The main scientific objectives are to study:
- the seasonal and interannual variability of the upper ocean heat content and the heat balance in the Southern Ocean
- the dynamics of the different thermal and haline fronts
- the transport and structure of the Antarctic Circumpolar Current
- the surface and subsurface salinity, and its relation to the freshwater budget of the Southern Ocean
- the variations of surface eddy heat, salt and momentum fluxes across the ACC
This programme, based on high density XBT measurements and continuous surface thermosalinograph data started in 1992-93, and an integral part of CLIVAR. There are very few choke points in the Southern Hemisphere at which to measure the circulation of the Southern Ocean, and the inter-ocean exchange between the Pacific, the Indian and the Atlantic Oceans. The three main choke points are at Drake Passage between Tierra Del Fuego and the Antarctic Peninsula, south of South Africa and south of Tasmania.
Systematic high, density XBT launching, reaching a depth of 800 metres, is performed at the beginning, middle and end of each austral summer. XBTs are launched during three return trips of the Astrolabe:
- R0: mid October - early November : in late spring during the start of the summer heating cycle.
- R1: December or R2: January, during mid summer.
- R4: mid February - early March : in late summer, at the start of the autumn cooling.
One or two observers onboard deploy the XBT probes every 2 hours (25 naut. miles or 40 km) from 44°S to 66°S, and every hour in the frontal zone from 48°S to 54°S (12 naut. miles or 20 km).
Surface salinity and temperature are recorded continuously using a Seabird Thermosalinograph during all 5 return trips of the ASTROLABE between 15 October and 7 March.
XBT measurements started in October 1992, and thermosalinograph measurements started in October 1993.
SURVOSTRAL surface salinity data distribution between 1993 and 2004.