Research ArticleOCEANOGRAPHY

Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water

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Science Advances  18 Apr 2018:
Vol. 4, no. 4, eaap9467
DOI: 10.1126/sciadv.aap9467
  • Fig. 1 Water properties on the Sabrina Coast.

    Map (B) of the Sabrina Coast [red rectangle in (A)] with bathymetry and coastline overlaid (81). Oceanographic stations where conductivity-temperature-depth (CTD) and oxygen isotope measurements have been collected are shown in red, whereas moorings T1, T2, and T3 are shown in blue. Black dashed lines are contours of the 2014 annual sea ice production (in meters) in the Dalton Polynya (24). Time series of conservative temperature Θ and absolute salinity SA, low-pass filtered using a fourth-order 40-hour Butterworth filter, are shown for T1 (C and D), T2 (E and F), and T3 (G and H) with indicated depths. A 30-day low-pass filter is shown in black dashed lines for salinity time series at the shallowest instrument of each mooring, whereas the thin black lines represent SA = 34.55 g kg−1 (that is, upper salinity limit for WW). The black dashed lines in (C), (E), and (G) are the surface freezing temperatures for a salinity of 34.4 g kg−1.

  • Fig. 2 Freshwater input on the Sabrina Coast.

    (A) Vertical profiles of conservative temperature Θ, oxygen isotope δ18O, and absolute salinity SA from station 21 [black square in (C)]. The vertical black (red) dashed line represents SA18O) used to estimate the meteoric water fraction relative to deep WW (see Materials and Methods). The dashed blue line is Θ = −1.75°C, the upper temperature limit for WW. (B) Vertical profiles at station 21 of the fractions of meteoric water (magenta), sea ice melt (green), and meteoric water relative to deep WW (gray) (see Material and Methods). (C) Vertically averaged meteoric water fraction above the MCDW layer. The black dashed line delimits the Dalton Polynya, defined as the area where the annual sea ice production is larger than 3 m. (D) Meters of meteoric water. (E) Same as (C) but relative to deep WW. (F) Meters of meteoric water accumulated above the MCDW layer since the commencement of mixed-layer retreat.

  • Fig. 3 Mixed-layer evolution in Antarctic polynyas.

    Modeled temporal evolution of mixed-layer depth (A) and absolute salinity SA (B) in the Dalton Polynya. In red, we show the case with no meteoric water included. In black, meteoric water is included by reducing the surface salt flux by 35%. Shaded areas represent uncertainty in the model output related to uncertainty in sea ice production [±25% (37)], whereas blue bars indicate the range of observed WW properties. (C and D) Same as (A) and (B) for the Amundsen Polynya. Surface salt flux is reduced by 75% in the black line case. Blue bars are based on WW variability in different years (6, 12, 25). (E and F) Same as (A) and (B) for the Cape Darnley Polynya but only showing the no–meteoric water case because it reproduces observations of DSW formation (20). Note that the y axis stops at the full depth of the ocean, different in (A), (C), and (E) (figs. S1 to S3).

  • Fig. 4 Impact of glacial meltwater on dense water formation and shelf stratification.

    On warm continental shelves, as those on the Sabrina Coast and in the Amundsen Sea (A), MCDW drives rapid ice shelf basal melt. The large volume of glacial meltwater prevents DSW formation in polynyas downstream of the meltwater outflow. MCDW remains in the bottom layer throughout the year in the polynya and further downstream, where it can access the ice shelf cavities. On cold continental shelves, the ice shelf cavities are filled by cold shelf waters, and basal melt rates are low. Glacial meltwater input is not sufficient to suppress winter convection in polynyas downstream of the meltwater outflow, as seen at Cape Darnley Polynya (B), allowing formation of DSW, the precursor to Antarctic Bottom Water.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Dalton Polynya.
    • fig. S2. Amundsen Polynya.
    • fig. S3. Cape Darnley Polynya.
    • table S1. Moorings on the Sabrina Coast.
    • Reference (82)

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