Decay of eddies at the South-West Indian Ridge

  • Jonathan V. Durgadoo Department of Oceanography, University of Cape Town
  • Isabelle J. Ansorge Department of Oceanography, University of Cape Town
  • Beverly A. de Cuevas Marine Systems Modelling Group, National Oceanography Centre, University of Southampton
  • Johann R.E. Lutjeharms Department of Oceanography, University of Cape Town
  • Andrew C. Coward Marine Systems Modelling Group, National Oceanography Centre, University of Southampton
Keywords: eddy kinetic energy hotspot, Southern Ocean, DEIMEC, OCCAM, Prince Edward Islands


The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a high-resolution ocean model. The model’s representation of the dynamic nature of this region is fully characteristic of observations. On average, 3–4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5–2.5 months after genesis and have depths of 2000 m – 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature–salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour.

Author Biography

Jonathan V. Durgadoo, Department of Oceanography, University of Cape Town
Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR)


1. Froneman PW, Ansorge IJ, Vumazonke L, et al. Physical and biological variability in the Antarctic Polar Frontal Zone: Report on research cruise 103 of the MV SA Agulhas. S Afr J Sci. 2002;98:534–536.

2. Ansorge IJ, Lutjeharms JRE. Eddies originating at the South-West Indian Ridge. J Mar Syst. 2003;39:1–18.

3. Pollard RT, Read JF. Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge. J Geophys Res. 2001;106:2881–2898.

4. Durgadoo JV, Ansorge IJ, Lutjeharms JRE. Oceanographic observations of eddies impacting the Prince Edward Islands, South Africa. Antarc Sci. 2010;22:211–219.

5. Ansorge IJ, Lutjeharms JRE, Swart NC, Durgadoo JV. Observational evidence for a cross frontal heat pump in the Southern Ocean. Geophys Res Lett. 2006;33:L19601.

6. Bernard ATF, Ansorge IJ, Froneman PW, Lutjeharms JRE, Bernard KS, Swart NC. Entrainment of Antarctic euphausiids across the Antarctic Polar Front by a cold eddy. Deep Sea Res I. 2007;54:1841–1851.

7. Swart NC, Ansorge IJ, Lutjeharms JRE. Detailed characterisation of an Antarctic eddy in the subantarctic. J Geophys Res. 2008;113:C01009.

8. Ansorge IJ, Lutjeharms JRE. Twenty-five years of physical oceanographic research at the Prince Edward Islands. S Afr J Sci. 2000;96:557–565.

9. Williams RG. Modification of ocean eddies by air-sea interaction. J Geophys Res. 1988;93:15523–15533.

10. Pacanowski RC, Dixon K, Rosati A. The GFDL Modular Ocean Model user guide. No. 2. Princeton, NJ: GFDL Ocean Group; 1990.

11. Webb DJ, De Cuevas BA, Richmond CS. Improved advection schemes for ocean models. J Atmos Oceanic Technol. 1998;15:1171–1187.<1171:IASFOM>2.0.CO;2

12. Large WG, McWilliams JC, Doney SC. Ocean vertical mixing: A review and a model with a nonlocal boundary layer parameterisation. Rev Geophys. 1994;32:363–403.

13. Coward AC, De Cuevas BA. The OCCAM 66 level model: Physics, initial conditions and external forcing. Intern Doc no. 99. Southampton: Southampton Oceanography Centre; 2005.

14. Kalnay E, Kanamitsu M, Kistler R, et al. The NCEP/NCAR reanalysis project. Bull Am Meteorol Soc. 1996;77:437–495.<0437:TNYRP>2.0.CO;2

15. Spencer RW. Global oceanic precipitation from the MSU during 1979–91 and comparisons to other climatologies. J Clim. 1993;6:1301–1326.<1301:GOPFTM>2.0.CO;2

16. Xie P, Arkin PA. Analyses of global monthly precipitation using gauge observations, satellite estimates and numerical model predictions. J Clim. 1996;9:840–858.<0840:AOGMPU>2.0.CO;2

17. Rossow WB, Schiffer RA. ISCCP Cloud data products. Bull Am Meteorol Soc. 1991;72:2–20.<0002:ICDP>2.0.CO;2

18. Levitus S, Boyer TP, Conkright ME, et al. World ocean database volume 1: Introduction. NOAA Atlas NESDIS 18. Washington, DC: US Government Printing Office; 1998.

19. Isern-Fontanet J, Garcia-Ladona E, Font J. Identification of marine eddies from altimetric maps. J Atmos Oceanic Technol. 2003;20:772–778.<772:IOMEFA>2.0.CO;2

20. Jeong J, Hussain F, Font J. On the identification of a vortex. J Fluid Mech. 1995;285:69–94.

21. Park Y-H, Charriaud E, Fieux M. Thermohaline structure of the Antarctic surface water/winter water in the Indian sector of the Southern Ocean. J Mar Syst. 1998;17:5–23.

22. Whitworth T, Nowlin WD. Water masses and currents of the Southern Ocean at the Greenwich meridian. J Geophys Res. 1987;92:6462–6476.

23. Ansorge IJ, Lutjeharms JRE. Direct observations of eddy turbulence at a ridge in the Southern Ocean. Geophys Res Lett. 2005;32:L14603.

24. Morrow R, Birol F, Griffin D, Sudre J. Divergent pathways of cyclonic and anti-cyclonic ocean eddies. Geophys Res Lett. 2004;31:L24311.

25. Gordon AL, Taylor HW. Heat and salt balance within the cold waters of the world ocean. In: Numerical models of ocean circulation. Washington DC: National Academy of Sciences, 1975; p. 54–56.

26. De Szoeke R, Levine M. The advective flux of heat by mean geostrophic motions in the Southern Ocean. Deep Sea Res I. 1981;28:1057–1085.

27. Sloyan BM, Rintoul SR. The Southern Ocean limb of the global deep overturning circulation. J Phys Oceanogr. 2001;31:143–173.<0143:TSOLOT>2.0.CO;2

28. Nel DC, Lutjeharms JRE, Pakhomov EA, Ansorge IJ, Ryan PG, Klages NTW. Exploitation of mesoscale oceanographic features by grey-headed albatross Thalassarche chrysostoma in the southern Indian Ocean. Mar Ecol Prog Ser. 2001;217:15–26.