How upstream ice shelves affect Dense Water formation: Insights from FESOM2 Experiments

The Southern Ocean is the source of the Antarctic Bottom Water (AABW), a dense water mass that occupies 60 % of the global ocean. AABW is formed in different places around Antarctica as a mixture of dense shelf waters (DSW) and Circumpolar Deep Water (CDW).

CDW occupies the deep Southern Ocean at depths between 200-1500 m, and is considerably warmer (temperature higher than 0°C) than the water masses found at similar depths over the continental shelf. CDW is carried eastward around the continent by the Antarctic Circumpolar Current (ACC) but close to the shelf break it flows westward within the Antarctic Slope Current (ASC).

The ASC is a quasi-circumpolar feature that starts in the Bellingshausen Sea and vanishes next to the Antarctic Peninsula in the Weddell Sea. The ASC is connected to the flooding of the continental shelf with CDW modified by the interaction with local surface waters (modified CDW – mCDW) and the presence of mCDW over the continental shelf can affect the sea ice formation and represents a risk for the ice shelves fringing the Antarctic continent. 

The presence of warmer mCDW and freshwater resulting from the ice shelf melt reduce the sea ice production rates, a crucial part of DSW formation. When the ocean freezes salt (brine) is rejected into the water increasing local density which creates an instability and can trigger convection and produce DSW, when less sea ice is formed the deep convection potential is reduced; the DSW formed this way is termed HSSW.

HSSW can flow offshore and slide down the continental slope mixing with ambient waters along its path until it reaches the equilibrium depth as AABW. HSSW can also flow underneath the ice shelf cavity and reach the grounding line where the freezing temperature is lower than at the surface due to the pressure effect. There, it causes melting and the mixture with glacial melt water generates the supercooled (colder than surface freezing temperature) and slightly fresher Ice Shelf Water (ISW). ISW can mix with ambient waters forming a dense water type that sinks when it reaches the continental slope producing another type of AABW.

When the DSW leaves the continental shelf they are carried westward by the ASC together with CDW and lighter waters influenced by the ice shelf melt water. Part of the waters on the continental shelf are advected in the same direction by the Antarctic Coastal Current (ACoC), a westward flow observed in various sites around Antarctica. The waters transported by the ACoC and ASC can have an impact on the neighbouring basins. 

Starting from a Finite volumE Sea Ice-Ocean Model (FESOM2) that is able to reproduce the above-mentioned key characteristics around the Antarctic continent  we prepared 3 experiments to investigate the effects of the ice shelves upstream from 3 key DSW formation sites: the Filchner-Ronne Ice Shelf, Amery Ice Shelf and Ross Ice Shelf. We will present the experiment design and preliminary results.