Standing Eddies in Glacial Fjords and Their Role in Fjord Circulation and Melt

Abstract

Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: (1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation; (2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies are an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.

Parameterizing Subglacial Discharge in Modeling Buoyancy Driven Flow in Tidewater Glacier Fjords

“…(2015) also exhibited eddying/meandering motions, as does the more recent realistic Greenlandic Fjord simulations of Zhao et al. (2023).…”

“…In addition, both Carroll et al (2017) and Mix/NOP show a return flow back toward buoyancy source. While both parameterizations evolve to similar solutions in the far field, difference at the near field could alter submarine melt rates since they are strongly influenced by flow velocities in the vicinity of the glacier (Holland & Jenkins, 1999;Jenkins, 2011;Slater et al, 2018;Zhao et al, 2023). Moreover, recent results (Sutherland et al, 2019) indicate that existing parameterizations for submarine melt considerably underestimate the melt rate and thus forthcoming parameterizations of submarine melt are likely to become even more sensitive to near-field flows and thus quite sensitive to the choice of outflow parameterization.…”

“…Numerous studies have used BPT to force fjord scale simulations (Bao & Moffat, 2023; Cowton et al., 2015; Hager et al., 2022; Oliver et al., 2020; Slater et al., 2017; Zhao et al., 2022, 2023). For example, Cowton et al.…”

“…Numerous studies have used BPT to force fjord scale simulations (Bao & Moffat, 2023;Cowton et al, 2015;Hager et al, 2022;Oliver et al, 2020;Slater et al, 2017;Zhao et al, 2022Zhao et al, , 2023. For example, Cowton et al (2015) prescribed the outflowing volume flux as vertical mass transport into a single grid cell and was implemented by prescribing vertical convergence/divergence at the top/bottom boundary of each grid cell, which is equivalently vertical velocity for a fixed grid cell surface.…”

“…Furthermore, our parameterization provides an estimate of vertical structure of the detrained flow and thus uses physical reasoning to determine the vertical distribution of the outflow in the model grid. Previous studies either force the outflow in a single grid that prescribe unrealistic high vertical convergence/divergence that set prohibitive restrictions set by the Courant‐Friedrichs‐Lewy (CFL) condition (Cowton et al., 2015) or arbitrarily spreading the outflowing plume across a number of grids to prevent such computational restrictions (Zhao et al., 2023).…”