The ANDREX project seeked to assess the role of the Weddell gyre in driving the southern closure of the meridional overturning circulation, in ventilating the deep global ocean, and in sequestering carbon and nutrients in the global ocean abyss.

To do this, the ANDREX team, a group of German collaborators and a group of US collaborators set out to obtain measurements of hydrography, velocity and a range of ventilation tracers and biogeochemical substances along the Weddell gyre’s rim. These measurements were organized into three surveys: a US CLIVAR section (a WOCE I6S repeat) between South Africa and Antarctica along 30°E in January–February 2008; the ANDREX section proper along the gyre’s northern edge between 30°E and the tip of the Antarctic Peninsula, in December–February 2009. The ANDREX section was aborted approximately half-way due to a medical evacuation, and was completed in March–April 2010 on board the RRS James Clark Ross. Finally, the box is close by the German Sr04 section (January–April 2005) in the Southwestern Weddell Sea between Kapp Norvegia and the tip of the Antarctic Peninsula.

ANDREX logo

NOC PI: Prof. Sheldon Bacon

Dates
–
Funding

NERC

Aim

Specifically, ANDREX aims to use a combination of inverse estimation techniques and tracer and biogeochemical analyses to:

  • Quantitatively assess the import / export routes of Circumpolar Deep Water / Antarctic Bottom Water (AABW) to / from the Weddell gyre.
  • Obtain an optimal, self-consistent estimate of the rates of ventilation and AABW formation in the Weddell gyre.
  • Quantify the heat and freshwater budgets of the Weddell gyre, and determine the freshwater inputs to the gyre (sea ice melt, precipitation, and glacial ice).
  • Quantify the nutrient and carbon cycles of the Weddell gyre.
  • Determine the rate of sequestration of anthropogenic carbon into the deep Weddell gyre.
  • Investigate the extent to which ventilation in, and AABW export from, the Weddell gyre have varied on interannual to decadal time scales.

ANDREX has produced the following key results:

  • The Antarctic Bottom Water (AABW) exported from the Weddell Sea has been steadily freshening over the last decade, at least in part as a result of increased glacial freshwater input in the region off the disintegrating Larsen Ice Shelf.
  • The AABW entering the eastern Weddell gyre from the Indian sector has warmed and lightened over the last decade, seemingly as a result of enhanced entrainment of warm mid-depth waters off the Amery Ice Shelf.
  • The mechanism by which the wind controls the properties of AABW exported from the Weddell gyre centrally involves modulation of turbulent diapycnal mixing in the Orkney Passage.
  • Our view of the circulation of the Weddell Sea and its climatic role has been significantly revised: the region plays a lesser role in the formation of Antarctic Bottom Water than previously thought, and instead recycles and exports Antarctic Bottom Water formed in the Indian sector of the Southern Ocean.
  • The Weddell gyre has for the first time been shown to be a significant sink of natural and anthropogenic carbon.

Publications

Palmer, M., D. Gomis, M. Flexas, G. Jordà, L. Jullion, T. Tsubouchi and A.C. Naveira Garabato, (2012) Water mass pathways and transports over the South Scotia Ridge west of 50°W. Deep Sea Research I, 59: 8–24. https://doi.org/10.1016/j.dsr.2011.10.005

Meredith, M.P., A.L. Gordon, A.C. Naveira Garabato, P. Abrahamsen, B.A. Huber, L. Jullion and H.J. Venables (2011) Synchronous intensification and warming of Antarctic Bottom Water outflow from the Weddell Gyre. Geophysical Research Letters, 38: L03603. https://doi.org/10.1029/2010GL046265

Jullion, L., S.C. Jones, A.C. Naveira Garabato and M.P. Meredith (2010) Wind-controlled export of Antarctic Bottom Water from the Weddell Sea. Geophysical Research Letters, 37: L09609. https://doi.org/10.1029/2010GL042822

Jullion, L., A.C. Naveira Garabato, M.P. Meredith, P.A. Holland, P. Courtois and B.A. King (2013) Decadal Freshening of the Antarctic Bottom Water Exported from the Weddell Sea. Journal of Climate, 26(20): 8111–25. https://doi.org/10.1175/JCLI-D-12-00765.1