Preprints
https://doi.org/10.5194/os-2022-18
https://doi.org/10.5194/os-2022-18
 
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11 Apr 2022
11 Apr 2022
Status: a revised version of this preprint was accepted for the journal OS and is expected to appear here in due course.

Exceptional freshening and cooling in the eastern subpolar North Atlantic caused by reduced Labrador Sea surface heat loss

Alan D. Fox1, Arne Biastoch2,3, Stuart A. Cunningham1, Neil Fraser1, Patricia Handmann2, N. Penny Holliday4, Clare Johnson1, Torge Martin2, Marilena Oltmanns4, Willi Rath2, Siren Rühs2,a, Alejandra Sanchez-Franks4, and Christina Schmidt2,b Alan D. Fox et al.
  • 1Scottish Association for Marine Science, Oban, UK
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 3Kiel University, Kiel, Germany
  • 4National Oceanography Centre, Southampton, UK
  • anow at: Institute for Marine and Atmospheric research Utrecht, Utrecht University, Netherlands
  • bnow at: Climate Change Research Centre and the Australian Centre for Excellence in Antarctic Science, University of New South Wales, Sydney, NSW, Australia

Abstract. Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Published theories for the mechanisms driving the freshening include: reduced transport of saltier, warmer surface waters northwards from the subtropics associated with reduced meridional overturning; shifts in the pathways of fresher, cooler surface water from the Labrador Sea driven by changing patterns of wind stress; and the eastward expansion of the subpolar gyre. Using output from a high-resolution hindcast model simulation, we propose that the primary cause of the exceptional freshening and cooling is reduced surface heat loss in the Labrador Sea. Tracking virtual fluid particles in the model backwards from the eastern subpolar North Atlantic between 1990 and 2020 shows the major cause of the freshening and cooling to be an increased outflow of relatively fresh and cold surface waters from the Labrador Sea; with a minor contribution from reduced transport of warmer, saltier surface water northward from the subtropics. The cooling, but not the freshening, produced by changing proportions of source waters is mitigated by reduced along-track heat loss to the atmosphere in the North Atlantic Current. We analyse modelled boundary exchanges and water mass transformation in the Labrador Sea to show that since 2000, while inflows of lighter surface waters remain steady, the increasing output of these waters is due to reduced surface heat loss in the Labrador Sea beginning in the early 2000s. Tracking particles further upstream reveals the primary source of the increased volume of lighter water transported out of the Labrador Sea is increased recirculation of water, and therefore longer residence times, in the upper 500–1000 m of the subpolar gyre.

Alan D. Fox et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2022-18', Anonymous Referee #1, 05 May 2022
    • AC1: 'Reply on RC1', Alan Fox, 05 Jul 2022
  • RC2: 'Comment on os-2022-18 - Reviewer 2', Anonymous Referee #2, 12 May 2022
    • AC2: 'Reply on RC2', Alan Fox, 05 Jul 2022
  • RC3: 'Comment on os-2022-18', Anonymous Referee #3, 15 May 2022
    • AC3: 'Reply on RC3', Alan Fox, 05 Jul 2022
  • EC1: 'Comment on os-2022-18', Karen J. Heywood, 19 Jun 2022
  • EC2: 'Comment on os-2022-18', Karen J. Heywood, 05 Jul 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2022-18', Anonymous Referee #1, 05 May 2022
    • AC1: 'Reply on RC1', Alan Fox, 05 Jul 2022
  • RC2: 'Comment on os-2022-18 - Reviewer 2', Anonymous Referee #2, 12 May 2022
    • AC2: 'Reply on RC2', Alan Fox, 05 Jul 2022
  • RC3: 'Comment on os-2022-18', Anonymous Referee #3, 15 May 2022
    • AC3: 'Reply on RC3', Alan Fox, 05 Jul 2022
  • EC1: 'Comment on os-2022-18', Karen J. Heywood, 19 Jun 2022
  • EC2: 'Comment on os-2022-18', Karen J. Heywood, 05 Jul 2022

Alan D. Fox et al.

Alan D. Fox et al.

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Executive editor
In this paper, a new hypothesis is put forward to explain the remarkable freshening of the Subpolar North Atlantic that peaked in 2016. The hypothesis emphasizes the increased surface circulation in the Labrador Sea region, following a reduction in heat loss to the atmosphere during the period 2000-2013. This new hypothesis, supported by the analysis of a very high resolution ocean model, contrasts with other studies that concluded to an overwhelming role of the reduced Atlantic Meridional Overturning Circulation in the freshening. In the present study, the AMOC decline is shown to account for not more than 27% of the freshening. Overall, this paper points out that surface properties of the Subpolar North Atlantic Ocean, which are important for European climate, cannot be understood in terms of AMOC variability only, but rather depend on the horizontal circulation at the ocean surface and on regional air-sea exchanges.
Short summary
Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Using output from a high-resolution hindcast model simulation, we propose that the primary cause is reduced surface cooling in the Labrador Sea, leading to increased outflow of relatively fresh and cold water in the upper ocean and so to freshening and cooling of the eastern subpolar North Atlantic.