Assessment of oscillation feature in sub-shelf melting from idealized coupled ice sheet ocean models

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• Chen Zhao, University of Tasmania
• Wednesday 01 December 2021, 10:00-11:00
• https://ukri.zoom.us/j/96463295770; BAS seminar room 2.

If you have a question about this talk, please contact Dr. Shenjie Zhou.

Changes in ocean-driven melting have a key influence on the stability of ice shelves, the mass loss from the ice sheet, ocean circulation and global sea level rise. Coupled ice sheet – ocean models have a critical role in examining the processes governing basal melting and understanding future ice sheet evolution. However, as a new approach, coupled ice-sheet/ocean systems comes with new challenges, and the impacts of solutions implemented to date have not been quantified. Here we use a recently developed coupling framework, FISOC , between the ocean model ROMS and ice-sheet model Elmer/Ice to investigate the impact of coupled modelling strategies on the simulated basal melt in an idealised ice shelf cavity, based on the Marine Ice Sheet–Ocean Model Intercomparison Project (MISOMIP) setup. An emergent feature was that the spatial-averaged basal melt rates (3.56 m yr−1) oscillated with an amplitude∼0.7 m yr−1 and approximate period of ∼6 years between year 30 and 100, depending on the experimental design. The oscillation of the spatial-averaged basal melt rates emerged as a feature in the coupled system and the stand-alone ocean model using a prescribed change of cavity geometry. This feature was also observed in several other models under the MISOMIP design experiments. It was found that the oscillation feature is closely related to the discretised row-by-row ungrounding of the ice sheet, exposing new ocean, and is likely strengthened by a combination of melt-plume feedback and/or melt-geometry feedback near the grounding line, and the frequent coupling of ice geometry and ocean evolution in a couple system. Sensitivity tests were done to determine the origin of the feature and show that the oscillation behaviour is insensitive to the choice of coupling interval, vertical resolution in the ocean model, tracer properties of immediately ungrounded cells by the retreating ice sheet, or the dependency of friction velocities to the vertical resolution. We locally conserve absolute tracer values when adjusting the ice draft to obtain a physically plausible tracer evolution. Furthermore, a conservative, parabolic splines reconstruction of vertical derivatives was chosen to defeat the numerical overshoot introduced by large vertical gradients near the grounding line, which may be instructive for future model configurations.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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