The densest waters in the Atlantic overturning circulation, Antarctic Bottom Water (AABW), originate in the Weddell Sea, as Weddell Sea Deep Water. They circulate within the Weddell Sea, and some can escape the Weddell Sea and flow into the Scotia Sea, from where they can penetrate into the Atlantic.
These dense waters are changing rapidly. In the Atlantic, AABW has been observed to be warming rapidly, with measurable warming having reached the North Atlantic in the space of just a few decades. Significant interannual variability is also present in AABW layers, superposed on the long-term warming trend. This makes clear identification of the warming more difficult in some places, and adds a layer of complexity to the system which needs to be understood. The causes of the warming trend in AABW have so far not been determined, primarily due to a lack of data with which to analyse the problem.
As part of the BAS Polar Oceans team’s long-term monitoring programme, we are repeating a hydrographic section annually. This section was first occupied in 1995 as part of the World Ocean Circulation Experiment, where the cruise was designated A23; this name has stuck, and is used for the subsequent repeats. In total, parts of the section have been occupied nine times from 1995 to 2015. Our current efforts concentrate on the northern part of the section, from the northern Weddell Sea, crossing South Scotia Ridge, and finishing at the shelf break near South Georgia. This is an ideal location for monitoring long-term changes in AABW as it leaves the Weddell Sea, and as it circulates within the Weddell Gyre. Annual occupations of this section are needed to disentangle the interannual variability separately from the long-term warming trend, and hence understand the causes of both.
At each station we lower a CTD (conductivity-temperature-depth) profiler on a wire from the surface to within 10 m of the seabed. As we raise the instruments, we close bottles to obtain water samples from different depths to calibrate the conductivity sensors, vital for measuring minute changes in salinity, and occasionally to measure other chemical parameters. We also have a Lowered Acoustic Doppler Current Profiler (LADCP) attached to the rosette, giving us a snapshot of the currents throughout the water column at each station.
|Year||Cruise metadata and cruise report||Ship||Chief Scientist|
|1995||JR010||RRS James Clark Ross||Karen Heywood & Brian King|
|1999||JR040||RRS James Clark Ross||Karen Heywood|
|2005||33RO200501||RV Ronald H. Brown||Rik Wanninkhof & Scott Doney|
|2010||JR235||RRS James Clark Ross||Mike Meredith|
|2012||JR272A||RRS James Clark Ross||Hugh Venables|
|2013||JR281 (JR272B)||RRS James Clark Ross||J-B Sallée|
|2013-2014||33RO20131223||RV Ronald H. Brown||Rik Wanninkhof & Leticia Barbero|
|2014||JR299 (JR272C)||RRS James Clark Ross||Andrew Meijers|
|2015||JR310 (JR272D)||RRS James Clark Ross||Povl Abrahamsen|
|2016||JR15006||RRS James Clark Ross||Andrew Meijers|
Johnson, G. C., and S. C. Doney (2006), Recent western South Atlantic bottom water warming, Geophys. Res. Lett., 33, L14614, doi:10.1029/2006GL026769.
Jullion, Loïc, Naveira Garabato, Alberto C., Meredith, Michael P., Holland, Paul R., Courtois, Peggy, King, Brian A., (2013) Decadal freshening of the Antarctic Bottom Water exported from the Weddell Sea. Journal of Climate, 26. 8111-8125. doi:10.1175/JCLI-D-12-00765.1
Meredith, Michael P., Naveira Garabato, Alberto C., Gordon, Arnold L., Johnson, Gregory C., (2008) Evolution of the deep and bottom waters of the Scotia Sea, Southern Ocean, during 1995-2005. Journal of Climate, 21. 3327-3343. doi:10.1175/2007JCLI2238.1
Meredith, Michael P., Juillon, Loic, Brown, Peter J., Naveira Garabato, Alberto C., Couldrey, Matthew P., (2014) Dense waters of the Weddell and Scotia Seas: recent changes in properties and circulation. Philosophical Transactions of the Royal Society of London, A, 372. 20130041. doi:10.1098/rsta.2013.0041