Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf

Abstract

The instability and accelerated melting of the Antarctic Ice Sheet are among the foremost elements of contemporary global climate change1,2. The increased freshwater output from Antarctica is important in determining sea level rise1, the fate of Antarctic sea ice and its effect on the Earth’s albedo4,5, ongoing changes in global deep-ocean ventilation6, and the evolution of Southern Ocean ecosystems and carbon cycling7,8. A key uncertainty in assessing and predicting the impacts of Antarctic Ice Sheet melting concerns the vertical distribution of the exported meltwater. This is usually represented by climate-scale models3,4,5,6,7,8,9 as a near-surface freshwater input to the ocean, yet measurements around Antarctica reveal the meltwater to be concentrated at deeper levels10,11,12,13,14. Here we use observations of the turbulent properties of the meltwater outflows from beneath a rapidly melting Antarctic ice shelf to identify the mechanism responsible for the depth of the meltwater. We show that the initial ascent of the meltwater outflow from the ice shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. We use an idealized ocean circulation model to show that this mechanism is relevant to a broad spectrum of Antarctic ice shelves. Our findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models.

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Figure 1: Map of the study region.
Figure 2: Transect along the PIIS calving front.
Figure 3: Transect along the main outflow from the PIIS calving front.
Figure 4: Schematic of the meltwater outflow from beneath the PIIS.

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Acknowledgements

The iSTAR programme is supported by the Natural Environment Research Council of the UK (grants NE/J005703/1, NE/J005746/1 and NE/J005770/1). A.C.N.G. acknowledges the support of a Philip Leverhulme Prize, the Royal Society and the Wolfson Foundation. We are grateful to the scientific party, crew and technicians on the RRS James Clark Ross for their hard work during data collection.

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Authors

Contributions

A.C.N.G. and A.F. designed and conducted the data analysis, with contributions from P.D. and L.C.B. L.B. designed and conducted the idealized model experiments. K.J.H. led the JR294/295 research cruise. All authors contributed to the scientific interpretation of the results.

Corresponding author

Correspondence to Alberto C. Naveira Garabato.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks C. Stevens and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Tidal flows near the PIIS calving front.

Major axis of tidal ellipse for each individual tidal constituent from a mooring deployed in the area of the main meltwater outflow from the PIIS (Fig. 1), at nominal depths of 310 m (measured with an acoustic Doppler current profiler, black symbols) and 671 m (measured with a current meter, red symbols). Tidal ellipses are computed using harmonic analysis33. The amplitude of each ellipse’s major axis is shown by dots, and error estimates are displayed as bars. The 2-year-record-mean flow speed for each of the two instruments is indicated in the inset.

Extended Data Figure 2 Turbulent dissipation along the PIIS calving front.

Rate of dissipation of turbulent kinetic energy (ε, colour scale) along transects S1A and S1B, displayed as a function of potential temperature θ and salinity S. The loci of the three distinct water masses in the region are indicated by the labels in italics (CDW, Circumpolar Deep Water; GMW, glacially modified water; WW, Winter Water).

Extended Data Figure 3 Cross-transect velocity and relative vorticity along the main outflow from the PIIS calving front.

a, Cross-transect velocity (v, colour scale) along transect S2, with positive values directed northeastward. Potential temperature contours are shown at intervals of 0.2 °C (see Fig. 3a). b, Ratio of relative vorticity ζ to planetary vorticity f along the same transect (colour scale), where . Areas of positive potential vorticity (indicative of overturning instabilities) are outlined as in Fig. 3e. The outline shading denotes the instability type (GRV, gravitational; SYM, symmetric; CTF, centrifugal; see Methods). The characteristic vertical extent of the PIIS is shown by the grey rectangle at the right-hand axis of each panel.

Extended Data Figure 4 Assessment of geostrophic balance along the main outflow from the PIIS calving front.

a, Cross-transect velocity (v, colour scale) along transect S2, with positive values directed norththeastward. Neutral density (in kg m−3) is indicated by black contours. b, Profiles of transect-mean cross-transect velocity v and geostrophic velocity gvel, relative to the depth-averaged velocity.

Extended Data Figure 5 Initial condition in the Main simulation of an idealized meltwater outflow from beneath an Antarctic ice shelf.

The initial temperature distribution is shown by the colour shading, and the locations of the unstable restoring region (representing the meltwater outflow) and of the stable restoring region (representing ambient offshore conditions) are indicated.

Extended Data Figure 6 Evolution of the idealized meltwater outflow in the Main experiment.

Distributions of the along-domain velocity (a), relative vorticity normalized by the planetary vorticity (c), potential vorticity (e) and passive tracer concentration (g), 2 h after the start of the simulation. Panels b, d, f and h show the same variables as a, c, e and g, respectively, 120 h after the start of the simulation. Temperature contours are shown at intervals of 0.2 °C in all panels. Units are indicated next to the colour bars.

Extended Data Figure 7 Evolution of the idealized meltwater outflow in the Perturbation experiments with reduced and enhanced forcings.

Distributions of the along-domain velocity (a), relative vorticity normalized by the planetary vorticity (c), potential vorticity (e) and passive tracer concentration (g), 120 h after the start of the reduced forcing (0.5 K) simulation. Panels b, d, f and h show the same variables as a, c, e and g, respectively, 120 h after the start of the enhanced forcing (1.5 K) simulation. Temperature contours are shown at intervals of 0.2 °C in all panels. Units are indicated next to the colour bars.

Extended Data Figure 8 Vertical distribution of passive tracer on day 1 for varying planetary rotation.

a, Horizontally integrated tracer concentration (A) (normalized to a common value, A0) as a function of depth for the Main experiment with realistic rotation ( f = 1.4 × 10−4 s−1), and for the two Rotation experiments with weak ( f = 1 × 10−5 s−1) and no ( f = 0) rotation. b, Domain-mean tracer concentration (normalized to a common value) in temperature bins of width 0.05 K. The depth (a) or temperature (b) of neutral buoyancy for the simulated outflow is shown as a horizontal black line.

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Garabato, A., Forryan, A., Dutrieux, P. et al. Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf. Nature 542, 219–222 (2017). https://doi.org/10.1038/nature20825

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