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Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean

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Science Advances  25 Jan 2017:
Vol. 3, no. 1, e1601426
DOI: 10.1126/sciadv.1601426
  • Fig. 1 Location of stations from the three I08S occupations in the southeastern Indian Ocean (filled squares), local bottom depths (blue to brown shading), and mean eddy kinetic energy field (rainbow map).

    The dashed line is a schematic view of AABW circulation based on the studies by Rintoul (5) and Orsi et al. (16). Bottom depths are 2-min bathymetry (40). Eddy kinetic energy (EKE) field is from 20 years of multisatellite altimetry data. Only EKE values larger than 250 cm2/s2 are plotted. South of 63°S, the 1994 I08S occupation is augmented by data from the Australian SR03C line. AAB, Australian-Antarctic Basin.

  • Fig. 2 Regional long-term changes in temperature and salinity.

    Mean differences in conservative temperature (ΔΘ) (A) and Absolute Salinity (ΔSa) (B) between 1994 and 2016. Red curves represent the PET region (64°S to 60°S), green curves represent the Australian-Antarctic Basin (60°S to 45°S), and blue curves represent the SAB (45°S to 30°S). Shading indicates 95% confidence intervals (CIs) on the means.

  • Fig. 3 Spatiotemporal patterns of the AABW rates of changes.

    Rates of change in conservative temperature (ΔΘ) (A and B), Absolute Salinity (ΔSa) (C and D), and potential density referenced to 4000 dbar (Δσ4) (E and F) in the AABW domain. Left and right columns show differences between 2007 to 2016 and 1994 to 2016, respectively. AABW domain is defined as the region where θ < 0°C in any I08S occupation. (A and B) Black curves are σ4 = 45.9 and 46.1 kg/m3 for 2016. Blue curves are the same isopycnals for 2007 in (A) and 1994 in (B). Thick green curves indicate γn = 28.27 kg/m3 in 2016 (left plots) and 1994 (right plots). Gray shading indicates bottom topography.

  • Fig. 4 AABW changes north of 64°S.

    Histograms of AABW changes north of 64°S (includes both the PET and the Australian-Antarctic Basin) for conservative temperature (ΔΘ) and Absolute Salinity (ΔSa) between 2007 and 2016 (A and C) and 1994 and 2016 (B and D). Blue indicates negative changes (cooling/freshening), red indicates positive changes (warming/salinification), and white indicates small/zero change. The y axis shows the percentage of grid cells that fall into each interval (bin) (x axis) normalized by the total number classified as AABW (θ < 0°C).

  • Fig. 5 Conservative temperature–Absolute Salinity (Θ − Sa) relationships in 1994 (green), 2007 (orange), and 2016 (pink) in the Australian-Antarctic Basin (60°S to 45°S).

    Colored solid curves indicate means computed on isopycnals, and dashes are the respective minima and maxima. Black curves are σ4 densities. Gray shading indicates Θ < 0°C (AABW region).

  • Fig. 6 AABW changes in the Australian-Antarctic Basin by decades.

    Changes in conservative temperature (ΔΘ) (A) and Absolute Salinity (ΔSa) (B) for the AABW in the Australian-Antarctic Basin (60°S to 45°S). Reddish shadings are used for the differences between 2007 and 2016, and blue curves are used for differences between 1994 and 2007. Vertical black lines mark zero change. (C) Changes in Absolute Salinity between 2007 and 2016 and between 1994 and 2007 averaged (density space) over the Australian-Antarctic Basin. Dashed curves are 95% CIs on the means.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/1/e1601426/DC1

    table S1. Details about the I08S (95°E) occupations in the Indian sector of the Southern Ocean.

    table S2. AABW mean conservative temperature, Absolute Salinity, and respective envelopes (minimum and maximum values) in 1994, 2007, and 2016 for the PET (64°S to 60°S) and the Australian-Antarctic Basin (60°S to 45°S).

    fig. S1. Snapshots of the Australian-Antarctic Basin: Sea surface height anomaly and absolute geostrophic currents from multisatellite altimetry.

    fig. S2. Rates of change in conservative temperature and Absolute Salinity in density space (neutral density).

    fig. S3. Rates of change in steric height in the Australian-Antarctic Basin.

  • Supplementary Materials

    This PDF file includes:

    • table S1. Details about the I08S (95°E) occupations in the Indian sector of the Southern Ocean.
    • table S2. AABW mean conservative temperature, absolute salinity, and respective envelopes (minimum and maximum values) in 1994, 2007, and 2016 for the PET (64°S to 60°S) and the Australian-Antarctic Basin (60°S to 45°S).
    • fig. S1. Snapshots of the Australian-Antarctic Basin: Sea surface height anomaly and absolute geostrophic currents from multisatellite altimetry.
    • fig. S2. Rates of change in conservative temperature and absolute salinity in density space (neutral density).
    • fig. S3. Rates of change in steric height in the Australian-Antarctic Basin.

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