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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); σ0 ∼ 26.5) are exported—originating in the Atlantic and Agulhas Retroflection regions; (b) SW of Australia, where 6.5 Sv of medium SAMW ( σ0 ∼ 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 ( σ0 ∼ 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. [ABSTRACT FROM AUTHOR]

Subantarctic Mode Water (SAMW) is the name given to the relatively deep surface mixed layers found directly north of the Subantarctic Front in the Southern Ocean, and their extension into the thermocline as weakly stratified or low potential vorticity water masses. The objective of this study is to begin an investigation into the mechanisms controlling SAMW formation, through a heat budget calculation. ARGO profiling floats provide estimates of temperature and salinity typically in the upper 2,000 m and the horizontal velocity at various parking depths. These data are used to estimate terms in the mode water heat budget; in addition, mode water circulation is determined with ARGO data and earlier ALACE float data, and climatological hydrography. We find a rapid transition to thicker layers in the central South Indian Ocean, at about 70°S, associated with a reversal of the horizontal eddy heat diffusion in the surface layer and the meridional expansion of the ACC as it rounds the Kerguelen Plateau. These effects are ultimately related to the bathymetry of the region, leading to the seat of formation in the region southwest of Australia. Upstream of this region, the dominant terms in the heat budget are the air–sea flux, eddy diffusion, and Ekman heat transport, all having approximately equal importance. Within the formation area, the Ekman contribution dominates and leads to a downstream evolution of mode water properties. [ABSTRACT FROM AUTHOR]

The vertical structures of three warm-core Leeuwin Current eddies are described, as well as their temporal evolution determined from altimetric data. These warm-core eddies have large positive sea-level anomalies, although their surface temperature fields are not necessarily warmer than the ambient waters. These eddies were observed some 350 km offshore from the Leeuwin Current in September 2000, around 25–30°S, and their vertical structure and temporal evolution show they were formed at the coast in May, when the Leeuwin Current is strongest. After separation from the current, the warm-core eddies drifted WNW following isopycnal contours, and were strongly steered by bathymetry. These eddies penetrated to at least 1500 m depth; the strongest eddy, Eddy B, influenced isopycnals to 2500 m. Eddy B appeared to be trapped and intensified by a Leeuwin Current squirt that directed warm, low-salinity water offshore from August to October. The average annual eddy heat and salt fluxes for waters warmer than 8°C are estimated at 0.004 PW and 3×105 kg s−1, respectively. The magnitude of these eddy fluxes is 20–30% of the annual mean poleward heat and salt flux in the eastern Indian Ocean, and is within the range of values observed for Agulhas eddies entering the South Atlantic. [Copyright &y& Elsevier]

Abstract: Hydrological data from the western coast of Australia to 110°E are used together with direct velocity observations to investigate the water mass distribution and the associated circulation. High spatial variability reflects advective features related to the turbulent character of the Leeuwin region. The correlation between the sea surface temperature and the sea surface salinity distributions and the upper layer circulation depicts a southward warm and fresh Leeuwin Current along the coast with several branchings and three anticyclonic eddies offshore. The strongest eddy includes a thick surface mixed layer of 220m and perturbs the hydrological structure down to more than 2000m. The net transports through the four sections are northward in the upper layers with larger transports through the southern sections, which imply a westward transport just north of Abrolhos Islands. In the intermediate and deep-water layers, the net transport is southward across the southern section and divergent, implying an eastern inflow around 29°S. The data confirm that Subantarctic Mode Water is spreading north while the Antarctic Intermediate Water appears to recirculate to the south along the eastern rim of the Perth Basin. [Copyright &y& Elsevier]