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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 02 Nov 2018

Research article | 02 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

Svein Østerhus1, Rebecca Woodgate2, Héðinn Valdimarsson3, Bill Turrell4, Laura de Steur5, Detlef Quadfasel6, Steffen M. Olsen7, Martin Moritz6, Craig M. Lee2, Karin Margretha H. Larsen8, Steingrímur Jónsson9, Clare Johnson10, Kerstin Jochumsen6, Bogi Hansen8, Beth Curry2, Stuart Cunningham10, and Barbara Berx4 Svein Østerhus et al.
  • 1NORCE Norwegian Research Centre, Bergen, Norway
  • 2University of Washington, Seattle, USA
  • 3Marine and Freshwater Research Institute, Reykjavik, Iceland
  • 4Marine Scotland Science, Marine Laboratory, Aberdeen, UK
  • 5Royal Netherlands Institute for Sea Research NIOZ, Texel, The Netherlands
  • 6Institut für Meereskunde, Universität Hamburg, Germany
  • 7Danish Meteorological Institute, Copenhagen, Denmark
  • 8Faroe Marine Research Institute, Tórshavn, Faroe Island
  • 9Marine and Freshwater Research Institute & University of Akureyri, Iceland
  • 10Scottish Association for Marine Science, Oban, UK

Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Into this region, water enters through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow) and then modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat, salt, and other substances into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, has the observational coverage become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993–2015. The total AM-import (oceanic inflows plus freshwater) is found to be 9.1±0.7Sv (1Sv=106m3s−1) and has a seasonal variation of amplitude close to 1Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remaining two thirds leaving as overflow. The overflow is mainly produced from modified Atlantic inflow and around 70% of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater, but is still ~2/3rds from modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s and this allows us to estimate trends for the AM-exchanges as a whole. At the 95% level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM-inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM-exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the Meridional Overturning Circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM-exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflows through the Denmark Strait, the overflow across the Iceland-Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

Svein Østerhus et al.
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Svein Østerhus et al.
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Short summary
Two decades of observations of the Arctic Mediterranean (AM) exchanges shows that the exchanges have been stable in terms of volume transport during a period when many other components of the global climate system have changed. The total AM-import is found to be 9.1 Sv and has a seasonal variation of amplitude close to 17 Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remain two thirds leaving.
Two decades of observations of the Arctic Mediterranean (AM) exchanges shows that the exchanges...