Ilulissat Icefjord Upper-Layer Circulation Patterns Revealed Through GPS-Tracked Icebergs

The Greenland Ice Sheet has undergone rapid mass loss over the last four decades, primarily through solid and liquid discharge at marine-terminating outlet glaciers. The acceleration of these glaciers is in part due to the increase in temperature of ocean water in contact with the glacier terminus. However, quantifying heat transport to the glacier through fjord circulation can be challenging due to iceberg abundance, which threatens instrument survival and fjord accessibility. Here we utilize iceberg movement to infer upper-layer fjord circulation, as freely floating icebergs (i.e., outside the melange region) behave as natural drifters. In the summers of 2014 and 2019, we deployed transmitting GPS units on a total of 13 icebergs in Ilulissat Icefjord, an iceberg-rich and historically data-poor fjord in west Greenland, to quantify circulation over the upper 0-250 m of the water column. We find that the direction of upper-layer fjord circulation is strongly impacted by the timing of tributary meltwater runoff, while the speed of this circulation changes in concert with glacier behavior, which includes increases and decreases in glacier speed and meltwater runoff. During periods of increased meltwater runoff entering from tributary fjords, icebergs at these confluences deviated from their down-fjord trajectory, even reversing up-fjord, until the runoff pulse subsided days later. This study demonstrates the utility of iceberg monitoring to constrain upper-layer fjord circulation, and highlights the importance of including tributary fjords in predictive models of heat transport and fjord circulation. The Greenland Ice Sheet has been rapidly losing mass over the last four decades, primarily at its edges through glacier melting and iceberg calving into fjords. Warming ocean water in contact with the glacier terminus can accelerate mass loss. However, quantifying the currents that transport this warm ocean water are challenging to constrain due to the abundance of icebergs in the near-terminus region. Here, we track freely floating icebergs, natural drifters, to infer surface circulation (0-250 m depth) in an iceberg-rich fjord. In the summers of 2014 and 2019, we deployed GPS units on 13 icebergs in Ilulissat Icefjord, a historically data-poor fjord in west Greenland. We find the direction of currents to be strongly impacted by tributary fjord runoff, with changes in iceberg trajectory coinciding with runoff pulses from these tributary fjords. We find the circulation speed to be most closely associated with glacier speed and meltwater runoff from the glacier at the head of Ilulissat Icefjord. This study highlights the utility of using icebergs to infer surface circulation and the importance of including tributary fjords in future circulation models. We used 13 on-iceberg GPS units to constrain upper-layer (0-250 m) circulation in Ilulissat Icefjord, west GreenlandDeviations in down-fjord iceberg trajectory coincide with tributary meltwater flux, in both location and timingThe speed of upper-layer circulation changes in concert with glacier behavior, including glacier speed and meltwater runoff