Ocean Stratification and Low Melt Rates at the Ross Ice Shelf Grounding Zone

Abstract

Ocean-driven melting of ice shelves is a primary mechanism for ice loss from Antarctica.However, due to the difficulty in accessing the sub-ice shelf ocean cavity, the relationship between ice shelf melting and ocean conditions is poorly understood, particularly near the grounding zone, where the ice transitions from grounded to floating. We present the first borehole oceanographic observations from the grounding zone of the Ross Ice Shelf, Antarctica's largest ice shelf by area. Contrary to predictions that tidal currents near grounding zones mix the water column, we found that Ross Ice Shelf waters were vertically stratified. Current velocities at middepth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle. The observed stratification resulted in low melt rates near this portion of the grounding zone, inferred from phase-sensitive radar observations. These melt rates were generally <10 cm/year, which is lower than average for the Ross Ice Shelf (~20 cm/year). Melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing. Stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting. Since diffusive convection influences vertical heat and salt fluxes differently than shear-driven turbulence, this process may affect ice shelf melting and merits further consideration in ocean models of sub-ice shelf circulation.Plain Language Summary Ice shelf melting is an important player in ice loss from the Antarctic Ice Sheet, affecting sea level rise. Ice shelf melting is controlled by ocean properties and processes, but sparse observations of the sub-ice shelf ocean cavity limit our understanding of these controls and thus limit our ability to predict sea level rise. This study presents rare ocean observations deep below the largest ice shelf by area, the Ross Ice Shelf, far from the open ocean. The observed ocean setting is surprisingly quiescent, and waters are cold, around À2°C. This study also presents new, highly localized ice shelf melting measurements at the site that show that these ocean conditions lead to slow ice shelf melting of only centimeters per year. These observations reveal the ways in which the Ross Ice Shelf contrasts with rapidly melting ice shelves affected by warmer seawater elsewhere in West Antarctica. Thus, they adds nuance to our scientific understanding of ice-ocean interactions around the Antarctic continent.Ice shelf basal melting is driven by the flux of heat from sub-ice shelf cavity water masses to the ice shelf base. On broad spatial scales, basal melt rates increase with the thermal driving, the difference between water BEGEMAN ET AL.7438

Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

“…The drag coefficient, C d , is a key parameter in the three‐equation parameterization of ice shelf basal melting and is used to relate the free stream velocity beyond the turbulent boundary to the friction velocity at the ice shelf‐ocean interface through a quadratic drag law (Jenkins et al, ). C d is poorly constrained by observations, however, and is often used to tune models to match observed melt rates (e.g., Begeman et al, ; Dutrieux et al, ). In reality, the drag coefficient is likely to be highly spatially and temporally variable (Gwyther et al, ).…”

“…Current parameterizations of basal melting, however, are poorly constrained by observations, and different parameterizations can yield model melt rates that vary by up to a factor of 5 (Dansereau et al, 2014). The canonical three-equation melt rate parameterization (Holland & Jenkins, 1999) has only been calibrated for a single location (Jenkins et al, 2010) and often requires regional tuning to match observed melt rates (Begeman et al, 2018;Dutrieux et al, 2014). Furthermore, the three-equation parameterization is known to perform poorly when predicting the melt rate in the presence of strong stratification (Kimura et al, 2015) or a buoyancy-driven plume flowing along a sloped ice shelf base (McConnochie & Kerr, 2017).…”

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

“…This site is within the grounding zone as determined by the limits of ice flexure from differential synthetic aperture radar interferometry (Marsh et al, ). The ice shelf thickness at the site was 757 ± 1 m, and the ocean cavity was 10 m thick (Begeman et al, ). We deployed a sensor string containing a pressure sensor and 30 thermistors on 16 January 2015.…”

“…We deployed a sensor string containing a pressure sensor and 30 thermistors on 16 January 2015. The pressure sensor hung 761 m from the ice surface, roughly middepth in the ocean cavity, within a water mass of almost constant potential density (Begeman et al, ). The thermistors were distributed throughout the ice shelf with a maximum spacing of 100 m and a minimum spacing of 1 m near the ice shelf bottom.…”

Biogeochemical Connectivity Between Freshwater Ecosystems beneath the West Antarctic Ice Sheet and the Sub‐Ice Marine Environment

“…The GZ site was 4.8 km downstream from the physical grounding line of the WIS (Figure 1). The ice cover at the GZ was~760 m thick, and the underlying marine water column was~10 m deep (Begeman et al, 2018;Christianson et al, 2016).…”