Topographic Shelf Waves Control Seasonal Melting Near Antarctic Ice Shelf Grounding Lines

Sun, Sainan; Hattermann, Tore; Pattyn, Frank; Nicholls, Keith W.; Drews, Reinhard; Berger, Sophie

doi:10.1029/2019gl083881

Cited by 28 publications

(23 citation statements)

References 64 publications

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“…Internal tidal oscillations at subinertial frequencies can therefore only reach the ice shelf as baroclinic Kelvin or topographic Rossby waves that require a vertical wall or sloping bottom, respectively (Gill, 1982). While Sun et al (2019) attributed observed subinertial diurnal oscillations in the basal melt rate along the grounding line of the broad Roi Baudouin Ice Shelf, Antarctica to topographic Rossby waves, we posit that the steep and narrow Petermann Fjord may more likely facilitate baroclinic Kelvin waves. We speculate that the diurnal baroclinic waves are generated by the diurnal barotropic tide traveling across the fjord sill, approximately 65 km seaward of our study site.…”

Section: Discussioncontrasting

confidence: 67%

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

et al. 2020

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A set of collocated, in situ oceanographic and glaciological measurements from Petermann Gletscher Ice Shelf, Greenland, provides insights into the dynamics of under-ice flow driving basal melting. At a site 16 km seaward of the grounding line within a longitudinal basal channel, two conductivity-temperature (CT) sensors beneath the ice base and a phase-sensitive radar on the ice surface were used to monitor the coupled ice shelf-ocean system. A 6 month time series spanning 23 August 2015 to 12 February 2016 exhibited two distinct periods of ice-ocean interactions. Between August and December, radar-derived basal melt rates featured fortnightly peaks of ∼15 m yr −1 which preceded the arrival of cold and fresh pulses in the ocean that had high concentrations of subglacial runoff and glacial meltwater. Estimated current speeds reached 0.20-0.40 m s −1 during these pulses, consistent with a strengthened meltwater plume from freshwater enrichment. Such signals did not occur between December and February, when ice-ocean interactions instead varied at principal diurnal and semidiurnal tidal frequencies, and lower melt rates and current speeds prevailed. A combination of estimated current speeds and meltwater concentrations from the two CT sensors yields estimates of subglacial runoff and glacial meltwater volume fluxes that vary between 10 and 80 m 3 s −1 during the ocean pulses. Area-average upstream ice shelf melt rates from these fluxes are up to 170 m yr −1 , revealing that these strengthened plumes had already driven their most intense melting before arriving at the study site. Plain Language Summary Petermann Gletscher is a large glacier in northern Greenland that transports 4% of the ice sheet by area into the ocean. At its marine terminus it extends into a 16 km wide and 50 km long floating ice shelf. The ice shelf has retreated by 30% over the last decade, likely due to increasing melting of its base. Regions of faster melting result in channels carved into the ice base, where ice thicknesses are half that of the surrounding ice. Here, we investigate the cause of this melting using 6 months of oceanographic and glaciological measurements from August 2015 to February 2016 in the ice shelf's central basal channel. We find that under-ice ocean current speeds played the primary role in mixing ocean heat upward and controlling melting in the channel. During the first 3 months of data, freshwater periodically flowed from land into the ocean beneath the ice shelf, causing currents to accelerate and melting to increase. This behavior changed in the second 3 months, when freshwater discharge stopped and tides instead drove weaker currents and low melting. Hence, the timing of subglacial freshwater outflow into the ocean controlled when the ice shelf experienced the strongest melting in its central basal channel.

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Section: Discussioncontrasting

confidence: 67%

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

et al. 2020

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“…There is an abrupt increase in seafloor depth between the two sites (Horgan et al, ) that may excite diurnal topographic vorticity waves (Mueller et al, ). These tide‐topography interactions may be very sensitive to frequency (e.g., Sun et al, ) and thus may explain Feature 1. However, we would expect to detect these vorticity waves as enhanced diurnal currents at our site, which is less than 1 km away from the topography, given that these waves have slow radiation of energy away from the generating topography.…”

Section: Discussionmentioning

confidence: 99%

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

et al. 2020

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Mass loss from the Antarctic ice sheet is sensitive to conditions in ice shelf grounding zones, the transition between grounded and floating ice. To observe tidal dynamics in the grounding zone, we moored an ocean pressure sensor to Ross Ice Shelf, recording data for 54 days. In this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice, yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone. This implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kPa with respect to glaciostatic pressure at the ice shelf base. Phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides, an effect that has not been incorporated into ocean models of tidal currents below ice shelves. These tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters, according to linear elastic fracture mechanics. We find anomalously high tidal energy loss at the K 1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography. These observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub-ice shelf cavities.Plain Language Summary One of the challenges for sea level rise prediction is understanding how the Antarctic ice sheets and the Southern Ocean interact. Ocean tides are an important component of this interaction, influencing ice shelf melting and the flow rate of grounded ice toward the coast. We report new observations relevant to this interaction: tidally varying ocean pressures where the ice first goes afloat to become an ice shelf. These tidal ocean pressure variations influence tidal currents below the ice shelf, and we propose that they also push seawater beneath the ice inland of the ice shelf and extend fractures at the ice shelf base. This study identifies tidal processes that may affect melt and fracture near the inland edge of ice shelves, a highly sensitive zone for ice dynamics.

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“…This ponded water can disrupt the structural integrity of the ice shelf and make it vulnerable to hydrofracturing (Glasser and Scambos, 2008). Basal melt rates close to 0 in winter to 10 m a −1 in summer were observed near in the grounding zone, with large seasonal and tidal variations (Sun and others, 2019). These melt rates are controlled by topographic waves generated at the ice-shelf break.…”

Section: Eastern Dml (24°e To 45°e)mentioning

confidence: 99%

Characteristics of ice rises and ice rumples in Dronning Maud Land and Enderby Land, Antarctica

et al. 2020

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Ice rises and rumples, locally grounded features adjacent to ice shelves, are relatively small yet play significant roles in Antarctic ice dynamics. Their roles generally depend upon their location within the ice shelf and the stage of the ice-sheet retreat or advance. Large, long-stable ice rises can be excellent sites for deep ice coring and paleoclimate study of the Antarctic coast and the Southern Ocean, while small ice rises tend to respond more promptly and can be used to reveal recent changes in regional mass balance. The coasts of Dronning Maud Land (DML) and Enderby Land in East Antarctica are abundant with these features. Here we review existing knowledge, presenting an up-to-date status of research in these regions with focus on ice rises and rumples. We use regional datasets (satellite imagery, surface mass balance and ice thickness) to analyze the extent and surface morphology of ice shelves and characteristic timescales of ice rises. We find that large parts of DML have been changing over the past several millennia. Based on our findings, we highlight ice rises suitable for drilling ice cores for paleoclimate studies as well as ice rises suitable for deciphering ice dynamics and evolution in the region.

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Topographic Shelf Waves Control Seasonal Melting Near Antarctic Ice Shelf Grounding Lines

Cited by 28 publications

References 64 publications

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

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

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

Characteristics of ice rises and ice rumples in Dronning Maud Land and Enderby Land, Antarctica

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