Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf

The instability and accelerated melting of the Antarctic Ice Sheet are among the foremost elements of contemporary global climate change1, 2. The increased freshwater output from Antarctica is important in determining sea level rise1, the fate of Antarctic sea ice and its effect on the Earth’s albedo4, 5, ongoing changes in global deep-ocean ventilation6, and the evolution of Southern Ocean ecosystems and carbon cycling7, 8. A key uncertainty in assessing and predicting the impacts of Antarctic Ice Sheet melting concerns the vertical distribution of the exported meltwater. This is usually represented by climate-scale models3–5, 9 as a near-surface freshwater input to the ocean, yet measurements around Antarctica reveal the meltwater to be concentrated at deeper levels10, 11, 12, 13, 14. Here we use observations of the turbulent properties of the meltwater outflows from beneath a rapidly melting Antarctic ice shelf to identify the mechanism responsible for the depth of the meltwater. We show that the initial ascent of the meltwater outflow from the ice shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. We use an idealized ocean circulation model to show that this mechanism is relevant to a broad spectrum of Antarctic ice shelves. Our findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models

Remotely induced warming of Antarctic Bottom Water in the eastern Weddell gyre

Four repeat hydrographic sections across the eastern Weddell gyre at 30°E reveal a warming (by ~0.1°C) and lightening (by ~0.02–0.03 kg m−3) of the Antarctic Bottom Water (AABW) entering the gyre from the Indian sector of the Southern Ocean between the mid-1990s and late 2000s. Historical hydrographic and altimetric measurements in the region suggest that the most likely explanation for the change is increased entrainment of warmer mid-depth Circumpolar Deep Water by cascading shelf water plumes close to Cape Darnley, where the Indian-sourced AABW entering the Weddell gyre from the east is ventilated. This change in entrainment is associated with a concurrent southward shift of the Antarctic Circumpolar Current's (ACC) southern boundary in the region. This mechanism of AABW warming may affect wherever the ACC flows close to Antarctica