Export of Ice Sheet Meltwater from Upernavik Fjord, West Greenland

Abstract

Meltwater from Greenland is an important freshwater source for the North Atlantic Ocean, released into the ocean at the head of fjords in the form of runoff, submarine melt and icebergs. The meltwater release gives rise to complex in-fjord transformations that result in its dilution through mixing with other water masses. The transformed waters, which contain the meltwater, are exported from the fjords as a new water mass “Glacially Modified Water” (GMW). Here we use summer hydrographic data collected from 2013 to 2019 in Upernavik, a major glacial fjord in northwest Greenland, to describe the water masses that flow into the fjord from the shelf and the exported GMWs. Using an Optimum Multi-Parameter technique across multiple years we then show that GMW is composed of 57.8 ±8.1% Atlantic Water, 41.0 ±8.3% Polar Water, 1.0 ±0.1% subglacial discharge and 0.2 ±0.2% submarine meltwater. We show that the GMW fractional composition cannot be described by buoyant plume theory alone since it includes lateral mixing within the upper layers of the fjord not accounted for by buoyant plume dynamics. Consistent with its composition, we find that changes in GMW properties reflect changes in the AW and PW source waters. Using the obtained dilution ratios, this study suggests that the exchange across the fjord mouth during summer is on the order of 50 mSv (compared to a freshwater input of 0.5 mSv). This study provides a first order parameterization for the exchange at the mouth of glacial fjords for large-scale ocean models.

Characteristic Depths, Fluxes, and Timescales for Greenland's Tidewater Glacier Fjords From Subglacial Discharge‐Driven Upwelling During Summer

“…Our discharge estimate of 140-960 m 3 s −1 and plume neutral buoyancy depth of 86-213 m are in a similar range, but our plume upwelling flux of 18-47 mSv is 2-5 times smaller than the observed export flux. Muilwijk et al (2022) have similarly estimated that in Upernavik Fjord, plume upwelling comprises only around 60% of the exported water mass. These studies highlight the importance of fjord-scale mixing processes beyond the plume, including lateral recirculation, winds, tides, flow-topography interactions and iceberg-driven mixing (Beaird et al, 2018;Carroll et al, 2017;Mortensen et al, 2014;Slater et al, 2018;Straneo & Cenedese, 2015).…”

“…Muilwijk et al. (2022) have similarly estimated that in Upernavik Fjord, plume upwelling comprises only around 60% of the exported water mass. These studies highlight the importance of fjord‐scale mixing processes beyond the plume, including lateral recirculation, winds, tides, flow‐topography interactions and iceberg‐driven mixing (Beaird et al., 2018; Carroll et al., 2017; Mortensen et al., 2014; Slater et al., 2018; Straneo & Cenedese, 2015).…”

“…Our results suggest that over half is exported below 100 m depth (Figure 3c), a fact that should be represented in ocean models examining the impact of Greenland freshwater on the ocean. Indeed, the upwelling fluxes quantified here could be combined with estimates of mixing in fjords beyond the plume (Beaird et al., 2018; Muilwijk et al., 2022) to provide boundary conditions for large‐scale ocean models that do not resolve fjords. Finally, plumes may impact fjord primary productivity if upwelling reaches into the photic zone, which has a typical depth of at most 50 m (Mascarenhas & Zielinski, 2019; Meire et al., 2017).…”

Impact of icebergs on the seasonal submarine melt of Sermeq Kujalleq

“…Moreover, the height reached by the plume along the glacier terminus has the potential to increase undercutting and thus impact the shape of the terminus, which again can cause further changes in the calving of the glacier front (D. A. Slater et al, 2017a;Slater et al, 2021). For marineterminating glaciers, rigid iceberg mélange is interpreted to provide mechanical support, or buttressing, that suppresses calving (Joughin et al, 2008;Amundson et al, 2010;Burton et al, 2018;Joughin et al, 2020), since observations indicate a correlation between rigid iceberg mélange in front of a glacier and suppressed calving (Joughin et al, 2020).…”

An Improved and Observationally‐Constrained Melt Rate Parameterization for Vertical Ice Fronts of Marine Terminating Glaciers

“…Our parameterization can readily be used in ocean circulation models that do not explicitly resolve heat transfer across the ice‐ocean interface, but resolve fjord dynamics. Implementation in ocean models that do not resolve fjord dynamics will require development of a suitable parameterization of the circulation within the fjord, followed by the transformation of subglacial discharge and melt water, and subsequent export from the fjord (Muilwijk et al., 2022). The improved quantification of submarine melting in numerical models based on our parameterization will enable the scientific community to address pressing questions related to a changing climate around Greenlands coastal margins.…”