A23 repeat section

A23 repeat section

Start date
1 January, 2001
End date
17 July, 2425

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.

YearCruise metadata and cruise reportShipChief Scientist
1995JR010RRS James Clark RossKaren Heywood & Brian King
1999JR040RRS James Clark RossKaren Heywood
200533RO200501RV Ronald H. BrownRik Wanninkhof & Scott Doney
2010JR235RRS James Clark RossMike Meredith
2012JR272ARRS James Clark RossHugh Venables
2013JR281 (JR272B)RRS James Clark RossJ-B Sallée
2013-201433RO20131223RV Ronald H. BrownRik Wanninkhof & Leticia Barbero
2014JR299 (JR272C)RRS James Clark RossAndrew Meijers
2015JR310 (JR272D)RRS James Clark RossPovl Abrahamsen
2016JR15006RRS James Clark RossAndrew Meijers

 

The circulation of Weddell Sea Deep Water (yellow arrows) in the Weddell & Scotia seas, based on a figure from Meredith et al. (2008). The part of the A23 section that is repeated annually is highlighted in red
The circulation of Weddell Sea Deep Water (yellow arrows) in the Weddell & Scotia seas, based on a figure from Meredith et al. (2008). The part of the A23 section that is repeated annually is highlighted in red

 

Caption: Sections of potential temperature from the nine first occupations of the A23 section.
Caption: Sections of potential temperature from the nine first occupations of the A23 section.

References:

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

Povl Abrahamsen

Observational Oceanographer

Polar Oceans team

Andrew Meijers

Deputy Science Leader, Polar Oceans

Polar Oceans team