Mixing at the fringes of a rapidly melting Antarctic ice shelf

The processes regulating the physical exchanges across the front of the Pine Island Ice Shelf (one of the most rapidly melting Antarctic ice shelves) in the Amundsen Sea are investigated through the analysis of an extensive set of full-depth hydrographic, velocity and microstructure measurements, obtained as part of the ISTAR expedition in February 2014, and an idealised high-resolution numerical circulation model. The outflows of Ice Cavity Water (ICW), which contains elevated concentrations of meltwater from the ice shelf, are shown to be subject to very intense small-scale turbulent mixing within 5 kilometres of the ice front. The rates of turbulent kinetic energy dissipation and diapycnal mixing in the outflows are enhanced by up to four orders of magnitude relative to those in surrounding waters, which are characterised by turbulence levels typical of the open ocean. An investigation of the causes of the intensified turbulence reveals that it is sustained by centrifugal instability of the ICW outflows. The instability drives a secondary ageostrophic circulation that induces rapid lateral mixing between ICW and pycnocline waters offshore, thereby preventing the ICW outflows from reaching the upper-ocean mixed layer and directly influencing surface climate in the region. The significance of this process for the fate of meltwater outflows from other Antarctic ice shelves will be discussed.