Research ArticleOCEANOGRAPHY

Highly variable upper and abyssal overturning cells in the South Atlantic

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Science Advances  05 Aug 2020:
Vol. 6, no. 32, eaba7573
DOI: 10.1126/sciadv.aba7573
  • Fig. 1 Array locations and simple schematic of water mass overturning transports.

    Study area with shaded colors representing oxygen concentrations (μmol kg−1) at the surface and along the 34.5°S SAMBA line from the World Ocean Atlas 2018 annual climatology (38). The tongues of elevated oxygen concentrations along the western and eastern boundaries identify different branches of recently ventilated NADW as it crosses 34.5°S (white contours). Locations of the nine SAMBA moorings used for computing the zonally integrated meridional geostrophic flow are represented as red squares. Conceptual schematics of the volume transport across 34.5°S and of the upper and abyssal Atlantic overturning cells are indicated on the left side of the panel. Colors indicate the classes of seawater density (red, shallow and intermediate; green, deep; blue, abyssal).

  • Fig. 2 Volume transports within the upper and abyssal cells: Mean structure and temporal variability.

    (A) Estimate of the September 2013 to July 2017 time-mean basin-wide vertically integrated (or cumulative) volume transport as a function of depth at 34.5°S with the upper cell highlighted in red and the abyssal cell in blue. The gray layer at the bottom of the panel represents the layer of unobserved flow below the deepest common depth of the interior moorings. The red and blue dots highlight the time-mean pressure interfaces where the zonally integrated meridional flow changes from northward to southward within the upper and abyssal cells, respectively. The black dot indicates the transition between the upper and abyssal cells. (B) Temporal anomalies of the upper cell transport (defined positive northward). (C) Temporal anomalies of the abyssal cell transport (defined positive southward). (B and C) Gray shading indicates the estimated daily accuracy as derived in the Supplementary Materials. Vertical dashed lines (B and C) illustrate events affecting both cells (July 2015) or only one of the cells (June 2016). Horizontal dashed lines (B and C) indicate anomalies exceeding the time-mean value by ±1 standard deviation; positive anomalies correspond to stronger cell transports, and negative anomalies indicate weaker transports. Sv, sverdrup.

  • Fig. 3 Energy distributions and seasonal anomalies of the MOC volume transports.

    Variance-preserving spectra of the upper cell (A, red) and abyssal cell (B, blue) volume transports. Spectra were determined using Welch’s averaged periodogram method and a 2-year window allowing 1 year of overlap. Red and blue shading areas indicate the 95% confidence limits based on the methods in (29), with colors matching the corresponding time series. Vertical black dashed lines indicate the annual, semi-annual, and 3-month periods (from left to right). Shaded gray area illustrates the 12- to 20-day period band, with high energy in the upper cell and low energy in the abyssal cell. Seasonal cycle of the upper cell (C, positive northward) and abyssal cell (D, positive southward) volume transport time series. Error bars indicated in the upper left represent ±1 standard error (see the Supplementary Materials for details), with colors matching the corresponding time series.

Supplementary Materials

  • Supplementary Materials

    Highly variable upper and abyssal overturning cells in the South Atlantic

    M. Kersalé, C. S. Meinen, R. C. Perez, M. Le Hénaff, D. Valla, T. Lamont, O. T. Sato, S. Dong, T. Terre, M. van Caspel, M. P. Chidichimo, M. van den Berg, S. Speich, A. R. Piola, E. J. D. Campos, I. Ansorge, D. L. Volkov, R. Lumpkin, S. L. Garzoli

    Download Supplement

    This PDF file includes:

    • Data and methods
    • Transports accuracy estimates
    • Fig. S1
    • Tables S1 and S2
    • References

    Files in this Data Supplement:

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