Integration of Passive Tracers in a Three-Dimensional Ice Sheet Model - Towards an Interdisciplinary Approach in Earth System Science - page 151

transect

Ice sheet- SIA applies SSA+SIA Ice shelf - SSA applies

A

B

C

SIA

SSA

0 800 1600 2400 3200 4000 4800

km

Fig. 2 Transition between the shallow shelf (SSH) and shallow ice (SIA) approximations. Where
ice is grounded SIA applies and ice flow is governed by overburden pressure, surface slope and
bedrock topography, floating shelf ice is governed by the SSH. Basal friction is set to zero and ice
flow is governed by longitudinal stress. The area close to the grounding line is represented by a
combined SSH+SIA term (depicted by the shaded coloring). A Transect through ABC present day
topography/bathymetry starting from Larssen-Shelf through Filchner-Ronne and Ross-Shelf.
B Velocities along the transect ABC

underneath the circumantarctic ice shelves can play an important role in the
dynamics of the grounding line and the adjacent glaciers feeding the ice shelves
(Hellmer et al. 2012 ; Joughin et al. 2014 ). Basal melting of the shelf is parametrized
according to the approach of Beckmann and Goosse ( 2003 ). A 3D temperature
distribution assembled from the World Ocean Atlas (2009) was compiled to drive
the melting process at the respective depth of the ice shelf bottom. For the control
run, Antarctic present-day (PD) geometry was used as a constraint to the equilib-
rium state. Even though PD Antarctica is not in equilibrium (Antarctica is loosing
mass and will be subject to increased rates of mass loss based on climate projec-
tions) it is a reasonable zeroth order topographic constraint. Optimal parameteri-
zation was achieved by varying the parameter space spanned by basal friction,
Beckmann-Goosse parameter and the flow enhancement factor. The parameter-set
which corresponded to the closest representation of the PD surface topography and
mass balance was chosen to be the default parameterization for the transient model
runs. Even though it is unlikely, that basal conditions remained constant throughout
glacial interglacial cycles, lack of proxy data shedding light on paleo-bedrock
conditions prohibits a more elaborate approach. Figure 3 illustrates the climato-
logical and topographic forcings used in the PD run. The PD run was carried out
until ice volume reached a stable state, which took approximately 50,000 model
years. Figure 4 exhibits the equilibrium topographic features of the tuned PD-run