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. For example, in 2016-2018, data have revealed a temporary reversal in the trend in the deep Scotia and Weddell seas. 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 underlying causes of the warming trend in AABW, and the temporary return of denser bottom waters, 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 13 times from 1995 to 2020. 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 northern limb of the Weddell Gyre. Annual occupations of this section are needed to disentangle the interannual variability separately from the long-term warming trend, and hence better understand the causes of both. In 2016-2021, this is being done as part of the ORCHESTRA programme.
At each station we lower a CTD (conductivity-temperature-depth) profiler mounted in a round frame (rosette) on a conducting wire from the surface to within 10 m of the seabed. As we raise the instruments, we remotely trigger spring-loaded bottles on the rosette (Niskin bottles) to close, letting us 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 such as oxygen isotope ratios, dissolved oxygen concentrations, and inorganic nutrients. 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.
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. https://doi.org/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. https://doi.org/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