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

Abstract

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 5 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, 10 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 M sf response on the ice 15 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.

Observations of Tidal Melt and Vertical Strain at the Filchner‐Ronne Ice Shelf, Antarctica

“…The strongest and cleanest signal in the internal reflectors is that of the semidiurnal M 2 constituent. For FRIS, ice shelf tilt has been shown to explain semidiurnal tidal variations in ice shelf flow (Makinson et al, 2012;Rosier & Gudmundsson, 2019). M 2 being the strongest signal in the internal reflectors is therefore consistent with M 2 having the largest amplitude in the ocean tidal model (Figure 2a).…”

“…Rosier and Gudmundsson (2018) suggested a flexural softening mechanism that should give rise to these double frequencies at significant levels. However, Rosier and Gudmundsson (2019) concluded that this mechanism may not be important at FRIS.…”

Tidal Grounding‐Line Migration Modulated by Subglacial Hydrology

“…The grounding‐line position as defined here represents the position downstream of which a significant volume of water is retained between the ice and the bed, lubricating the contact such that the till exerts very little basal drag on the flow of the ice. Rosier and Gudmundsson (2020) highlight the sensitive role that variations in drag in the grounding zone play in setting the velocity response of ice shelves. In this way, modulations of horizontal ice velocity can be understood through the modulation of this position of decreased contact.…”

Tidal Modulation of Buoyant Flow and Basal Melt Beneath Petermann Gletscher Ice Shelf, Greenland

“…While these remotely sensed measurements reveal large-scale trends in ice sheet variability, their coarse temporal resolutions do not capture variations across the broad range of time scales over which ocean processes influence ice sheet change. High temporally resolved, long-term surface-based measurements of ice thickness from phase-sensitive radars (e.g., Vaňková et al, 2020) and ice velocity from global positioning systems (e.g., Rosier & Gudmundsson, 2020) have exposed some of these processes on both grounded and floating ice.…”