The global ocean and climate systems are strongly influenced by physical oceanographic processess within the Southern Ocean1. In particular, the exchange of water between subtropical North Atlantic Deep Water and the Antarctic Circumpolar Current controls the rate at which the latter upwells and mixes2. Despite its significance, the details of this exchange are poorly understood. Acoustic imaging of the water column can reveal the detailed thermohaline structure3. Here we present a subsurface acoustic image, acquired in October 1998, that crosses the Sub-Antarctic Front in the South Atlantic Ocean, where the two water masses converge and shear past each other. We find that down to a depth of 2.5 km, the vertical boundary between the North Atlantic Deep Water and the Antarctic Circumpolar Current is sharp and mass exchange is negligible. Below this depth, where cross-track velocities converge, we detect a prominent swirling structure that is 500 m high and 10 km wide. We analyse prestack acoustic records, which suggest that this structure rotates at an average speed of 0.3±0.1 m s−1 about a horizontal axis. We suggest that the structure could either be a thermohaline intrusion created by frontal instability processes, or—more speculatively—a localized and intermittent overturning event.
Subscribe to Journal
Get full journal access for 1 year
only $15.58 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Marshall, J. & Speer, K. Closure of the meridional overturning circulation through Southern Ocean upwelling. Nature Geosci. 5, 171–180 (2012).
Jullion, L., Heywood, K. J., Naveira Garabato, A. C. & Stevens, D. P. Circulation and water mass modification in the Brazil–Malvinas confluence. J. Phys. Oceanogr. 40, 845–864 (2010).
Ruddick, B., Song, H., Dong, C. & Pinheiro, L. Water column seismic images as maps of temperature gradient. Oceanography 22, 192–205 (2009).
Orsi, A. H., Whitworth, T. III, Worth, D. & Nowlin, W. D. Jr On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea. Res. I 42, 641–673 (1995).
Naveira Garabato, A. C., Polzin, K. L., King, B. A., Heywood, K. J. & Visbeck, M. Widespread intense turbulent mixing in the Southern Ocean. Science 303, 210–213 (2004).
Piola, A. R. & Georgi, D. Circumpolar properties of Antarctic Intermediate Waters and Subantarctic Mode Water. Deep Sea Res. 29, 687–711 (1982).
Peterson, R. G. & Whitworth, T. III The Subantarctic and Polar Fronts in relation to deep water masses through the Southwestern Atlantic. J. Geophys. Res. 94, 10817–10838 (1989).
Arhan, M., Heywood, K. J. & King, B. A. The deep waters from the Southern Ocean at the entry to the Argentine Basin. Deep-Sea Res. II 46, 475–499 (1999).
Klaeschen, D., Hobbs, R. W., Krahmann, G., Papenberg, C. & Vsemirnova, E. Estimating movement of reflectors in the water column using seismic oceanography. Geophys. Res. Lett. 36, L00D03 (2009).
Sheen, K. L., White, N. J., Caulfield, C. P. & Hobbs, R. W. Estimating geostrophic shear from seismic images of oceanic structure. J. Atmos. Oceanic Technol. 28, 1149–1154 (2011).
Pinheiro, L.M. et al. Detailed 2-D imaging of the Mediterranean outflow and meddies off W Iberia from multichannel seismic data. J. Marine. Syst. 79, 89–100 (2010).
Krahmann, G., Brandt, P., Klaeschen, D. & Reston, T. Mid-depth internal wave energy off the Iberian Peninsula estimated from seismic reflection data. Geophys. Res. Lett. 113, C12016 (2008).
Sheen, K. L., White, N. J. & Hobbs, R. W. Estimating mixing rates from seismic images of oceanic structure. Geophys. Res. Lett. 36, L00D04 (2009).
Bower, A. S., Rossby, H. T. & Lillibridge, J. L. The Gulf Stream—barrier of blender? J. Phys. Oceanogr. 15, 24–32 (1985).
Sundermeyer, M. A., Ledwell, J. R., Oakey, N. S. & Greenan, B. J. W. Stirring by small-scale vortices caused by patchy mixing. J. Phys. Oceanogr. 35, 1245–1262 (2005).
Smith, K. S. & Ferrari, R. The production and dissipation of compensated thermohaline variance by mesoscale stirring. J. Phys. Oceanogr. 39, 2477–2501 (2009).
Woods, J. D., Onken, R. & Fischer, J. Thermohaline intrusions created isopycnally at oceanic fronts are inclined to isopycnals. Nature 322, 446–449 (1986).
MacVean, M. K. & Woods, J. D. Redistribution of scalars during upper ocean frontogenesis: A numerical model. Q. J. R. Meteorol. Soc. 106, 293–311 (1980).
Klein, P., Treguier, A-M. & Hua, B. L. Three-dimensional stirring of thermohaline fronts. J. Marine Res. 56, 589–612 (1998).
Arhan, M., Carton, X., Piola, A. & Zenk, W. Deep lenses of circumpolar water in the Argentine Basin. J. Geophys. Res. 107, 3007 (2002).
Riley, J. J. & Lindborg, E. Stratified turbulence: A possible interpretation of some geophysical turbulence measurements. J. Atmos. Sci. 65, 2416–2424 (2008).
Lindborg, E. The energy cascade in a strongly stratified fluid. J. Fluid Mech. 550, 207–242 (2006).
Brethouwer, G., Billant, P., Lindborg, E. & Chomaz, J-M. Scaling analysis and simulation of strongly stratified turbulent flows. J. Fluid Mech. 585, 343–368 (2007).
Molemaker, M. J & McWilliams, J. C. Local balance and cross-scale flux of available potential energy. J. Fluid Mech. 645, 295–314 (2010).
Kunze, E., Firing, E., Hummon, J. M., Chereskin, T. K. & Thurnherr, A. M. Global abyssal mixing inferred from lowered ADCP shear and CTD strain profiles. J. Phys. Oceanogr. 36, 1553–1576 (2006).
Wells, M., Cenedese, C. & Caulfield, C. P. The relationship between flux coefficient and entrainment ratio in density currents. J. Phys. Oceanogr. 40, 2713–2727 (2010).
Thorpe, S. A. Turbulence and mixing in a Scottish loch. Phil. Trans. R. Soc. Lond. 286A, 125–181 (1977).
Dillon, T. M. Vertical overturns: A comparison of Thorpe and Ozmidov length scales. J. Geophys. Res. 87, 9601–9613 (1982).
Alford, M. H. Sustained, full-water-column observations of internal waves and mixing near mendicino escarpment. J. Phys. Oceanogr. 40, 2643–2660 (2010).
Klymak, J. M. & Moum, J. N. Oceanic isopycnal slope spectra. Part II: Turbulence. J. Phys. Oceanogr. 37, 1232–1245 (2007).
K.L.S. was supported by the NERC UK and by Schlumberger Cambridge Research. Seismic acquisition was financially supported by the NERC. We thank P. Christie, A. Crosby, T. Hesse, K. Heywood, R. Jones, S. Jones, D. Koenitz, D. Lyness, A. Naveira Garabato, C. Richardson, S. Thorpe and C. Trowell for their help. S. Jones and R. Hardy discussed methods of velocity analysis with us. J. B. Sallée provided an incisive review. Department of Earth Sciences contribution number esc.2511.
The authors declare no competing financial interests.
About this article
Cite this article
Sheen, K., White, N., Caulfield, C. et al. Seismic imaging of a large horizontal vortex at abyssal depths beneath the Sub-Antarctic Front. Nature Geosci 5, 542–546 (2012). https://doi.org/10.1038/ngeo1502
Scientific Reports (2019)
Nature Communications (2019)
Characterization of the submesoscale energy cascade in the Alboran Sea thermocline from spectral analysis of high-resolution MCS data
Geophysical Research Letters (2016)
Carbon isotope offsets between benthic foraminifer species of the genusCibicides(Cibicidoides) in the glacial sub-Antarctic Atlantic
GEOPHYSICAL ANALYSIS OF ABNORMAL SEISMIC (OCEANOGRAPHY) REFLECTION CHARACTERISTICS OF OCEANIC BOTTOM BOUNDARY LAYER
Chinese Journal of Geophysics (2016)