Mechanics and dynamics of pinning points on the Shirase Coast, West Antarctica

Ice rises and rumples, sites of localised ice-shelf grounding, modify ice-shelf flow by generating lateral and basal shear stresses, upstream compression, and downstream tension. Studies of pinning points typically quantify this role indirectly, through related metrics such as a buttressing number. Here, we quantify the dynamic effects of pinning points directly, by comparing model-simulated stress states in the Ross Ice Shelf (RIS) with and without a specific set of pinning points located downstream of the MacAyeal and Bindschadler ice streams (MacIS and BIS, respectively). Because ice properties are only known indirectly, the experiment is repeated with different realisations of the ice softness. While longitudinal stretching, and thus ice velocity, is smaller with the pinning points, flow resistance generated by other grounded features is also smaller. Conversely, flow resistance generated by other grounded features increases when the pinning points are absent, providing a non-local control on the net effect of the pinning points on ice-shelf flow. We find that an ice stream located directly upstream of the pinning points, MacIS, is less responsive to their removal than the obliquely oriented BIS. This response is due to zones of locally higher basal drag acting on MacIS, which may itself be a consequence of the coupled ice-shelf and ice-stream response to the pinning points. We also find that inversion of present-day flow and thickness for basal friction and ice softness, without feature-specific, a posteriori adjustment, leads to the incorrect representation of ice rumple morphology and an incorrect boundary condition at the ice base. Viewed from the perspective of change detection, we find that, following pinning point removal, the ice shelf undergoes an adjustment to a new steady state that involves an initial increase in ice speeds across the eastern ice shelf, followed by decaying flow speeds, as mass flux reduces thickness gradients in some areas and increases thickness gradients in others. Increases in ice-stream flow speeds persist with no further adjustment, even without sustained grounding-line retreat. Where pinning point effects are important, model tuning that respects their morphology is necessary to represent the system as a whole and inform interpretations of observed change.

Diagnosing surfzone impacts on inner-shelf flow spatial variability using realistic model experiments with and without surface gravity waves

Rip currents are generated by surfzone wave breaking and are ejected offshore inducing inner-shelf flow spatial variability (eddies). However, surfzone effects on the inner-shelf flow spatial variability have not been studied in realistic models that include both shelf and surfzone processes. Here, these effects are diagnosed with two nearly identical twin realistic simulations of the San Diego Bight over summer to fall where one simulation includes surface gravity waves (WW) and the other that does not (NW). The simulations include tides, weak to moderate winds, internal waves, submesoscale processes, and have surfzone width Lsz of 96(±41) m (≈ 1 m significant wave height). Flow spatial variability metrics, alongshore root mean square vorticity, divergence, and eddy cross-shore velocity, are analyzed in a Lsz normalized cross-shore coordinate. At the surface, the metrics are consistently (> 70%) elevated in the WW run relative to NW out to 5Lsz offshore. At 4Lsz offshore, WW metrics are enhanced over the entire water column. In a fixed coordinate appropriate for eddy transport, the eddy cross-shore velocity squared correlation betweenWWand NW runs is < 0.5 out to 1.2 km offshore or 12 time-averaged Lsz. The results indicate that the eddy tracer (e.g., larvae) transport and dispersion across the inner-shelf will be significantly different in the WW and NW runs. The WW model neglects specific surfzone vorticity generation mechanisms. Thus, these inner-shelf impacts are likely underestimated. In other regions with larger waves, impacts will extend farther offshore.

Treatment of ice-shelf evolution combining flow and flexure

We develop a two-dimensional, plan-view formulation of ice-shelf flow and viscoelastic ice-shelf flexure. This formulation combines, for the first time, the shallow-shelf approximation for horizontal ice-shelf flow (and shallow-stream approximation for flow on lubricated beds such as where ice rises and rumples form), with the treatment of a thin-plate flexure. We demonstrate the treatment by performing two finite-element simulations: one of the relict pedestalled lake features that exist on some debris-covered ice shelves due to strong heterogeneity in surface ablation, and the other of ice rumpling in the grounding zone of an ice rise. The proposed treatment opens new venues to investigate physical processes that require coupling between the longitudinal deformation and vertical flexure, for instance, the effects of surface melting and supraglacial lakes on ice shelves, interactions with the sea swell, and many others.

Storm-driven across-shelf oceanic flows into coastal waters

The North Atlantic Ocean and northwest European shelf experience intense low-pressure systems during the winter months. The effect of strong winds on shelf circulation and water properties is poorly understood as observations during these episodes are rare, and key flow pathways have been poorly resolved by models up to now. We compare the behaviour of a cross-shelf current in a quiescent period in late summer, with the same current sampled during a stormy period in midwinter, using drogued drifters. Concurrently, high-resolution time series of current speed and salinity from a coastal mooring are analysed. A Lagrangian analysis of modelled particle tracks is used to supplement the observations. Current speeds at 70 m during the summer transit are 10–20 cm s−1, whereas on-shelf flow reaches 60 cm s−1 during the winter storm. The onset of high across-shelf flow is identified in the coastal mooring time series, both as an increase in coastal current speed and as an abrupt increase in salinity from 34.50 to 34.85, which lags the current by 8 d. We interpret this as the wind-driven advection of outer-shelf (near-oceanic) water towards the coastline, which represents a significant change from the coastal water pathways which typically feed the inner shelf. The modelled particle analysis supports this interpretation: particles which terminate in coastal waters are recruited locally during the late summer, but recruitment switches to the outer shelf during the winter storm. We estimate that during intense storm periods, on-shelf transport may be up to 0.48 Sv, but this is near the upper limit of transport based on the multi-year time series of coastal current and salinity. The likelihood of storms capable of producing these effects is much higher during positive North Atlantic Oscillation (NAO) winters.

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

An extensive network of GPS sites on the Filchner–Ronne Ice Shelf and adjoining ice streams shows strong tidal modulation of horizontal ice flow at a range of frequencies. A particularly strong (horizontal) response is found at the fortnightly (Msf) frequency. Since this tidal constituent is absent in the (vertical) tidal forcing, this observation implies the action of some non-linear 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 across the entire Filchner–Ronne Ice Shelf. The cause of the tidal ice flow 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 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 observed long-period response. We find that only tidally induced lateral migration of grounding lines can generate a sufficiently strong long-period Msf response on the ice shelf to match observations. Furthermore, we show that the observed horizontal short-period semidiurnal 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.

Storm-driven across-shelf oceanic flows into coastal waters

The North Atlantic Ocean and Northwest European shelf experience intense low-pressure systems during the winter months. The effect of strong winds on shelf circulation and water properties is poorly understood as observations during these episodes are rare, and key flow pathways have been poorly resolved by models up to now. We compare the behaviour of a cross-shelf current in a quiescent period in late summer, with the same current sampled during a stormy period in mid-winter, using drogued drifters. Concurrently, high-resolution time-series of current speed and salinity from a coastal mooring are analysed. A Lagrangian analysis of modelled particle tracks is used to supplement the observations. Current speeds at 70 m during the summer transit are 10–20 cm s−1, whereas on-shelf flow reaches 60 cm s−1 during the winter storm. The onset of high across-shelf flow is identified in the coastal mooring time-series, both as an increase in coastal current speed and as an abrupt increase in salinity from 34.50 to 34.85, which lags the current by 8 days. We interpret this as the wind-driven advection of outer-shelf (near-oceanic) water towards the coastline, which represents a significant change from the coastal water pathways which typically feed the inner shelf. The modelled particle analysis supports this interpretation: particles which terminate in coastal waters are recruited locally during the late summer, but recruitment switches to the outer shelf during the winter storm. We estimate that during intense storm periods, on-shelf transport may be up to 0.48 Sv, but that this is near the upper limit of transport based on the multi-year time series of coastal current and salinity. The likelihood of storms capable of producing these effects is much higher during NAO-positive winters.