RRS Discovery Cruise DY113, 3 February – 13 March 2020. Repeat hydrographic measurements on GO-SHIP lines SR1b and A23

Cruise DY113 comprised occupations of two repeat hydrographic sections, SR1b across Drake Passage from Burdwood Bank to Elephant Island, and A23 from the northern Weddell Sea across the Scotia Sea to South Georgia. Ocean physical measurements are made on these two sections annually funded by NERC as National Capability, currently through the ORCHESTRA (Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports) programme, in order to monitor and understand variability of Antarctic Circumpolar Current transports and Antarctic Bottom Water properties and volumes. In addition to the 62 CTD/LADCP casts on SR1b and A23, a CTD survey was made over 17 sites in Cumberland Bay, South Georgia, and a section along the North Scotia Ridge also occupied on cruise JR299 was revisited, bringing the total to 104 CTD/LADCP casts including one test cast and one other repeat. Water column samples were collected for calibration of CTD salinity and dissolved oxygen (most stations) as well as for measurements of oxygen isotopes (SR1b, A23, Cumberland Bay), nutrient (N and Si) isotopes (SR1b), nutrient (NO2+NO3, NO3, Si, P; SR1b, A23) concentrations, microplastics (SR1b, A23, Cumberland Bay), and environmental DNA (SR1b). Standard underway measurements including underway surface ocean and meteorological data and upper ocean vessel-mounted current measurements were collected throughout, while multibeam swath bathymetry data was recorded on the transit between SR1b and A23 (south of the South Orkney Islands), in Cumberland Bay, and on previously-unsurveyed parts of the North Scotia Ridge transect and between there and the Falkland Islands. Four standard Argo autonomous profiling floats were also deployed, two on SR1b and two on A23