Spatial distribution of Antarctic meltwater governs Southern Ocean deep convection and shelf warming feedback

Abstract

Increasing Antarctic ice sheet mass loss is anticipated to become a major player in Southern Ocean and global climate change. Since most climate models are lacking an interactive ice sheet, numerous freshwater-release scenarios have been conducted recently, in which the effect of melting of ice shelves and calving of icebergs on the ocean and climate system is studied by prescribing a freshwater flux to the high latitude Southern Ocean. The Southern Ocean Freshwater Input from Antarctica (SOFIA, https://sofiamip.github.io) initiative is designed to reconcile these studies and quantify model uncertainty. In the framework of SOFIA we conduct experiments with the Flexible Ocean and Climate Infrastructure (FOCI) model, which consists of NEMO3.6-LIM 0.5˚ ocean-sea ice and ECHAM6.3-JSBACH 1.8˚ atmosphere-land components. We study the effect of freshwater input (0.1 Sv) along the Antarctic coast (antwater) versus a wide-spread, iceberg melt-like input field south of 60˚S (60Swater) under pre-industrial climate control conditions. A small ensemble of eight members each also serves to demonstrate a significant effect by centennial-scale internal variability on the magnitude of the Southern Ocean’s response to the freshwater. Besides responses like surface cooling, sea ice expansion, deep ocean warming, weakening of the Antarctic Circumpolar Current, which are robust across models and experiments, we find two intriguing differences between antwater and 60Swater experiments: In three of the eight ensemble members of antwater, large-scale open ocean deep convection emerges in the central Weddell Gyre, which is absent from the reference run without freshwater perturbation and the eight ensemble runs of 60Swater. This can be linked to the spin-up of the Weddell Gyre in the experiments, increasing the doming of isotherms, but being counterbalanced by surface freshening in the gyre center in 60Swater. Further, the zonal mean warming of >1˚C at mid depth in the Weddell Sea sector present in all experiments spills onto the continental shelf in antwater whereas it resides below the shelf break in 60Swater. This gives rise to the assumption that the spatial distribution of the freshwater has the potential to drive or limit a positive melt feedback loop associated with warming on the shelf.