Exploring mechanisms responsible for tidal modulation in flow of the Filchner-Ronne Ice Shelf

• Main Authors: Rosier, Sebastian H. R., Gudmundsson, G. Hilmar
• Format: Text
• Language: English
• Published: 2019
• Subjects: Ronne Ice Shelf; Filchner Ronne Ice Shelf; Filchner-Ronne Ice Shelf; Ice Shelf
• Online Access: https://doi.org/10.5194/tc-2019-79; https://www.the-cryosphere-discuss.net/tc-2019-79/

An extensive network of GPS sites on the Filchner-Ronne Ice Shelf and adjoining ice streams show strong tidal modulation of horizontal ice flow at a range of frequencies. A particularly strong (horizontal) response is found at the fortnightly ( M sf ) frequency. Since this tidal constituent is absent in the (vertical) tidal forcing, this observation implies the action of some nonlinear mechanism. Another striking aspect is the strong amplitude of the flow perturbation, causing a periodic reversal in the direction of ice shelf flow in some areas, and a 10–20 % change in speed at grounding lines. No model has yet been able to reproduce the quantitative aspects of the observed tidal modulation on the Filchner-Ronne Ice Shelf. The cause of the tidal response has therefore remained an enigma, indicating a serious limitation in our current understanding of the mechanics of large-scale ice flow. A further limitation of previous studies is that they have all focused on isolated regions and interactions between different areas have, therefore, not been fully accounted for. Here, we conduct the first large-scale ice-flow modelling study to explore these processes using a viscoelastic rheology and realistic geometry of the entire Filchner-Ronne ice shelf, where the best observations of tidal response are available. We evaluate all the relevant mechanisms that have hitherto been put forward to explain how tides might affect ice-shelf flow and compare our results with observational data. We conclude that, while some are able to generate the correct general qualitative aspects of the tidally-induced perturbations in ice flow, most of these mechanisms must be ruled out as being the primary cause of the large observed nonlinear response. We find that only tidally-induced lateral migration of grounding lines can generate a sufficiently strong long-periodic Msf response on the ice shelf to match observations. Furthermore, we show that the observed short-periodic diurnal tidal motion, causing twice-daily flow reversals at the ice front, can be generated through a purely elastic response to basin-wide tidal perturbations in the ice shelf slope. This model also allows us to quantify the effect of tides on mean ice flow and we find that the Filchner-Ronne Ice Shelf flows on average ~ 21 % faster than it would in the absence of large ocean tides.