Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near‐glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill‐driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non‐glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016–2017 in LeConte Bay, Alaska, with a three‐dimensional numerical model of the fjord to quantify sill‐driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume‐driven buoyancy forcing, sill‐generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%–72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re‐entrained into the subglacial discharge plume at the fjord head. As a result, near‐terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.
Abstract. The role of icebergs in narrow fjords hosting marine-terminating glaciers in Greenland is poorly understood, even though iceberg melt results in asubstantial freshwater flux that can exceed the subglacial discharge. Furthermore, the melting of deep-keeled icebergs modifies the verticalstratification of the fjord and, as such, can impact ice–ocean exchanges at the glacier front. We model an idealised representation of thehigh-silled Ilulissat Icefjord in West Greenland with the MITgcm ocean circulation model, using the IceBerg package to study the effect of submarineiceberg melt on fjord water properties over a runoff season, and compare our results with available observations from 2014. We find the subglacialdischarge plume to be the primary driver of the seasonality of circulation, glacier melt and iceberg melt. Furthermore, we find that melting oficebergs 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 watersand by modifying the incoming ambient waters. Through the combination of cooling and causing the subglacial-discharge-driven plume to equilibratedeeper, icebergs suppress glacier melting in the upper layer, resulting in undercutting of the glacier front. Finally, we postulate that the impactof submarine iceberg melt on the neutral buoyancy depth of the plume is a key mechanism linking the presence of an iceberg mélange with theglacier front, without needing to invoke mechanical effects.
more » « less- Award ID(s):
- 1948482
- PAR ID:
- 10485679
- Publisher / Repository:
- The Crysophere EGU
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 17
- Issue:
- 1
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 371 to 390
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. Frontal ablation has caused 32 %–66 % of Greenland Ice Sheet mass loss since 1972, and despite its importance in driving terminus change, ocean thermal forcing remains crudely incorporated into large-scale ice sheet models. In Greenland, local fjord-scale processes modify the magnitude of thermal forcing at the ice–ocean boundary but are too small scale to be resolved in current global climate models. For example, simulations used in the Ice Sheet Intercomparison Project for CMIP6 (ISMIP6) to predict future ice sheet change rely on the extrapolation of regional ocean water properties into fjords to drive terminus ablation. However, the accuracy of this approach has not previously been tested due to the scarcity of observations in Greenland fjords, as well as the inability of fjord-scale models to realistically incorporate icebergs. By employing the recently developed IceBerg package within the Massachusetts Institute of Technology general circulation model (MITgcm), we here evaluate the ability of ocean thermal forcing parameterizations to predict thermal forcing at tidewater glacier termini. This is accomplished through sensitivity experiments using a set of idealized Greenland fjords, each forced with equivalent ocean boundary conditions but with varying tidal amplitudes, subglacial discharge, iceberg coverage, and bathymetry. Our results indicate that the bathymetric obstruction of external water is the primary control on near-glacier thermal forcing, followed by iceberg submarine melting. Despite identical ocean boundary conditions, we find that the simulated fjord processes can modify grounding line thermal forcing by as much as 3 °C, the magnitude of which is largely controlled by the relative depth of bathymetric sills to the Polar Water–Atlantic Water thermocline. However, using a common adjustment for fjord bathymetry we can still predict grounding line thermal forcing within 0.2 °C in our simulations. Finally, we introduce new parameterizations that additionally account for iceberg-driven cooling that can accurately predict interior fjord thermal forcing profiles both in iceberg-laden simulations and in observations from Kangiata Sullua (Ilulissat Icefjord).
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Abstract Iceberg discharge is estimated to account for up to 50% of the freshwater flux delivered to glacial fjords. The amount, timing, and location of iceberg melting impacts fjord‐water circulation and heat budget, with implications for glacier dynamics, nutrient cycling, and fjord productivity. We use Sentinel‐2 imagery to examine seasonal variations in freshwater flux from open‐water icebergs in Sermilik Fjord, Greenland during summer and fall of 2017–2018. Using iceberg velocities derived from visual‐tracking and changes in total iceberg volume with distance down‐fjord from Helheim Glacier, we estimate maximum average two‐month full‐fjord iceberg‐derived freshwater fluxes of ~1,060 ± 615, 1,270 ± 735, 1,200 ± 700, 3,410 ± 1,975, and 1,150 ± 670 m3/s for May–June, June–July, July–August, August–September, and September–November, respectively. Fluxes decrease with distance down‐fjord, and on average, 86–91% of iceberg volume is lost before reaching the fjord mouth. This method provides a simple, invaluable tool for monitoring seasonal and interannual iceberg freshwater fluxes across a range of Greenlandic fjords.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/tc-17-371-2023
