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Abyssal ocean overturning shaped by seafloor distribution

Abstract

The abyssal ocean is broadly characterized by northward flow of the densest waters and southward flow of less-dense waters above them. Understanding what controls the strength and structure of these interhemispheric flows—referred to as the abyssal overturning circulation—is key to quantifying the ocean’s ability to store carbon and heat on timescales exceeding a century. Here we show that, north of 32° S, the depth distribution of the seafloor compels dense southern-origin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2.5 kilometres. Unless ventilated from the north, the overlying mid-depths (1 to 2.5 kilometres deep) host comparatively weak mean meridional flow. Backed by analysis of historical radiocarbon measurements, the findings imply that the geometry of the Pacific, Indian and Atlantic basins places a major external constraint on the overturning structure.

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Figure 1: Density surfaces, seafloor areas and the ocean’s overturning.
Figure 2: Depth and density distributions of seafloor area over 32° S–48° N.
Figure 3: Pacific seafloor and radiocarbon distributions.
Figure 4: Sketch of a volume V of waters denser than γ, bounded by the density surface A(γ) and latitudes ys and yn.
Figure 5: Density fluxes and dianeutral transports within 32° S–48° N.
Figure 6: Schematic abyssal overturning circulation north of 32° S.

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Acknowledgements

We thank P. Barker and J. Dunn for their help with the mapping and for providing the distance look-up tables. We also thank J. Nycander, A. Melet, M. Nikurashin and J. Goff for sharing their published datasets. C.d.L., R.M.H. and T.J.McD. gratefully acknowledge Australian Research Council support through grant FL150100090.

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C.d.L. designed and conducted the analysis. G.M., F.R., R.M.H. and T.J.McD. contributed to analysis and presentation choices and to the scientific interpretation of results. C.d.L. prepared the manuscript. G.M., F.R., R.M.H. and T.J.McD. assisted in the writing.

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Correspondence to C. de Lavergne.

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

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Extended data figures and tables

Extended Data Figure 1 Bottom density contrasts and basin masks.

a, Shaded bottom neutral density field, with the 4 km bathymetric contour overlaid in black. b, Basin masks employed for Extended Data Figs 2, 3, 4. The dianeutral circulation, which is essentially confined to the near-bottom, can largely be tracked from the bottom density distribution. The four main northward paths of AABW are identified in the western Indian, eastern Indian, western Pacific and western Atlantic oceans. The southeastern Pacific, east of the East Pacific Rise, hosts inflow of Circumpolar Deep Water71,72 (see Extended Data Fig. 2). Large bottom density differences across connected, AABW-ventilated sub-basins largely reflect the efficient consumption of the densest through- and overflowing waters at deep straits and sills. The Chain (CFZ), Romanche (RFZ) and Vema (VFZ) fracture zones linking the abyssal eastern and western Atlantic are indicated.

Extended Data Figure 2 Seafloor and radiocarbon distributions of the whole Pacific and the southeastern Pacific.

The whole Pacific (shown in ac) comprises the main Pacific (shown in Fig. 3) and the southeastern Pacific (shown in df) domains defined in Extended Data Fig. 1b. a, d, Zonally summed seafloor areas as a function of latitude and depth. b, e, Zonally summed incrop areas as a function of latitude and pseudo-depth. c, f, Along-density zonal mean radiocarbon content (Δ14C) as a function of latitude and pseudo-depth. Panels with a pseudo-depth y axis have density contoured in black every 0.1 kg m−3 for γ ≥ 27.5 kg m−3. White curves depict the local northward–southward and diabatic–adiabatic transition levels inferred from the incrop area distribution, as in Fig. 3. The cluster of large incrop areas within 28–28.06 kg m−3 south of 20° N seen in b originates in the southeastern Pacific, which hosts a secondary circulation branch71,72 separate from the main abyssal overturning cell (Methods).

Extended Data Figure 3 Indian seafloor and radiocarbon distributions.

a, d, g, j, Zonally summed seafloor areas as a function of latitude and depth. b, e, h, k, Zonally summed incrop areas as a function of latitude and pseudo-depth. c, f, i, l, Along-density zonal mean radiocarbon content (Δ14C) as a function of latitude and pseudo-depth. Note that the colour scales differ from those in Fig. 3. Panels with a pseudo-depth y axis have density contoured in black every 0.1 kg m−3 for γ ≥ 27.5 kg m−3. White curves in all panels except gi depict the local northward–southward and diabatic–adiabatic transition levels inferred from the incrop area distribution, as in Fig. 3. In the Central Indian Basin (gi), whose abyss is not fed from the south but instead through gaps in the Ninety East Ridge, the white curves correspond to the peak and weak incrop density surfaces based on the total sub-basin incrop profile. Sub-basin masks are shown in Extended Data Fig. 1b.

Extended Data Figure 4 Atlantic seafloor and radiocarbon distributions.

a, d, Zonally summed seafloor areas as a function of latitude and depth. b, e, Zonally summed incrop areas as a function of latitude and pseudo-depth. c, f, Along-density zonal mean radiocarbon content (Δ14C) as a function of latitude and pseudo-depth. Notice that the colour scales differ from those in Fig. 3. Panels with a pseudo-depth y axis have density contoured in black every 0.1 kg m−3 for γ ≥ 27.5 kg m−3. In the western Atlantic (ac), white curves depict the local northward–southward and diabatic–adiabatic transition levels inferred from the incrop area distribution, as in Fig. 3. In the eastern Atlantic (df), whose abyss is not fed from the south but instead through ridge gaps in the vicinity of the Equator, white curves correspond to peak and weak incrop density surfaces based on the total sub-basin incrop profile. Sub-basin masks are shown in Extended Data Fig. 1b.

Extended Data Figure 5 Simplified sketch of the density transformation associated with scenarios S1 and S2.

a, Scenario S1 is the case of a uniform geothermal density sink or, equivalently, of a mixing-driven density flux that is uniform in the interior. Density loss occurs in a thin bottom boundary layer. The net density loss within a density layer, and therefore the dianeutral transport across it, is proportional to the layer’s incrop area. b, Scenario S2 is the case of bottom-enhanced turbulence where density loss in a thin bottom layer is compensated by an equal and opposite density gain above that layer. Density loss and density gain layers are sketched with equal thicknesses to illustrate this compensation: a density layer undergoes net density loss if the red area dominates over the blue area, and conversely. As an approximate rule, a density layer loses density in proportion to its incrop area but gains density in proportion to the incrop area of its underlying neighbour. Consequently, the larger the increase (or decrease) of incrop area with height, the larger the net density loss (or gain) of a density layer, and therefore the larger the net upwelling (or downwelling) transport across it.

Extended Data Figure 6 Bottom-intensified mixing scenarios.

32° S–48° N density profiles of the total density flux (a, d), the density flux averaged over density surfaces (b, e) and total dianeutral transports (c, f), under various scenarios for the local intensity of bottom-intensified mixing. Specifically, we set the magnitude of local density-flux profiles proportional to (see also labels and Methods): characteristics of the large-scale topography (ac, orange and red); characteristics of small-scale abyssal hills (ac, dark and pale blue); bottom energy fluxes into internal waves (df, orange and red); and powers of the bottom buoyancy frequency N (df, dark and pale blue). Here ‘slope2’ refers to the squared topographic slope. Absolute values of density fluxes are chosen such that the peak upwelling rate is 25 × 106 m3 s−1 for all scenarios. Only the structure—as opposed to the magnitude—of dianeutral transports thus warrants interpretation.

Extended Data Figure 7 Silicic acid distributions.

Along-density zonal mean silicic acid concentration41 as a function of latitude and pseudo-depth in the main Pacific (a), southeastern Pacific (b), western Indian (c), Arabian (d), central Indian (e), eastern Indian (f), western Atlantic (g) and eastern Atlantic (h) domains. Density is contoured in black every 0.1 kg m−3 for γ ≥ 27.5 kg m−3. White curves mark the peak and weak incrop levels, as in Extended Data Figs 2, 3, 4. Domains are defined in Extended Data Fig. 1b.

Extended Data Figure 8 Example radiocarbon map and underlying data.

Δ14C mapped (a) and measured (b) on the 28.045 kg m−3 neutral density surface. Dark grey patches in a are portions of the density surface that are unmapped owing to insufficient data density. Similar maps have been constructed for 140 density surfaces spanning the world ocean’s neutral density range13.

Extended Data Figure 9 Sub-basin profiles of incrop area.

Total incrop area as a function of density (ac) and pseudo-depth at 32° S (df), shown for labelled subdomains of the Pacific (a, d), Atlantic (b, e) and Indian (c, f) oceans. The abyssal Arabian and central Indian sub-basins exchange with the Southern Ocean via the western and eastern Indian oceans, respectively. We therefore use the 32° S pseudo-depth of the western and eastern Indian oceans for the (orange) Arabian and (pale blue) central Indian profiles in f, respectively. Sub-basin masks are shown in Extended Data Fig. 1b.

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de Lavergne, C., Madec, G., Roquet, F. et al. Abyssal ocean overturning shaped by seafloor distribution. Nature 551, 181–186 (2017). https://doi.org/10.1038/nature24472

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