FRIS Revisited in 2018: On the Circulation and Water Masses at the Filchner and Ronne Ice Shelves in the Southern Weddell Sea

Markus A Janout,Michael Schröder,Torsten Kanzow,Tore Hattermann,Svein Østerhus,Lukrecia Stulic,Jürgen Sültenfuss,Oliver Huhn,Svenja Ryan,Hartmut H Hellmer

doi:10.1029/2021jc017269

Abstract

AbstractThe Filchner‐Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near‐freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW‐sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer‐based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two‐year travel time, and the other located in the central Filchner Trough following a ∼six year‐long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW‐derived ISW (Ronne‐mode) or more locally derived Berkner‐HSSW (Berkner‐mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW‐cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne‐ISW preconditions Berkner‐HSSW production. The negligible density difference between Berkner‐ and Ronne‐mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.

Highlights

The Antarctic continent stores the largest amount of freshwater on Earth
This paper provides a 2018-status update on the hydrographic conditions along the entire length of Filchner-Ronne Ice Shelf (FRIS) in the southern Weddell Sea, based on a tightly spaced CTD and noble gas tracer survey
The conditions found off RIS were similar to those previously described, including a strong presence of High Salinity Shelf Water (HSSW) in Ronne Depression, a core of outflowing Ice Shelf Water (ISW) and an inflow of modified Warm Deep Water (mWDW) from Central Trough (Nicholls et al, 2003)

Summary

Introduction

The Antarctic continent stores the largest amount of freshwater on Earth. the Antarctic ice sheets and glaciers are losing mass, leading to a cumulative sea level rise of 14 mm since 1979 (Rignot et al, 2019). The “dense” continental shelves of the Ross and Weddell Seas are wide and dominated by near-freezing conditions (Nicholls et al, 2009) and characterized by moderate ice shelf basal melt rates (Paolo et al, 2015; Rignot et al, 2019). The Filchner-Ronne Ice Shelf (FRIS), Antarctica's largest ice shelf by volume, is located south of such a “dense” wide continental shelf in the Weddell Sea. The circulation and oceanographic conditions on this southern Weddell Sea shelf are dominated by a saline water mass named High Salinity Shelf Water (HSSW). HSSW is at surface-freezing temperature (∼−1.9°C) and forms as a result of sea ice production on the continental shelf and in coastal polynyas, in particular in the southwestern Weddell Sea off Ronne Ice Shelf (RIS, Haid & Timmermann, 2013). The resulting modified water mass is colder and fresher and more

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