Heat, Salt, and Freshwater Budgets for a Glacial Fjord in Greenland

Abstract

In Greenland's glacial fjords, heat and freshwater are exchanged between glaciers and the ocean. Submarine melting of glaciers has been implicated as a potential trigger for recent glacier acceleration, and observations of ocean heat transport are increasingly being used to infer the submarine melt rates. The complete heat, salt, and mass budgets that underlie such methods, however, have been largely neglected. Here, a new framework for exploring glacial fjord budgets is developed. Building on estuarine studies of salt budgets, the heat, salt, and mass transports through the fjord are decomposed, and new equations for calculating freshwater fluxes from submarine meltwater and runoff are presented. This method is applied to moored records from Sermilik Fjord, near the terminus of Helheim Glacier, to evaluate the dominant balances in the fjord budgets and to estimate freshwater fluxes. Throughout the year, two different regimes are found. In the nonsummer months, advective transports are balanced by changes in heat/salt storage within their ability to measure; freshwater fluxes cannot be inferred as a residual. In the summer, a mean exchange flow emerges, consisting of inflowing Atlantic water and outflowing glacially modified water. This exchange transports heat toward the glacier and is primarily balanced by changes in storage and latent heat for melting ice. The total freshwater flux increases over the summer, reaching 1200 6 700 m 3 s 21 of runoff and 1500 6 500 m 3 s 21 of submarine meltwater from glaciers and icebergs in August. The methods and results highlight important components of fjord budgets, particularly the storage and barotropic terms, that have been not been appropriately considered in previous estimates of submarine melting.

Greenland Iceberg Melt Variability from High-Resolution Satellite Observations

“…We generally find the highest melt rates near Koge Bugt and Helheim glaciers in the southeast (>0. The observed large-scale spatial patterns in melt rate generally follow expected variations based on regional differences in subsurface ocean temperatures (e.g., Straneo et al (2012)) and surface meltwater runoff (e.g., van den Broeke et al (2016)), 15 which drives summertime fjord circulation (Jackson et al, 2016). There are, however, some notable exceptions.…”

“…CC BY 4.0 License. depth) than in the horizontal plane (i.e., along fjord) Bendtsen et al, 2015;Gladish et al, 2015;Jackson et al, 2016). As such, we expect to find pronounced variations in melt rates for icebergs that do and do not penetrate into the relatively warm and salty water masses found below ~100-200 m-depth around the ice sheet periphery (Straneo et al, 2012;Moon et al, 2017) but no discernible variations in melt rates with distance from the parent glacier.…”

“…The location where iceberg meltwater enters the ocean system is proving important for local to global ocean circulation (Luo et al, 2016), yet the spatial distribution of iceberg meltwater fluxes has been largely overlooked because it cannot be estimated from existing 5 hydrographic observations (Jackson et al, 2016). Where iceberg residence times can be estimated from, for example, iceberg tracking (Sulak et al, 2017), these data can be paired with remotely-sensed iceberg size and area distributions (Enderlin et al, 2016;Sulak et al, 2017) and empirical iceberg melt rates to estimate iceberg freshwater fluxes.…”

Glacier Calving in Greenland

“…Estimates of submarine melt rate have been derived primarily from fjord flux gate studies, involving the analysis of heat and salt budgets across hydrographic sections that may lie several kilometres down-fjord from the calving front [38][39][40][41]. These studies indicate that average calving front melt rates lie in the range of~1-10 m/ day, with the spread of values reflecting regional differences in ocean heat content and subglacial discharge, in addition to the uncertainties introduced by temporal variability in fjord circulation, incomplete sampling across fjord sections, and the input of freshwater from iceberg melt [42,43]. Furthermore, whole-fjord estimates made over short periods of time may mask considerable spatial and temporal variability, making the evaluation of the likely effects on calving processes difficult.…”

Influence of meltwater on Greenland Ice Sheet dynamics

“…The heat to drive submarine melting is supplied by waters from the subpolar North Atlantic and Arctic seas, whose circulation inside the fjords is a result of processes across a range of spatiotemporal scales [Straneo et al, 2010;Jackson et al, 2014;Jackson and Straneo, 2016]. Melt rates are affected by ocean temperature, stratification, and circulation in near-ice waters (<200 m from the terminus) [Jenkins et al, 2010].…”