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,

Abstract. Rapid retreat of Greenland's marine-terminating glaciers coincides with regional warming trends, which have broadly been used to explain these rapid changes. However, outlet glaciers within similar climate regimes experience widely contrasting retreat patterns, suggesting that the local fjord geometry could be an important additional factor. To assess the relative role of climate and fjord geometry, we use the retreat history of Jakobshavn Isbræ, West Greenland, since the Little Ice Age (LIA) maximum in 1850 as a baseline for the parameterization of a depth- and width-integrated ice flow model. The impact of fjord geometry is isolated by using a linearly increasing climate forcing since the LIA and testing a range of simplified geometries. We find that the total length of retreat is determined by external factors – such as hydrofracturing, submarine melt and buttressing by sea ice – whereas the retreat pattern is governed by the fjord geometry. Narrow and shallow areas provide pinning points and cause delayed but rapid retreat without additional climate warming, after decades of grounding line stability. We suggest that these geometric pinning points may be used to locate potential sites for moraine formation and to predict the long-term response of the glacier. As a consequence, to assess the impact of climate on the retreat history of a glacier, each system has to be analyzed with knowledge of its historic retreat and the local fjord geometry.

<p>Shoreward oceanic heat flux in deep channels on the continental shelf typically far exceeds that required to match observed ice shelf melt rates, suggesting other critical controls.  IN the present study we study the depth-independent (barotropic) and the density-driven (baroclinic) components of the flow of warm ocean water towards an ice shelf. Using observations from the Getz Ice Shelf system as well as geophysical laboratory experiments on a rotating platform, it is shown that the dramatic step shape of the ice front blocks the barotropic component, and that only the baroclinic component, typically much smaller, can enter the sub-ice cavity.  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. Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates.</p>

Abstract. Rapid acceleration and retreat of Greenland's marine-terminating glaciers during the last two decades have initiated questions on the trigger and processes governing observed changes. Destabilization of these glaciers coincides with atmosphere and ocean warming, which broadly has been used to explain the rapid changes. To assess the relative role of external forcing versus fjord geometry, we investigate the retreat of Jakobshavn Isbrae in West Greenland, where margin positions exist since the Little Ice Age maximum in 1850. We use a one-dimensional ice flow model and isolate geometric effects on the retreat 5 using a linear increase in external forcing.We find that the observed retreat of 43 km from 1850 until 2014 can only be simulated when multiple forcing parameterssuch as hydrofracturing, submarine melt and frontal buttressing by sea ice-are changed simultaneously. Surface mass balance, in contrast, has a negligible effect. While changing external forcing initiates retreat, fjord geometry controls the retreat pattern.Basal and lateral topography govern shifts from temporary stabilization to rapid retreat, resulting in a highly non-linear glacier 10 response. For example, we simulate a disintegration of a 15 km long floating tongue within one model year, which dislodges the grounding line onto the next pinning point. The retreat pattern loses complexity and becomes linear when we artificially straighten the glacier walls and bed, confirming the topographic controls.For real complex fjord systems such as Jakobshavn Isbrae, geometric pinning points predetermine grounding line stabilization and may therefore be used as a proxy for moraine build-up. Also, we find that after decades of stability and with constant 15 external forcing, grounding lines may retreat rapidly without any trigger. This means that past changes may precondition marine-terminating glaciers to reach tipping-points, and that retreat can occur without additional climate warming. Present-day changes and future projections can therefore not be viewed in isolation of historic retreat.

The vertical front of ice shelves represents a topographic barrier for barotropic currents that transport a considerable amount of heat towards the ice shelves. The blocking effect of the ice front on barotropic currents has recently been observed to substantially reduce the heat transport into the cavity beneath the Getz Ice Shelf. We use an idealized numerical model to study the vorticity dynamics of an externally forced barotropic current at an ice front and the impact of ice shelf thickness, ice front steepness, and ocean stratification on the volume flux entering the cavity. Our simulations show that thicker ice shelves block a larger volume of the barotropic flow, in agreement with geostrophic theory. However, geostrophy breaks locally at the ice front, where relative vorticity and friction become essential for the flow to cross the discontinuity in water column thickness. The flow entering the cavity accelerates and induces high basal melt rates in the frontal region. Tilting the ice front, as undertaken in sigma-coordinate models, reduces this acceleration as the flow is more geostrophic. Viscous processes—typically exaggerated in low-resolution models—break the potential vorticity constraint and bring the flow deeper into the ice shelf cavity. The externally forced barotropic current can only enter the cavity if the stratification is weak, as strong vertical velocities are needed at the ice front to squeeze the water column beneath the ice shelf. If the stratification is strong, vertical velocities are suppressed and the barotropic flow is almost entirely blocked by the ice front.

Abstract. Ice shelves in the Amundsen Sea are thinning rapidly as ocean currents bring warm water into the cavities beneath the floating ice. Although the reported melt rates for the Getz Ice Shelf are comparatively low for the region, its size makes it one of the largest freshwater sources around Antarctica, with potential consequences for, bottom water formation downstream, for example. Here, we use a 2-year-long novel mooring record (2016–2018) and 16-year-long regional model simulations to describe, for the first time, the hydrography and circulation in the vicinity of the ice front between Siple and Carney Island. We find that, throughout the mooring record, temperatures in the trough remain below 0.15 ∘C, more than 1 ∘C lower than in the neighboring Siple and Dotson Trough, and we observe a mean current (0.03 m s−1) directed toward the ice shelf front. The variability in the heat transport toward the ice shelf appears to be governed by nonlocal ocean surface stress over the Amundsen Sea Polynya region, and northward to the continental shelf break, where strengthened westward ocean surface stress leads to increased southward flow at the mooring site. The model simulations suggest that the heat content in the trough during the observed period was lower than normal, possibly owing to anomalously low summertime sea ice concentration and weak winds.

Increased oceanic heat transport toward the ice shelves has been the primary driver of ice-shelf thinning in the Amundsen Sea sector of West Antarctica during the last decades (

<p>The Filchner Trough on the continental shelf in the southern Weddell Sea is the gateway for warm water from off the continental shelf to flow towards the Filchner Ice Shelf. The warm water is steered southward along the eastern slope of the trough, potentially increasing basal melt rates of the ice shelf and leading to the formation of cold and dense Ice Shelf Water that overflows and contributes to the Antarctic Bottom Water. We present mooring time series from 2017 to 2021 in key inflow regions of modified Warm Deep Water onto the eastern continental shelf. Three moorings were placed across the eastern flank of the Filchner Trough close to the shelf break and captured the changes in the thickness of the northward-flowing Ice Shelf Water as well as the overlying southward warmer water. Another mooring was placed over the shallower eastern shelf and allowed a comparison between the two pathways of warm water onto the continental shelf. The four-year-long observations provide a better understanding of the processes that influence the seasonal and interannual variability in temperatures and circulation and possible changes in the flow of warm water towards the ice shelf. </p>