Bedrock bathymetry (m) in color and water column thickness (contours, 100 m intervals) of the (1/32)◦ (900 ± 30 m) horizontal resolution model. The white contour indicates the ice edge, the white crosses the position of three hypothetical drilling sites – source: Heimbach and Losch, 2012
Ocean measurements near the Pine Island Glacier made in 1994 indicate that the ice shelf extending out over the ocean has been melting and rapidly (Jacobs et al, 2011). More than this, near-shelf ocean transport and tracer calculations suggest that melting of the shelf’s underside has led to the formation and growth of an interior cavity with implications for enhanced further melting and reductions in the extent of the glacier grounding zone – where the ice anchors to the bed rock.
In a new paper, published in the Annals of Glaciology, long-time MITgcm users Patrick Heimbach (MIT) and Martin Losch (Alfred-Wegener-Institute fur Polar- und Meeresforschung, Bremerhaven, Germany) extend MITgcm’s adjoint capability to investigate the sensitivity of sub-ice-shelf melt rates under the Pine Island Ice Shelf, West Antarctica, to changes in the oceanic state.
What happens under an ice shelf extending over the ocean around Antarctica is hard to observe but is clearly critical to any detailed understanding of changes in ice volume and ocean temperature or salinity characteristics in that region with all the implications that holds for more global change.
While forward models are useful, adjoint models are an even more powerful tool to answer questions about sensitivity of a particular variable to ocean state.
Using adjoint code based on algorithmic differentiation (AD) of the MITgcm, Heimbach and Losch extend the adjoint model to include representation of the flow in a sub-ice-shelf cavity. By adding into the AD process the corresponding sub-ice-shelf cavity code (an implementation of a three-equation thermodynamic melt-rate parameterization to infer heat and freshwater fluxes at the ice-shelf/ocean boundary) the pair find the inferred sensitivities reveal timescales of 30– 60 days over which advective processes connect the shelf exit to the deep interior. Three-dimensional time-evolving patterns also emerge which Heimbach and Losch explain in terms of a combination of buoyancy forcing by inflowing water masses, cavity geometry and the effect of rotation and topography in steering the flow in the presence of prominent features in the bedrock bathymetry. Finally they report finding dominant sensitivity pathways over a particular sill, as well as ‘shadow regions’ of very low sensitivities.
To the extent that these transient patterns are robust, the authors report that they may carry important information for decision- making in observation deployment and monitoring. To find out more contact Patrick or Martin.
References:
Heimbach, P. and M. Losch (2012) Adjoint sensitivities of sub-ice-shelf melt rates to ocean circulation under the Pine Island Ice Shelf, West Antarctica, Annals of Glaciology 53(60), doi: 10.3189/2012/AoG60A025
Jacobs, S. S, A. Jenkins, C. F. Giulivi and P. Dutrieux (2011) Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf, Nature Geoscience 4, 519-523, doi: 10.1038/ngeo1188
Patrick Heimbach has been working with the MITgcm since 1998. He enjoys rock-climbing and playing the cello.
Martin has been using the MITgcm since 2001. He enjoys playing the piano.