Kalbung und Zerfall riesiger antarktischer Eisberge: eine kombinierte Modell- und Satellitenfernerkundungsstudie
Zusammenfassung der Projektergebnisse
Ice dynamics and basal melting: A combined approach of numerical modelling and analysis of remote sensing data showed that the most significant process in iceberg evolution is melting at the iceberg base. This process causes the main geometric modification of the iceberg during long periods of its drift and also represents the most significant impact of tabular icebergs on their environment. Strain thinning due to horizontal spreading is of minor importance and only comparable to the magnitude of thinning due to melting for icebergs located in direct vicinity of the ice shelf fronts, where the water temperatures are low. Ice deformation within a tabular iceberg is of a radial shape, as the driving force applies at the entire ice front. Thus, no or very little horizontal shear stress and associated fracturing exists. Considering the fact that the inherent ice dynamics do not play a major role in iceberg evolution, it might be problematic to regard tabular icebergs as a model for ice shelves under warming conditions. During ice shelf retreat, the stress distribution within the ice itself, which is related to the coupling to the embayment and eventual pinning points, is a governing factor for the stability of the ice body. During the final decay a very large amount of melt water is released into the ocean within a short period of time, as the contact surface of ice and ocean is increased. However, already during its drift the iceberg continuously releases cold freshwater. ICESat altimetry measurements together with a physical melting model approach showed that the melting of icebergs is not only determined by ocean temperature and salinity, but also to some extent by the drift state and the relative velocity between iceberg and ocean. Assuming a constant turbulent exchange parameter during the entire drift leads to an overestimation of iceberg melt water in the Weddell Sea. The results indicate that a large quantity of melt water is transported into the Scotia Sea or even further north, and helps to understand the relatively long survival of drifting tabular icebergs in these latitudes. As this study was focused on one particular iceberg, it is necessary to confirm the estimations of melt rates and associated turbulent exchange parameters by data from other icebergs, derived by satellite altimetry or, if possible, by radio echo sounding. In this context, also the magnitude of surface melting and its impact on iceberg freeboard needs to be further investigated. Grounding icebergs and related tremor events: By the combination of MODIS satellite imagery and ETOPO2 bathymetry data, it was possible to correlate single tremor events recorded at the HOPE seismic Station on South Georgia Island with collisions of the iceberg with the island shelf. The iceberg’s grounding or sliding along the continental slope was attributed to diffuse tremor events with a background sound structured into overtones of a fundamental mode. Superposed onto this background, gliding spectral peaks occur which are probably related to the stick-slip process between the iceberg and the island. Harmonic tremor with only a few pronounced, variable frequency maxima occur exclusively for the floating iceberg A-43G, which contains a major rift structure. The excitation of the tremor is probably due to coupling of hydrodynamic instabilities with the reservoir modes of the rift channel or oscillations of the channel walls, in analogy to tremor excitation in volcanic systems.
Projektbezogene Publikationen (Auswahl)
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(2005). Model experiments on large tabular iceberg evolution: ablation and strain thinning. Journal of Glaciology, 51(174), 363-372
Jansen, D., Sandhaeger, H., Rack, W.
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(2005). Model investigations of Larsen B Ice Shelf dynamics prior to the breakup. Forum for Research into Ice Shelf Processes (FRISP) Report No.16, 5-12
Sandhäger, H., Rack, W., Jansen, D.
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(2007). Basal melting of A-38B: a physical model constrained by satellite observations. Remote Sensing of the Environment, 111, 195-203
Jansen, D., Schodlok, M., Rack, W.
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Evolution of Tabular Icebergs. PhD Thesis, Bremen, 2009, pp. 128
D. Jansen
