AbstractTidal Currents under the Ross Ice Shelf ("Hello Down There")Laurie Padman Thick ice shelves insulate the underlying ocean from the usual dominant forcing from wind stress and air/sea buoyancy fluxes, leaving tides as the primary source of ocean motion affecting ice shelves. While tide-forced variation of ice shelf heights has been the primary interest in Antarctic tides to date, at least as far as the glaciological community is concerned, tidal currents also play a major role in the fate of ice shelves. We consider three issues: tides as a cause of melting at the ice shelf base; tides as a source of stress near the grounding line; and the tides’ role in rift expansion and the motion of recently calved icebergs. Under the Ross Ice Shelf (RIS), mean tidal current speeds vary from negligible to about 20 cm/s, with maximum values ("spring tides") about twice this. The highest sub-ice-shelf currents are along the Shirase and Siple Coasts, where the water column thickness under the RIS is small. The stress applied to the ice shelf base by a current of 20 cm/s is about the same as one gets with a wind speed of ~10 m/s. That is, unless the ice base is hydrodynamically smooth, we can expect significant turbulence near the ice base to result from tidal action. Tides therefore provide a mechanism for enhancing the flow of heat up to the ice base from "warm", high salinity shelf water (HSSW) flowing into the cavity along the seabed. The consequent basal melt may account for roughly half of the total ice mass loss from an ice shelf (the remainder being in infrequent large calving events or smaller disintegrations). From an oceanographic point of view, tides also help determine the actual properties of the HSSW source water at the ice front: for example, tides help maintain the Ronne Polynya at the front of the Ronne Ice Shelf, thus increasing the salinity and density of HSSW flowing into the Ronne cavity. This discussion raises the (unanswered) question: How effective can models of ocean/ice interaction under major ice shelves be when tidal currents are ignored? As well as this thermodynamic role of tides, stress at the ice base plays a mechanical role in ice shelf behavior. As one example, Bindschadler and others hypothesized that offshore-directed tidal stress near the outlet of Whillans Ice Stream was one agent responsible for "stick-slip" behavior of the lightly grounded ice of the adjacent ice plain. Nearer the ice front, tidal currents clearly influence the paths of recently calved icebergs, including their potential to collide with the remaining, uncalved front. Tides may also play a role in the development of the rifts that ultimately become the calving fronts. The latter process depends on both dynamics and thermodynamics: shear and strain of the tide-induced stress act mechanically on the developing rift, while turbulence associated with tidal currents helps determine the properties of the "mélange" that fills the rift during the divergent phase. |