Abstract: A common belief about tidal sedimentation is that tides are always larger near the equator and negligible at high latitudes. This belief appears to be based on equilibrium tidal theory that predicts the existence of two ocean–surface bulges centered at low latitudes; however, it is a misconception because this theory is a poor model for real-world tides. Instead, the tide behaves as a set of shallow-water waves that are guided around the world by the continents. Tidal ranges and tidal-current speeds increase as the tidal wave propagates onto and across continental shelves; especially large ranges and fast currents can occur in coastal embayments and in straits that join two larger bodies of water. Models of real-world tides today demonstrate that tides in shallow water (<100 m) have amplitude peaks at 50° N to 70° N and 50° S to 60° S that are associated with especially wide continental shelves and coastal embayments in which the tidal wave is close to resonance. The small tides characterizing most polar areas today are the result of local geomorphic features: the Arctic Ocean is too small to have its own tide and has only a small connection to the Atlantic Ocean that prevents effective northward propagation of the tidal wave, and Antarctica has narrow and deep continental shelves that do not accentuate the tide. Nevertheless, there are local areas in both the Arctic and Antarctic with favorable geomorphology that have macrotidal ranges. Thus, the latitudinal distribution of large tides is contingent on the plate-tectonic and sea-level history of the earth and changes over geologic time as the configuration of the ocean basins and the geometry of the flooded shelves change. The latitudinal variation of the strength of the Coriolis effect has a second-order influence on tidal dynamics, with the degree of tidal-range asymmetry across a basin potentially being larger at higher latitudes. The offshore extent of large coastal tidal ranges decreases at higher latitudes because the increased Coriolis effect leads to the tidal wave being more strongly banked-up against the shoreline. Diurnal, topographically trapped vorticity waves that can generate large tidal currents in shelf-edge water depths are also limited to middle to high latitudes. The presence of ice in polar areas also has an influence on tidal dynamics. Sea ice causes a small decrease in tidal range, whereas thick, floating ice shelves can cause dramatic increases in tidal range and tidal-current speeds, at least locally, as a result of the decrease in the cross-sectional area of the water beneath the ice shelves. Because coastal sedimentation is controlled by the relative importance of tidal currents and waves, the abundance of tide-dominated deposits might not reflect perfectly the latitudinal distribution of large tides. Thus, the small size of waves in the equatorial zone appears to cause preferential development of tide-dominated coastal zones near the equator, whereas wave dominance might be higher at midlatitudes because of the higher level of storminess, regardless of the latitudinal distribution of large tides.

Abstract The Institute Ice Stream (IIS) in West Antarctica may be increasingly vulnerable to melting at the grounding line through modifications in ocean circulation. Understanding such change requires knowledge of grounding-line boundary conditions, including the topography on which it rests. Here, we discuss evidence from new radio-echo sounding (RES) data on the subglacial topography adjacent to the grounding line of the IIS. In doing so, we reveal a previously unknown subglacial embayment immediately inland of the IIS grounding zone which is not represented in the Bedmap2 compilation. We discuss whether there is an open-water connection between the embayment and the ice-shelf cavity. The exact location of the grounding line over the embayment has been the subject of considerable uncertainty, with several positions being proposed recently. From our compilation of data, we are able to explain which of these grounding lines is most likely and, in doing so, highlight the need for accurate bed topography in conjunction with satellite observations to fully comprehend ice-sheet processes in this region and other vulnerable locations at the grounded margin of Antarctica.

Abstract A large volume of the East Antarctic Ice Sheet drains through the Totten Glacier (TG) and is thought to be a potential source of substantial global sea-level rise over the coming centuries. We show that the surface velocity and height of the floating part of the TG, which buttresses the grounded component, have varied substantially over two decades (1989–2011), with variations in surface height strongly anti-correlated with simulated basal melt rates ( r = 0.70, p < 0.05). Coupled glacier–ice shelf simulations confirm that ice flow and thickness respond to both basal melting of the ice shelf and grounding on bed obstacles. We conclude the observed variability of the TG is primarily ocean-driven. Ocean warming in this region will lead to enhanced ice-sheet dynamism and loss of upstream grounded ice.