Insights into ice stream dynamics through modelling their response to tidal forcing

Abstract:

The tidal forcing of ice streams at their ocean boundary can serve as a natural experiment to gain an insight into their dynamics and constrain the basal sliding law. A 3-D visco-elastic full Stokes model of coupled ice-stream ice-shelf flow is used to investigate the response of ice streams to the ocean tide. In agreement with previous results based on flow-line modeling and with a fixed grounding line position, we find that a non-linear basal sliding law alone can qualitatively reproduce long period modulation of tidal forcing found in field observations. Matching the large amplitude of these modulations with the inclusion of lateral drag however requires the addition of either grounding line migration or tidally modulated changes in till strength due to sub-glacial pressure variations. Further analysis of modeled ice stream flow shows a varying stress-coupling length scale of boundary effects upstream of the grounding line. We derive a visco-elastic stress coupling length scale from ice stream equations that depends on the forcing period and closely agrees with model output. To date, the modeling of an ice stream’s response to tidal forcing appears to be the only approach capable of giving quantitative information about the basal sliding law from in-situ observations.