The data analysed in this article are all publicly available, with the exception of updates to the UW Calibrated O 2 package, described below:

Ocean bathymetry data were obtained from the ETOPO1 1 Arc-Minute Global Relief Model114 at https://doi.org/10.7289/V5C8276M (accessed February 2017).

Sea ice concentration data were obtained for the period 1972–1977 from the NSIDC Nimbus-5 ESMR v1 product53 at https://doi.org/10.5067/W2PKTWMTY0TP (accessed February 2017); for the period 1978–2017 from the merged NASA Goddard v3 product54 at https://doi.org/10.7265/N59P2ZTG (accessed October 2018); for January 2018–February 2019 from the NOAA/NSIDC Near-Real-Time CDR v1 product56 at https://doi.org/10.7265/N5FF3QJ6 (accessed February 2019); and for the period 2002–2019 from the University of Bremen ASI AMSR-E and AMSR2 v5 products57,58 at https://seaice.uni-bremen.de/sea-ice-concentration/ (accessed February 2019).

Profiling float temperature and salinity measurements were obtained from the US-GODAE GDAC70 at http://www.usgodae.org/ftp/outgoing/argo (accessed October 2018). Dissolved oxygen measurements for floats 5904468 and 5904471 were obtained from the SOCCOM quality-controlled archive74 at https://doi.org/10.6075/J02J6968 (accessed January 2019) and for float 5903616 from the UW Calibrated O 2 package, v1.175 at http://runt.ocean.washington.edu/o2 (accessed August 2016), with updated dissolved oxygen profiles provided by R. Drucker (personal communication, August 2017).

Shipboard and elephant seal temperature and salinity measurements were obtained from the World Ocean Database 2018 prerelease77 with August 2018 additions at http://www.nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html (accessed October 2018).

Gridded climatological ocean temperature fields were obtained from the 2018 WAGHC91 at http://icdc.cen.uni-hamburg.de/1/daten/ocean/waghc (accessed January 2018).

The monthly SAM index98 was obtained for the period 1972–2019 at http://legacy.bas.ac.uk/met/gjma/sam.html (accessed February 2019).

Monthly and daily ERA-I atmospheric reanalysis fields99 were obtained for the period 1979–2018 using the Python MARS API, described at https://confluence.ecmwf.int/display/WEBAPI/ (accessed February 2019).

Queen Maud Land pressure records (see Methods section ‘Meteorological station records’) were obtained from the READER archive110 at http://legacy.bas.ac.uk/met/READER (accessed February 2019) and the NOAA-NCEI ISD111 at http://www.ncdc.noaa.gov/isd (accessed February 2019).