Formation of subantarctic mode water in the southeastern Indian Ocean

Abstract

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.

Aspects of the Antarctic Circumpolar Current dynamics investigated with drifter data

“…Positive (negative) values represent a density increase (decrease). As expected from the eddy 20 here that topography induces EHF important for local mode water production that adds up to the larger scale fluxes induced by air-sea fluxes (Sallée et al, 2006).…”

“…Vertical diffusion was discovered to be important in subtropical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12 al., 1999). In the weakly stratified Southern Ocean, isopycnals are almost vertical, so lateral eddy mixing becomes more relevant (Follows and Marshall, 1994;Marshall, 1997;Sallée et al, 2006). In previous observation-based studies of water-mass formations, the contribution of eddies was estimated from parameterization (Marshall, 1997;Sallée et al, 2006Sallée et al, , 2008aSallée et al, , 2010.…”

“…In the weakly stratified Southern Ocean, isopycnals are almost vertical, so lateral eddy mixing becomes more relevant (Follows and Marshall, 1994;Marshall, 1997;Sallée et al, 2006). In previous observation-based studies of water-mass formations, the contribution of eddies was estimated from parameterization (Marshall, 1997;Sallée et al, 2006Sallée et al, , 2008aSallée et al, , 2010. In this work, we estimated the EHF and their contribution to water-mass formation directly from drifter observations.…”

“…13; e.g., Sallée et al, 2006). Then, we infer subduction rate by averaging eddy heat fluxes over outcrop areas and computing the associated divergence and convergence of volume in a given outcrop (see Eqs.…”

“…Eddy contribution to water-mass transformation and subduction has often been ignored because of the difficulties in estimating their amount over a broad range of scales from direct measurements. However, Sallée et al (2006Sallée et al ( , 2008a underlined how it can play a relevant role in local mixed layer heat balance, and more recent papers showed that eddy heat fluxes (EHF) play a role in the subduction of water masses, contributing to set the global ocean ventilation and stratification (Karsten and Marshall, 2002b;LaCasce and Isachsen, 2010;Sallée et al, 2010).…”