Ocean Dynamics

, Volume 60, Issue 3, pp 563–583 | Cite as

Origin and mechanism of Subantarctic Mode Water formation and transformation in the Southern Indian Ocean

  • Ariane Koch-LarrouyEmail author
  • Rosemary Morrow
  • Thierry Penduff
  • Mélanie Juza


The sources and pathways of mode waters and lower thermocline waters entering the subtropical gyre of the Indian Ocean are examined. A Lagrangian analysis is performed on an eddy-admitting simulation of the Global Ocean performed by the DRAKKAR Group (NEMO/OPA), which captures the main observed features. We trace the subducted mode water’s pathways, identify their formation regions and trace whether their source waters come from the Atlantic, Pacific or Indian sectors of the Southern Ocean. Three main sites for mode waters ventilation in the Indian sector are identified with different circulation pathways and source water masses: (a) just north of Kerguelen, where 4.2 Sv of lighter Subantarctic Mode Waters (SAMW); σ 0 ∼ 26.5) are exported—originating in the Atlantic and Agulhas Retroflection regions; (b) SW of Australia, where 6.5 Sv of medium SAMW (σ 0 ∼ 26.6) are ventilated—originating in the southern and denser Agulhas Retroflection region; (c) SW of Tasmania and along the South Australian coast, where 3 Sv of denser SAMW (σ 0 ∼ 26.75) are ventilated—originating from three sources: Leeuwin Current waters, Tasman Sea (Pacific) waters and Antarctic Surface Waters. In all cases, modelled mode waters were last ventilated in the Indian Ocean just north of the deepest winter-mixed layers. For the waters subducted SW of Australia, the last ventilation site extends even further north. Waters ventilated in the deepest mixed layers north of the Subantarctic Front are then re-ventilated 5 years later southwest of Australia. The model results raise new hypotheses that revisit the classical picture of the SAMW formation and transformation, where a large homogeneous mixed layer is subducted and ‘slides’ equatorward, essentially maintaining the T/S characteristics acquired at the surface. Firstly, the last ventilation of the modelled mode waters is not in the region of the deepest mixed layers, as previously thought, but further north in regions of moderate meso-scale eddy activity. Secondly, the model shows for the first time a significant source region for Indian Ocean mode waters coming from deep winter-mixed layers along the south Australian coast. Finally, this analysis shows how the mode water characteristics are modified after subduction, due to internal eddy mixing. The simulation shows that resolved eddies have a strong impact on the mixed layer properties and that isopycnal eddy mixing also contributes to the generation of more homogeneous mode water characteristics in the interior.


Subantarctic mode water Formation Transformation Eddy mixing OGCM ARGO Winter mixed layers Pathways Southern Indian Ocean 


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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Ariane Koch-Larrouy
    • 1
    Email author
  • Rosemary Morrow
    • 1
  • Thierry Penduff
    • 2
    • 3
  • Mélanie Juza
    • 2
  1. 1.LEGOSToulouse Cedex 09France
  2. 2.Laboratoire des Ecoulements Géophysiques et IndustrielsGrenoble Cedex 9France
  3. 3.Department of OceanographyFlorida State UniversityTallahasseeUSA

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