Stress applied to the base of pack ice by ocean tidal currents is one component determining ice motion. An analysis of the motion of satellite-tracked drifters, and comparisons with CATS99.2, an updated version of our ocean tidal model [Robertson et al., 1998], shows that tidal currents affect ice motion along the Ronne Ice Front, the northern Filchner Trough, and the outer shelf and upper continental slope of the southern and western Weddell Sea. Over the remainder of the Weddell Sea, near-inertial response to wind stress, and direct forcing by tidal-band energy in the wind stress spectrum, account for most tidal-band ice motion. Ice divergence also shows energy at tidal frequencies, suggesting that tides can contribute to the oceanic heat loss to the atmosphere by an increase in lead fraction in tidally active areas. Along the southern and western continental shelf break and the Ronne Ice Front, the standard deviation of horizontal divergence calculated from our Weddell Sea ocean tides model is about 2x10^-6 s^-1, consistent with a mean tide-forced lead fraction of 3%. The diurnal and semidiurnal model divergence fields are shown in figure 2. Most of the horizontal tidal divergence along the shelf break is due to advection of the water column over sloping topography by the tides: continuity requires that the water column’s area change in inverse proportion to the water depth.

This work has been prepared as manuscript entitled "High-frequency ice motion and divergence in the Weddell Sea", J. Geophys. Res, 105, 3379-3400, 2000.

A paper with this title is in press at Journal of Geophys. Res. (Oceans), with an estimated publication date of July 2000. Authors are E. Rignot (JPL, Pasadena), L. Padman (ESR), D. MacAyeal (University of Chicago) and M Schmeltz (also at JPL).

ABSTRACT: Tides near and under floating glacier ice (e.g., ice shelves, glacier termini in fjords) can influence heat transport into the sub-ice-shelf cavity, mixing of the under-ice water column, and the calving and subsequent drift of tabular icebergs. It is extremely difficult to collect ocean data in these environments. Fortunately, free-surface displacement patterns associated with ocean variability below glacier ice are readily observed by differencing two synthetic-aperture radar (SAR) interferograms, each of which represents the combination of the displacement patterns associated with the time-varying vertical motion and the time-independent lateral ice flow. We present the pattern of net free-surface displacement for the iceberg calving regions of the Ronne and Filchner Ice Shelves in the southern Weddell Sea. The free-surface displacement variability for these regions is dominated by ocean tides, this assumption being validated by comparisons of the SAR-based displacement fields with ocean tidal models. The present limited amount of SAR interferometry for the ice shelves prevents us from using these data to independently generate tidal constituent fields. We show, however, that this procedure is theoretically possible, given a sufficient number of interferograms and carefully chosen satellite orbit parameters. The principal value of using SAR interferometry in this manner to observe ocean tides lies in the very high resolution obtained over the large region covered by each SAR image. The SAR interferometry of the Ronne Ice Shelf highlights a region of strong spatial gradients in differential displacement that are present, but much weaker, in the tidal models. The cause of this feature is believed to be a combination of the presence of diurnal-frequency topographic vorticity waves and proximity to the amphidromic points of semidiurnal tidal constituents.