Research ArticleOCEANOGRAPHY

Warm Circumpolar Deep Water transport toward Antarctica driven by local dense water export in canyons

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Science Advances  01 May 2020:
Vol. 6, no. 18, eaav2516
DOI: 10.1126/sciadv.aav2516
  • Fig. 1 Model simulation of the source locations and descending pathways of dense water.

    Surface water mass transformation across σ1 = 32.57 kg/m3 is shown in red colors. Bottom speed is shown in green colors in the descending pathways of dense water (where bottom density σ4 >46.105 kg/m3). Blue background shading shows bathymetry deeper than 4000 m. The black line represents the 1000-m isobath contour.

  • Fig. 2 Circumpolar overview of water mass transports across the 1000-m isobath.

    (A) The 1000-m isobath contour, with distances (in 103 km) around Antarctica marked with green circles. The blue box shows the Ross Sea region analyzed in Fig. 3. (B) Net offshore transport across the 1000-m isobath, cumulatively integrated upward through density space. The horizontal lines and colored labels show water mass definitions. (C) Transport across the 1000-m isobath, cumulatively summed around Antarctica. Colors show transport in different water masses (AASW, Antarctic Surface Water; CDW, Circumpolar Deep Water; DSW, Dense Shelf Water; and the total sum). Distances on the x axis are marked on the map in (A). The gray shading in (C) represents the regions where DSW descends the continental slope.

  • Fig. 3 DSW thickness and cross-slope transports in the Ross Sea.

    (A) Average thickness of DSW, showing the location of the overflows aligned with the three canyon features. The blue line shows the 1000-m isobath contour. Black lines show the bathymetry in 1000-m intervals, and gray lines show the 500-m bathymetry contour. (B) Downslope transport across the 1000-m isobath in the Ross Sea. A 100-km smoothing filter has been applied to the transports. Dark lines indicate a 10-year average in the control simulation, and pale lines show a 2-year average in the freshwater perturbation simulation.

  • Fig. 4 Spatial and temporal correlations between CDW and DSW transport in the Ross Sea.

    (A) Average potential temperature at 312-m depth. (B) Average meridional velocity at 312-m depth. Brown (green) shows downslope (upslope) flow. Note that the meridional velocity weakens when the overflow pathway deviates from north/south, as expected. The DSW overflow pathways (DSW thickness >240 m) are shown in (A) and (B) by blue hatched contours. Red contours in (A) and (B) show old waters representative of CDW approaching the shelf (normalized ideal age = 0.85; see Material and Methods for definition). (C) A 1-year time series of DSW and CDW transport crossing the 1000-m isobath in the Glomar Challenger Trough (3-hour temporal resolution). The gray lines have been smoothed over a 10-day window to highlight the lower-frequency variability. The correlation coefficient (r) is for the raw (3-hour frequency) data, with P < 0.01.

  • Fig. 5 Demonstration of canyon exchange mechanism.

    (A) The schematic view shows a canyon cross section looking toward the open ocean. Dense water hugs the left of the canyon as it descends and lowers the sea surface height above (exaggerated scale). The sea surface variation results in a barotropic pressure gradient that drives AASW and CDW onshore to the east. Directions of flow are indicated by arrow heads and tails. Interfacial form stress transfers momentum upward from the DSW layer to the CDW and AASW layers above. (B) Relationship between the 1000-m cross-isobath velocity in the CDW layer and the sea surface height gradient along the 1000-m isobath in the Glomar Challenger Trough. Each point shows a 3-hour average over a 1-year time period. The dashed line shows the expected geostrophic velocity resulting from the barotropic pressure gradient. Downslope velocities are defined as positive. The correlation coefficient (r) between ∂η/∂x and the simulated model velocity is given. (C) The along-isobath cumulative sum of the interfacial form stress divergence summed over middepths (300 to 700 m, blue), compared with cross-isobath water mass transports summed over isopycnal layers.

Supplementary Materials

  • Supplementary Materials

    Warm Circumpolar Deep Water transport toward Antarctica driven by local dense water export in canyons

    A. K. Morrison, A. McC. Hogg, M. H. England, P. Spence

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