The Antarctic ice sheet has been losing mass over the past decades through the accelerated flow of its glaciers conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (i.e., bed elevation drops in the inland direction) are potentially unstable, whereas subglacial ridges slow down the glacial retreat. Despite major advances in mapping subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution, and physically-based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response

Abstract. Observations show that the flow of Rutford IceStream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77-day tidal period (M sf ). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modelling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic fullStokes model of ice-stream flow to investigate this open issue. We find that the long period M sf modulation of icestream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼ 12 % increase in mean horizontal velocity of the adjoining ice stream. Observations of tidally induced variations in flow of ice streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

A potentially irreversible threshold in Antarctic ice shelf melting would be crossed if the ocean cavity beneath the large Filchner–Ronne Ice Shelf were to become flooded with warm water from the deep ocean. Previous studies have identified this possibility, but there is great uncertainty as to how easily it could occur. Here, we show, using a coupled ice sheet-ocean model forced by climate change scenarios, that any increase in ice shelf melting is likely to be preceded by an extended period of reduced melting. Climate change weakens the circulation beneath the ice shelf, leading to colder water and reduced melting. Warm water begins to intrude into the cavity when global mean surface temperatures rise by approximately 7 °C above pre-industrial, which is unlikely to occur this century. However, this result should not be considered evidence that the region is unconditionally stable. Unless global temperatures plateau, increased melting will eventually prevail.

Abstract. The tidal forcing of ice streams at their ocean boundary can serve as a natural experiment to gain an insight into their dynamics and constrain the basal sliding law. A nonlinear 3-D viscoelastic full Stokes model of coupled ice stream ice shelf flow is used to investigate the response of ice streams to ocean tides. In agreement with previous results based on flow-line modelling and with a fixed grounding line position, we find that a nonlinear basal sliding law can qualitatively reproduce long-period modulation of tidal forcing found in field observations. In addition, we show that the inclusion of lateral drag, or allowing the grounding line to migrate over the tidal cycle, does not affect these conclusions. Further analysis of modelled ice stream flow shows a varying stress-coupling length scale of boundary effects upstream of the grounding line. We derive a viscoelastic stresscoupling length scale from ice stream equations that depends on the forcing period and closely agrees with model output.