Preprints
https://doi.org/10.5194/os-2021-14
https://doi.org/10.5194/os-2021-14

  10 Feb 2021

10 Feb 2021

Review status: this preprint is currently under review for the journal OS.

An overlooked freshwater source contributed to the extreme freshening event in the eastern subpolar North Atlantic after 2014

Bogi Hansen1, Karin Margretha Húsgarð Larsen1, Hjálmar Hátún1, Steingrímur Jónsson2, Sólveig Rósa Ólafsdóttir3, Andreas Macrander3, William Johns4, N. Penny Holliday5, and Steffen Malskær Olsen6 Bogi Hansen et al.
  • 1Faroe Marine Research Institute, Tórshavn, Faroe Islands
  • 2University of Akureyri, Akureyri, Iceland
  • 3Marine and Freshwater Research Institute, Hafnarfjörður, Iceland
  • 4Department of Ocean Sciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, FL, USA
  • 5National Oceanography Centre, Southampton, UK
  • 6Research and Development, Danish Meteorological Institute, Copenhagen, Denmark

Abstract. Outflows of low-salinity waters from the Arctic to the upper layers of the subpolar North Atlantic (SPNA) are central in redistributing freshwater from river runoff, melting sea ice, and precipitation. They act to reduce shallow, as well as deep, convection; thereby affecting both biological production and the Atlantic Meridional Overturning Circulation. The two main sources of low-salinity water to the SPNA are the flows through the Canadian Arctic Archipelago and through the Denmark Strait. A potential additional source of low-salinity water is the shelf/slope region south of Iceland, mainly fed by Icelandic runoff. Normally this water passes into the Nordic Seas, but in some periods, it may instead flow into the upper layers of the central parts of the Iceland Basin in the eastern SPNA. This low-salinity water has previously been overlooked as a freshwater supply to the SPNA. Using a range of observational data sets, we show that the conditions for a diversion of this water mass from the south Iceland shelf into the Iceland Basin were favourable during the 2014–2018 period. In those years the Iceland Basin became extraordinarily fresh, characterized by surface salinity lower than previously seen in a 120-year long time series. The event is thought to have been mainly caused by unusual winter wind stress patterns that diverted freshwater from the western SPNA to the eastern basin and caused a zonal shift of the subpolar front. Here, we show that the low-salinity signal near the surface was locally reinforced in the central Iceland Basin by anomalous diversion of low-salinity water originating in the shallow shelf areas south of Iceland and that this can help explain why the surface salinity of the Iceland Basin became so exceptionally low. The diversion was generated by anomalous wind conditions over the Iceland Basin and caused slightly enhanced freshening of the warm waters crossing the Greenland-Scotland Ridge from the SPNA into the Nordic Seas. The low-salinity Icelandic-source water also increased the near-surface stratification and reduced the depth of convection in the Iceland Basin during two consecutive winters with reduced nutrient renewal of near-surface waters as a consequence. Although especially pronounced after 2014, this extra freshwater input probably occurs more generally, which may help explain why the central Iceland Basin may be an oligotrophic region, as has previously been suggested.

Bogi Hansen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-14', Anonymous Referee #1, 05 Mar 2021
    • AC2: 'Reply on RC1', Bogi Hansen, 09 Jun 2021
  • RC2: 'Comment on os-2021-14', Anonymous Referee #2, 13 Apr 2021
    • AC1: 'Reply to both referees', Bogi Hansen, 27 Apr 2021
    • AC3: 'Reply on RC2', Bogi Hansen, 09 Jun 2021

Bogi Hansen et al.

Bogi Hansen et al.

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Short summary
Compared to other freshwater sources, runoff from Iceland is small and usually flows into the Nordic Seas. Under certain wind conditions, it can, however, flow into the Iceland Basin and this occurred after 2014, when this region had already freshened from other causes. This explains why the surface freshening in this area became so extreme. The local and shallow character of this runoff allows it to have a disproportionate effect on vertical mixing, winter convection, and biological production.