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Abstract:
We report on observations from instruments deployed on a pair of moorings sited ~5 km apart, beneath Ronne Ice Shelf, Antarctica. Measurements of temperature, salinity and current velocities for the period from early 2015 to mid-2019 demonstrate strong variability at timescales from tidal to interannual. Here we focus on features that are a few days in length that we interpret as vortices streaming past the site. The intensity of the vortices is enhanced towards the ice-shelf base; they are in geostrophic equilibrium, have a radius (12 km), substantially larger than the estimated internal radius of deformation (~1500 m) and have a relative vorticity that is 30 to 40% of the local planetary vorticity. The velocity of the features, determined by correlating observations from instruments on the two moorings, is the same as that of the ambient water flow. The time series of basal melt rates, measured using a collocated downward-looking radar, shows the melt rate signal to be dominated by an approximate spring-neap variability, but with a significant response to the eddying flow. Although tidal activity clearly affects basal melt rates, as illustrated by the strong ~14-day variation, the net effect of the vortices is less obvious. Here we argue that the cyclonic and anticyclonic vortices ventilate the thermocline via Ekman pumping, thus increasing melting. Such eddy features are clearly a significant component of sub-ice shelf ocean variability, at least in the study area.
