Necessary Conditions for Warm Inflow Toward the Filchner Ice Shelf, Weddell Sea

Abstract

Understanding changes in Antarctic ice shelf basal melting is a major challenge for predicting future sea level. Currently, warm Circumpolar Deep Water surrounding Antarctica has limited access to the Weddell Sea continental shelf; consequently, melt rates at Filchner-Ronne Ice Shelf are low. However, large-scale model projections suggest that changes to the Antarctic Slope Front and the coastal circulation may enhance warm inflows within this century. We use a regional high-resolution ice shelf cavity and ocean circulation model to explore forcing changes that may trigger this regime shift. Our results suggest two necessary conditions for supporting a sustained warm inflow into the Filchner Ice Shelf cavity: (i) an extreme relaxation of the Antarctic Slope Front density gradient and (ii) substantial freshening of the dense shelf water. We also find that the on-shelf transport over the western Weddell Sea shelf is sensitive to the Filchner Trough overflow characteristics. Plain Language Summary The Weddell Sea continental shelf is presently filled with water masses that are too cold to melt the Filchner-Ronne Ice Shelf from below. If warmer offshore water masses gain access to the Weddell Sea continental shelf and flow into the ice shelf cavity, the ocean-driven melting would increase rapidly and cause ice shelf thinning. Increased ice shelf melting would supply the continental shelf with freshwater that influence sea ice production and generation of dense water masses and cause an increased discharge of grounded ice, which would contribute to global sea level rise. Two main factors currently prevent warm water from accessing the Weddell Sea continental shelf. First, the warm water is located deeper than the continental shelf and does not have direct access. Second, the Weddell Sea continental shelf is filled with dense water masses that block an inflow of warmer and lighter water. We use a regional ocean model to investigate what would happen if the warm offshore water is lifted higher up or if the dense water masses become fresher and lighter. We find that the Weddell Sea system is robust, and we need to make extreme changes to both factors to allow warm water access to the continental shelf.

Observed interannual changes beneath Filchner-Ronne Ice Shelf linked to large-scale atmospheric circulation

“…HSSW formation is also influenced by glacial meltwater inflow 46 . A sustained Berkner mode would eventually decrease the ISW density on the eastern part of the continental shelf, causing a potential feedback on the Berkner HSSW production and weakening the density barrier 11 that prevents modified WDW from entering the ice shelf cavity 7 , 18 . Although varying amounts of remotely sourced Ronne ISW and more locally produced Berkner HSSW have been observed in front of Filchner Ice Shelf (as indicated by blue and red letters in Fig.…”

“…At present, these outflows are dense enough to block the inflow of Warm Deep Water that circulates along the Weddell Sea continental shelf break 17 . For a warmer and wetter future climate, model simulations suggest that HSSW production is reduced far enough that the ISW density barrier will be eroded and warmer waters will be able to flush the FRIS cavity 11 , 18 . However, the uncertainties in these projections are large 19 and the likelihood of such a regime shift due to ongoing climate change remains largely unknown.…”

Two-timescale response of a large Antarctic ice shelf to climate change

“…Using an ice-ocean model forced with a climate change projection, Warm Deep Water (WDW) from offshore was simulated to enter the cavity through the Filchner Trough, dramatically increasing basal melt rates. A similar circulation change has been triggered in other ocean models by manually varying the cavity salinity or atmospheric forcing 11,12 . However, it remains unclear how easily this circulation change could occur in reality, and over what timescales.…”

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

“…ISW is characterized by below-surface freezing temperatures (<−1.9°C) and leaves the FRIS cavity via the Filchner Trough (Nicholls et al, 2009). These cold and dense waters prevent a large-scale inflow of Warm Deep Water (WDW), the Weddell Sea's derivative of CDW, which occupies the intermediate layer (200-1,200 m) of the Weddell Sea (Daae et al, 2020). This is not the case along the narrow continental shelf off Dronning Maud Land, where WDW mixes with the colder and fresher Eastern Shelf Water to result in modified (m)WDW, which seasonally penetrates the southern Weddell Sea through troughs intersecting the continental shelf (Ryan et al, 2017).…”