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Mass loss from the Antarctic Ice Sheet to the ocean has increased in recent decades, largely because the thinning of its floating ice shelves has allowed the outflow of grounded ice to accelerate 1,2. Enhanced basal melting of the ice shelves is thought to be the ultimate driver of change 2,3 , motivating a recent focus on the processes that control ocean heat transport onto and across the seabed of the Antarctic continental shelf towards the ice 4-6. However, the shoreward heat flux typically far exceeds that required to match observed melt rates 2,7,8 , suggesting other critical controls. Here we show that the depth-independent (barotropic) component of the flow towards an ice shelf is blocked by the dramatic step shape of the ice front, and that only the depth-varying (baroclinic) component, typically much smaller, can enter the sub-ice cavity. Our results arise from direct observations of the Getz Ice Shelf system and laboratory experiments on a rotating platform. A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf 9. Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates. Main text: The fate of the Antarctic Ice Sheet is the greatest remaining uncertainty when predicting future sea level 10. Estimates of its contribution to global sea-level rise range from none to a catastrophic > 5 cm/year 10-12 (4 m by the year 2100). The ice sheet drains into the ocean where it terminates in floating ice shelves, overlying vast sub-ice cavities. These buttress the flow of the ice sheet, regulating the speed at which it flows into the ocean 13. Rapid thinning of ice shelves in coastal regions with warm ocean water on the continental shelf is accelerating the outflow from the ice sheet 1,2. The perceived reason-although rarely observed directly 14is that ocean currents deliver more warm water to the ice shelf cavities, causing increased basal melt. These currents originate in a reservoir of warm and salty water, known as Circumpolar Deep Water (CDW) 15 , residing at 300-1000 m depth in the Southern Ocean. Substantial amounts of dense CDW are carried onto the continental shelf by various mechanisms 4-7,16 , but only a fraction of this is needed to explain observed basal melt rates 17. The CDW flows southward in deep troughs that crosscut the continental shelf 4,18-21. The currents are steered by the bathymetry and move with shallower water to the left of the flow direction 22-24 so southward transport occurs along the eastern, and northward on western,

An object-based method for automatic iceberg detection has been applied to Advanced Synthetic Aperture Radar images in the Amundsen Sea Embayment (ASE), Antarctica. The images were acquired between 1 January 2006 and 8 April 2012 under varying meteorological, oceanographic and sea-ice conditions. During this time period, the icebergs were counted (average 1370 ± 50) and their surface area was estimated (average 1537.5 km 2). The average surface area was about 2.5 times larger than the annual calved area (620 km 2), indicating that the average iceberg age in the ASE is about 2.5 years, which was confirmed by observed residence times based on drift tracks. Most of the ASE icebergs were less than 1500 m long, and almost 90% of them were smaller than 2 km 2. The proportion of small-and medium-sized icebergs (84.4%) was significantly higher than in the open ocean, where large icebergs (>10 km 2) account for nearly the whole iceberg surface area. The opposite was true for the freshly calved icebergs in the ASE. The data indicate that the creation of icebergs in the ASE is dominated by steady small-to medium-scale calving from ice shelves fringing the embayment. In addition, rare calving events of giant icebergs occur on a decadal timescale. There is also some import of icebergs from the Bellingshausen Sea further east along the coast, in particular after large calving events there.