Basal melting of tabular icebergs, model results

1. Basal melting of tabular icebergs,model results Daniela Jansen, Ralph Timmermann, Michael Schodlok, Wolfgang Rack, Henner Sandhäger Alfred Wegener Institute for Polar and Marine Research Bremerhaven, Germany

2. The role of tabular icebergs in the ice – ocean system Icebergs with main axis > 28 km: -1000 Gtice yr-1 Motivation Calving at the ice fronts represents the main mass loss of the Antarctic ice sheet Iceberg calving: -2000 Gtice yr-1 Basal melting: -550 Gtice yr-1 Jacobs et al., 1992 Melting icebergs provide cold freshwater during their drift and their final decay (Gladstone et al., 2001) Basal melting of tabular icebergs

3. Modeling iceberg evolution What we need: Geometry of an adequate iceberg Model for simulation of inherent ice dynamics Basal melting approach, ocean forcing data Observations for reliable description of evolution and validation of model results Outline Basal melting of tabular icebergs

4. Modeling iceberg evolution What we need: Geometry of an adequate iceberg Model for simulation of inherent ice dynamics Basal melting approach, ocean forcing data Observations for reliable description of evolution and validation of model results Outline Jansen, Sandhäger, Rack, 2005 Basal melting of tabular icebergs

5. 1. Evolution of A-38B: observation • Adequate iceberg for model studies: A-38B • Geometry and temperature profile is known (Filchner Ronne Thickness Map, Sandhäger et al., 2004; drill site, Grosfeld and Thyssen, 1994) • Subject to long-distance drift with changes in environmental conditions • Evolution is well documented by remote sensing data (Radarsat, Modis, ICESat) Calving and breaking apart of Iceberg A-38, October 1998 Basal melting of tabular icebergs

6. February 17th 2003 September 15th 2003 April 15th 2004 1. Evolution of A-38B: observation Image courtesy: MODIS Rapid Response Team NASA/GSFC Basal melting of tabular icebergs

7. 1. Evolution of A-38B: observation August 16th 2004 August 20th 2004 Image courtesy: MODIS Rapid Response Team NASA/GSFC September 28th 2004 Basal melting of tabular icebergs

8. Melting approach Based on two-dimensional model for ice shelf – ocean interaction (Hellmer and Olbers, 1989) Balance of heat Balance of salt In situ freezing temperature at the iceberg base 2. Melting approach Total heat flux across interface Heat consumed by melting Molecular diffusive heatflux through iceberg Total salt flux across interface Dilution caused by melting Basal melting of tabular icebergs

9. Melting approach Based on two-dimensional model for ice shelf – ocean interaction (Hellmer and Olbers, 1989) Balance of heat Balance of salt In situ freezing temperature at the iceberg base 2. Melting approach Total heat flux across interface Heat consumed by melting Molecular diffusive heatflux through iceberg Model input: Temperature and salinity of the ocean at the iceberg base TW, SW Turbulent exchange coefficients for temperature and saltT, S Total salt flux across interface Dilution caused by melting Basal melting of tabular icebergs

10. Melting approach Based on two-dimensional model for ice shelf – ocean interaction (Hellmer and Olbers, 1989) Balance of heat Balance of salt In situ freezing temperature at the iceberg base Model output: Temperature and salinity at the iceberg base TB, SB , meltrate 2. Melting approach Total heat flux across interface Heat consumed by melting Molecular diffusive heatflux through iceberg Total salt flux across interface Dilution caused by melting Basal melting of tabular icebergs

11. 2. Melting approach maximal/minimal draft of the iceberg Olbers et al.,1992. Hydrographic Atlas of the Southern Ocean Basal melting of tabular icebergs

12. 2. Melting approach Initial model run Filchner Ronne thickness map Basal melting of tabular icebergs

13. 3. ICESat data • ICESat GLAS data • Validation of basal melting • GLAS: Geoscience Laser Altimetry System • Data is available for • March/April 2003, Release 18 September/October 2003, Release 24 • February/March 2004, Release 26 • (Zwally et al., 2003/2004) • Three hits on iceberg A-38B Basal melting of tabular icebergs

14. 3. ICESat data • Initial model run with various temperature forcings • Ocean temperature based on measurements and model data (BRIOS) • Model results deviate from ICESat elevation at the thinner part • Initial geometry of the iceberg probably incorrect Initial model run Basal melting of tabular icebergs

15. 3. ICESat data • Geometry initialization • Initial geometry is based on a Filchner Ronne thickness map with ice shelf front from 1986 (Sandhäger et al. 2004) • Ice thickness probably changed significantly until calving of iceberg 1998 • First ICESat profile from March 2003 is used as an initialization for iceberg geometry Basal melting of tabular icebergs

16. 3. ICESat data New geometry Basal melting of tabular icebergs

17. 4. Melting approach: results March 2003 • Decrease of mean freeboard from 37m to 35 m • Decrease of mean thickness from 200m to 180 m Basal melting of tabular icebergs

18. 4. Melting approach: results October 2003 • Mean freeboard: 32 m • Mean thickness 155 m Basal melting of tabular icebergs

19. 4. Melting approach: results March 2004 • Profile is at a low angle to contour lines • Near to the iceberg edge • Fit is best in the part on which geometry correction is based • Mean freeboard 29 m, mean thickness 130m Basal melting of tabular icebergs

20. 4. Melting approach: results March 2004 • Bad fit, but • Profile is at a low angle to contour lines • Near to the iceberg edge • Fit is best in the part on which geometry correction is based Basal melting of tabular icebergs

21. 4. Melting approach: results • Melt rates • Turbulent exchange parameter T is determined through friction velocity at the ice/ocean boundary • High value in the Weddell Sea: Iceberg drift governed by sea ice drift, higher friction velocity • Lower value in the Scotia Sea, where the iceberg can drift with ocean current Weddell Sea Scotia Sea (m/s) Drift velocities derived from Antarctic Iceberg data base, D. Long Basal melting of tabular icebergs

22. 4. Melting approach: results • Melt rates • Turbulent exchange parameter T is determined through friction velocity at the ice/ocean boundary • High value in the Weddell Sea: Iceberg drift governed by sea ice drift, higher friction velocity • Lower value in the Scotia Sea, where the iceberg can drift with ocean current T = 1.2*10-4 ms-1 T = 0.5*10-4 ms-1 (m/s) Drift velocities derived from Antarctic Iceberg data base, D. Long Basal melting of tabular icebergs

23. 5. Summary and Outlook • Summary !!! • Iceberg basal melting can be simulated using an ice shelf melting approach • To obtain sound results accurate iceberg geometry is substantial • ICESat data enables validation of model results • Together with basal melting model ICESat could also provide information about the initial geometry of icebergs Basal melting of tabular icebergs

24. 5. Summary and Outlook • Summary !!! • Iceberg basal melting can be simulated using an ice shelf melting approach • To obtain sound results accurate iceberg geometry is substantial • ICESat data enables validation of model results • Together with basal melting model ICESat could also provide information about the initial geometry of icebergs • Outlook ??? • Calculate turbulent exchange coefficient from friction velocity? • Critical iceberg thickness determining final break up? • Can laterally inhomogeneous melting of grounded icebergs induce break up? Basal melting of tabular icebergs

25. 5. Summary and Outlook • Outlook ??? • Calculate turbulent exchange coefficient from friction velocity? • Critical iceberg thickness determining final break up? • Can laterally inhomogeneous melting of grounded icebergs induce break up? Basal melting of tabular icebergs