FLOSTRAL

R. M. Morrow, LEGOS, Toulouse, France (Rosemary.Morrow@cnes.fr)

The FLOSTRAL project aims to study the formation, circulation and modification of Subantarctic Mode Waters (SAMW) in the southern Indian Ocean. SAMW forms in the deep winter mixed layers on the equatorward side of the Subantarctic Front (SAF). Figure 1 shows that the formation zone stretches from the southeast Indian Ocean right across the south Pacific Ocean, and even into the southwest Atlantic just east of Drake Passage.  Once formed in the deep winter mixed layers, these SAMW are subducted and plunge into the subtropical gyre, where they circulate at around 300-800 m depth.

SAMW have water mass properties which are cool and fresh with a high CO2 content and low potential vorticity. They are strong indicators of climate change and carbon uptake. As the SAMW circulate within the subtropical gyre, their properties are gradually modified so that they are slightly warmer and saltier when they exit the subtropical gyre.  

As part of the FLOSTRAL project, we aim to :

1. determine the water mass characteristics of SAMW in the southern Indian subtropical gyre – their spatial distribution, and whether they undergo temporal evolution (any seasonal or interannual variations in their water mass properties).
2. determine their circulation pathways, and the origins of the different classes of SAMW
3. study their formation regions (deep winter mixed layers north of the SAF). Which mechanisms are dominant in setting their temperature and salinity characteristics (air-sea fluxes, horizontal advection by Ekman or geotrophic currents, diffusion and vertical entrainment, …)
4. identify the regions where the mode water characteristics change, and investigate whether mixing by mesoscale eddies or internal waves can cause these modifications.

To resolve these questions, the FLOSTRAL project aims to use a combination of satellite (altimetry, SST) and in-situ observations (ARGO floats, CTD measurements, surface drifters) and numerical model outputs (DRAKKAR, MERCATOR) over the period 2003-2005. As part of the project 30 profiling PROVOR floats were deployed in 2003-2004 as part of the international ARGO programme.

 First Results

ARGO float data from 2003 have been used to determine the water mass characteristics of the deep winter mixed layers in the southern Indian Ocean.  Figure 2 shows that during winter, there is a region (80-110°E; 45-55°S) in the southeast Indian Ocean with very deep winter mixed layers (300 – 500 m depth).  This region lies north of the Subantarctic Front (SAF) and south of the Subtropical Front (STF). SAMW formed in this region have distinct water mass properties, known as SouthEast Indian SAMW (SEISMAW).