanographic measurements near ice shelves should be planned with the appropriate spatial and temporal resolutions; an ideal campaign would have full-depth profiles spaced less than 1 km apart and cover an area with a radius greater than 10 km, with all data collected within 1-2 days. We realize that this is a difficult request.

Autonomous gliders are, in our opinion, the most appropriate tool for such an investigation. Stormy was able to autonomously complete each full down-and-up profile in TNB to 1000 m depth in approximately 3 hours with a speed over ground of close to 1 km per hour, and was able to repeatedly do so for several days in a row on a single charge. With more optimized battery performance, deployment lengths can stretch into weeks so that the sample area can be repeatedly surveyed. This combination of high spatial resolution over large distances allows a detailed resolution of submesoscale features with which a typical arrangement of ship-based profiles struggles. Meanwhile, the buoyancy-driven glider propulsion is a quiet and efficient ride for onboard sensors, making them an ideal platform for microstructure turbulence measurements. One downside of the autonomous nature is that sub-ice deployments lead to nervous hours (or days); our Slocum glider navigates via dead reckoning, which was combined with little detailed knowledge of the ice shelf bottom morphology and a personal lack of experience in implementing ice-avoidance algorithms. Despite this, our team was able to execute one of the few sub-ice shelf glider deployments and gathered the first glider-based microstructure turbulence measurements beneath an ice shelf.

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2. Pritchard, H., Ligtenburg, S. R., Fricker, H. A., Vaughan, D. G., van den Broeke, M. R., & Padman, L. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature 484, 502-505 (2012).
3. Dow, C. F., et al. Basal channels drive active surface hydrology and transverse ice shelf fracture. Sci. Adv. 4, eaao7212 (2018).
4. Moctezuma-Flores, M., Parmiggiani, F., Fragiacomo, C., & Guierrieri, L. Synthetic aperture radar analysis of floating ice at Terra Nova Bay-an application to ice eddy parameter extraction. J. Applied Remote Sensing 11, 026041 (2017).
5. Friedrichs, D. M., et al. Observations of submesoscale eddy-driven heat transport an an ice shelf calving front. Comm. Earth & Env. 3, 140 (2022).
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