Developing a coupled ice sheet-ocean model: challenges and progress with
terrain-following ocean coordinates
Abstract
The ice sheet-ocean modeling community is making large strides toward
developing coupled models capable of examining the interactions and
feedbacks between ice shelves and ocean along the Antarctic margin. We
present preliminary results and address some of the challenges that have
arisen during the development of a coupled ice sheet-ocean model. The
ice sheet model is icepack, a shallow-shelf finite element model written
in Python. The ocean model is the Regional Ocean Modelling System
(ROMS), a terrain-following vertical (sigma) coordinate model that has
been modified to interface with a moving ice shelf. These two models are
coupled in an online configuration using the Framework for Ice Sheet
Ocean Coupling (FISOC). The use of a model with sigma coordinates for
the ocean component introduces a simplification and a complication to
modeling a moving ice draft. The sigma coordinate system retains the
same number of vertical layers at any depth, eliminating the need to
convert grid cells between ice and water, when using a fixed grounding
line configuration. However, as the ice shelf draft evolves in time,
topographic configurations develop that induce pressure gradient errors
in ROMS. We quantify these errors in an idealized set-up with an
artificially changing ice draft following the ISOMIP+ geometry. We
compare results between an ice draft that is smoothed to meet standard
ROMS smoothing criteria (rx0, rx1) and a non-smoothed ice draft.
Finally, we present a simple parameterization in a buffer zone near the
grounding line that uses interpolated melt rates from the ocean model,
allowing us to maintain a steep ice topography in the ice model without
inducing pressure gradient errors in the short water column in the ocean
model. This model configuration will be applied to Pine Island Glacier
and used to examine present and possible future states of the ice
sheet-ocean system.