Unveiling the 3-dimensional fjord water circulation from iceberg tracking at a calving glacier in Greenland

Processes at the ice-ocean boundary are key in driving the rapid mass loss of the Greenland ice sheet. However, quantifying and understanding these processes remains challenging, particularly those occurring at the ice terminus beneath the fjord's water surface. Critical mechanisms include the melting of the submerged ice front by ocean water (oceanic melt), which influences the geometry of the glacier terminus and thus the glacier mass loss, and the outflow of subglacial meltwater. Subglacial meltwater can enforce a circulation cell within the fjord, drawing warm ocean water at depth towards the glacier front and enhancing oceanic melt. This circulation generates highly variable and opposing currents across different depths and over time. The harsh and highly dynamic environment makes direct observation and quantification of these circulations extremely difficult. Consequently, our understanding of key mechanisms is limited, restricting our ability to predict the future behaviour of the Greenland ice sheet. To bridge this knowledge gap, more comprehensive observations of circulation patterns near the calving front are crucial to improve our knowledge about the processes in the fjords.

This study exploits a unique time-series of terrestrial radar interferometry (TRI) acquisitions, complemented by fjord measurements, to investigate the tidewater outlet glacier Eqalorutsit Kangilliit Sermiat (EKaS) in South Greenland. A novel approach is applied to this dataset for inferring three-dimensional underwater fjord circulation with high temporal (minute-scale) and spatial (meter-scale) resolution over continuous periods lasting several weeks. An automated iceberg tracking method is employed to analyse the movement of icebergs of various sizes within the approximately 300 m deep fjord over time. TRI-derived elevation models are used to determine the above-water shapes of icebergs and estimate their submerged draft below the waterline. By linking the movements of icebergs with their draft, this study is able to extract the general water circulation patterns in the fjord at different depths, as icebergs of varying sizes are influenced by currents at distinct water layers. These findings are combined with fjord stratification data obtained from CTD profiles, providing a comprehensive understanding of the fjord's circulation dynamics.

The results of the inverted fjord circulation can later be compared with modelled fjord water circulation and combined with observations of glacier dynamics and calving derived from TRI acquisitions to obtain a comprehensive image of the “hidden” interplay between glacier and fjord circulation acting below the waterline. The findings are essential for understanding and predicting the role of oceanic forcing for the Greenland ice sheet mass loss and for assessing the implications for biodiversity within fjords in a changing climate.