Orkney Passage Long Term Monitoring

The densest waters in the Atlantic overturning circulation, Antarctic Bottom Water (AABW), originate in the Weddell Sea, as Weddell Sea Deep Water. A large proportion is exported northward to the Scotia Sea through a gap in the South Scotia Ridge called Orkney Passage.

The circulation of Weddell Sea Deep Water (yellow arrows) in the Weddell & Scotia seas, based on a figure from Meredith et al. (2008). The location of the Orkney Passage moorings is shown with red +’s.

As part of the BAS Polar Oceans team’s long-term monitoring programme, we have been observing this outflow using moored instruments since 2007, in collaboration with Lamont-Doherty Earth Observatory , who have been observing this region since 2004. We currently have four moorings across Orkney passage.

Each mooring consists of an anchor on the seabed, attached to a length of chain that is connected to a release mechanism. Above the release are lengths of wire, with glass and foam spheres for buoyancy. The moorings are designed to cover the extent of Weddell Sea Deep Water in the area; they do not extend to the surface. Since we cannot transmit data to the surface, we only receive data when we recover the moorings every second year. At the same time, we take measurements with the ship’s conductivity-temperature-depth (CTD) profiler across Orkney Passage, both at the mooring sites and between them.

The moorings are fitted with current meters to measure the flow of water through the passage, along with temperature and temperature/conductivity loggers to measure the properties – and changes in the properties – of the water.

Our long-term aim is to monitor the volume and properties of Weddell Sea Deep Water exported through Orkney Passage and gain better understanding of this region’s role in the large-scale ocean circulation. In the shorter term, we are also trying to improve our understanding of the shorter-term drivers on outflow and small-scale mixing processes around Orkney Passage. As part of this goal, the DynOPO project, ran from 2015 to 2019, deploying additional instruments on the existing moorings, as well as an additional mooring in 2015. These were recovered in 2017, when further measurements of mixing within Orkney Passage were made using instruments deployed from the ship and on NOC’s Autosub Long Range autonomous underwater vehicle (AUV).