Quantifying the feedback between Antarctic meltwater release and subsurface Southern Ocean warming

Abstract

The subsurface ocean around Antarctica is one of the primary drivers of mass loss from the Antarctic ice sheet through the basal melting of ice shelves. The resultant meltwater flux into the surrounding ocean can, mainly through reduced vertical mixing, further enhance subsurface ocean warming, inducing a positive feedback that amplifies mass loss and sea-level rise. This feedback is omitted in most sea-level projections though, as few Earth system models are fully coupled to an interactive model of the Antarctic ice sheet. Here, we quantify this feedback between Antarctic meltwater release and ocean warming using linear response functions in the Earth system model EC-Earth3. Increased meltwater release from five individual Antarctic ice-sheet regions is found to unambiguously warm the subsurface Southern Ocean at centennial timescales in EC-Earth3. This warming response is quantified in terms of linear ocean response functions. Combining these with linear response functions of the Antarctic mass loss and sea-level rise because of ocean warming allows for the quantification of the meltwater-ocean-warming feedback. Here, this feedback is calculated for ocean temperature projections from 14 CMIP6 Earth system models and linear response functions from 8 ice-sheet models. Using a fixed basal melt relation with ocean temperatures, the feedback enhances 21st century projections of the Antarctic sea-level contribution by approximately 80 %. However, the inclusion of this feedback necessitates a calibration of the basal melt relation in order to reproduce historical ice-mass loss. This calibration leads to a reduction in the basal melt parameter by 35 %, after which the 21st century sea-level enhancement due to the feedback decreases to a mere 5 %. We propose that a similar reduction in the basal melt parameter must be applied in ice-sheet model forcing when transitioning from a stand-alone setup to an ice-sheet-ocean coupled setup in which the meltwater-warming feedback is explicitly simulated.