Impact of icebergs on the seasonal submarine melt of Sermeq Kujalleq

Abstract

Abstract. The role of icebergs in narrow fjords hosting marine-terminating glaciers in Greenland is poorly understood, even though iceberg melt results in a substantial freshwater flux that can exceed the subglacial discharge. Furthermore, the melting of deep-keeled icebergs modifies the vertical stratification of the fjord and, as such, can impact ice–ocean exchanges at the glacier front. We model an idealised representation of the high-silled Ilulissat Icefjord in West Greenland with the MITgcm ocean circulation model, using the IceBerg package to study the effect of submarine iceberg melt on fjord water properties over a runoff season, and compare our results with available observations from 2014. We find the subglacial discharge plume to be the primary driver of the seasonality of circulation, glacier melt and iceberg melt. Furthermore, we find that melting of icebergs modifies the fjord in three main ways: first, icebergs cool and freshen the water column over their vertical extent; second, iceberg-melt-induced changes to fjord stratification cause the neutral buoyancy depth of the plume and the export of glacially modified waters to be deeper; third, icebergs modify the deep basin, below their vertical extent, by driving mixing of the glacially modified waters with the deep-basin waters and by modifying the incoming ambient waters. Through the combination of cooling and causing the subglacial-discharge-driven plume to equilibrate deeper, icebergs suppress glacier melting in the upper layer, resulting in undercutting of the glacier front. Finally, we postulate that the impact of submarine iceberg melt on the neutral buoyancy depth of the plume is a key mechanism linking the presence of an iceberg mélange with the glacier front, without needing to invoke mechanical effects.

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

“…The coolest water within the surface water mass remained closer to the glacier terminus (within the mélange to mid‐fjord region vs. extending the full length of the fjord in 2019; Figures 6c–6f), and the warmer and more saline basin water mass was ∼100 m thicker with an intermediary water‐basin water transition at about 400 m depth (vs. 500 m depth in 2019). While the interannual differences in temperature, salinity, and stratification are sizable, these results are consistent with previous work focused on the impact of mélange on fjord hydrography as iceberg presence is shown to alter upper layer hydrography toward cooler and fresher conditions (Davison et al., 2020; Hager et al., 2022; Kajanto et al., 2022).…”

“…Additionally, this study demonstrates the utility and caveats associated with using natural drifters (icebergs) to infer upper layer circulation. Recent advances in both regional‐scale and global climate models have highlighted the influence of icebergs on freshwater injection and ocean modification (Davison et al., 2020; Hager et al., 2022; Kajanto et al., 2022; Zhao et al., 2022), however we should continue to pursue the inclusion of complex geometry, non‐linear movement, and the influence of meltwater runoff on fjord circulation in systems thinking and modeling.…”

“…As this freshwater plume rises, the entrained warm ocean water is brought directly in contact with the glacier terminus, contributing to additional terminus melt (convective and ambient melt; Cenedese & Gatto, 2016; Slater et al., 2018). The plume continues to buoyantly rise until reaching equilibrium (e.g., Jenkins, 2011), and then flows down‐fjord (influenced by bathymetry, wind, iceberg presence, and shelf flow; Davison et al., 2020; Hager et al., 2022; Kajanto et al., 2022; Zhao et al., 2022) until entering the ocean (Figure 1b). While the majority of freshwater in Ilulissat Icefjord originates as subglacial discharge, meltwater also results from the abundance of icebergs in the fjord, and to a much lesser degree, the convective and ambient melting of the terminus.…”

“…The bathymetry and geometry of Ilulissat Icefjord play a dominant role in its unique fjord hydrography. The shallow sill constrains fjord‐shelf water exchange above the sill (Gladish et al., 2015; Sutherland, Straneo, & Pickart, 2014), trapping water on the leeward side (Beaird et al., 2017; Stigebrandt, 2012), which in turn obstructs density‐driven circulation and strongly influences the flow of water (e.g., Beaird et al., 2017; Davison et al., 2020; Hager et al., 2022; Kajanto et al., 2022). The shallow sill also acts as a physical barrier to deep‐keeled icebergs, forcing them to break apart at the fjord mouth (Beaird et al., 2017) prior to entering Disko Bay and the open ocean.…”

Climate Variability and Glacier Dynamics Linked to Fjord Productivity Changes Over the Last ca. 3300 Years in Nuup Kangerlua, Southwest Greenland

“…Iceberg discharge can have a positive impact on primary production by local upwelling (Duprat et al., 2016; Vernet et al., 2012; Wu & Hou, 2017), yet these findings are mainly from open ocean environments while not many studies exist from fjords (Kanna et al., 2018). Additionally, in fjords with marine‐terminating glaciers, iceberg melt is a significant source of freshwater and icebergs have been found to be important in modifying the fjord water column by contributing to cooling and freshening of the uppermost 50 m and increasing stratification (Kajanto et al., 2023). It is thus plausible that the KNS unstable front margin and high iceberg discharge would have modified the hydrographic conditions and that (due to a strong stratification and insufficient wind strength) only minimal nutrient delivery would have reached the surface waters thus limiting primary production.…”