Observation of ocean tides below the Filchner and Ronne Ice Shelves, Antarctica, using synthetic aperture radar interferometry: Comparison with tide model predictions

E Rignot,M Schmeltz,L Padman,D R Macayeal

doi:10.1029/1999jc000011

Abstract

Tides near and under floating glacial ice, such as ice shelves and glacier termini in fjords, can influence heat transport into the subice cavity, mixing of the under‐ice water column, and the calving and subsequent drift of icebergs. Free‐surface displacement patterns associated with ocean variability below glacial ice can be 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. By comparing SAR‐based displacement fields with ocean tidal models, the free‐surface displacement variability for these regions is found to be dominated by ocean tides. The inverse barometer effect, i.e., the ocean's isostatic response to changing atmospheric pressure, also contributes to the observed vertical displacement. The principal value of using SAR interferometry in this manner lies in the very high lateral resolution (tens of meters) obtained over the large region covered by each SAR image. Small features that are not well resolved by the typical grid spacing of ocean tidal models may contribute to such processes as iceberg calving and cross‐frontal ventilation of the ocean cavity under the ice shelf.

Highlights

Et al, 1989].The potentialformonitoringicesheetmotion with this techniquewas describedby Goldsteinet interferometry are strongly correlated with tions based on numerical ocean tidal models
Hartl et al [1994]developedthis ideafurtherby comparingoceantide heightpredictionswith the displacementevaluatedfrom differencingtwo SAR interferogramsfor a singlelocationnear the easternend of of extensive regions of ice shelf we demonstrate the method's value in providing more spatial detail of ocean tide variability than is available through classical oceantide measurementtechniquessuchas moored the Ronne Ice Shelf in the southern Weddell Sea
In this study we develop the methodology for using differential SAR interferometry to investigatethe ocean tide in the ice shelf covered portions of the southern

Summary

Relationship Between Tides and SAR Data

In Appendix A we describe how tidal displacement of the sea surface at the four times of SAR image acquisition determines the DSI-derived At/. With this as- I and ERS-2 satellitesusedto acquireour data) is an sumption the observedrange differencezl( introduced exact multiple of the 12.0000 hour period of the semidiabovemay be attributed to seasurfaceelevation change urnal solar tide $. In this circumstance the sea surface alone. Between the Hemmen Ice Rise and Lassiter Coast re- The practicality of such an inversion remains suspect, gionson the Ronne Ice Shelf This large-scalegradient becauseof its requirement for large numbers is consistent with the notion that the ocean tides prop- of independenDt SI fields(seeAppendixB) to capture. To understand the implications of the DSI-determined At/fields reported here

Ocean Tide Model Predictions

Discussion

Errors Thickness in Model

Nontidal Ocean Height Variability