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Arctic freshwater fluxes: sources, tracer budgets and inconsistencies

Arctic freshwater fluxes: sources, tracer budgets and inconsistencies
Arctic freshwater fluxes: sources, tracer budgets and inconsistencies

The net rate of freshwater input to the Arctic Ocean has been calculated in the past by two methods: directly, as the sum of precipitation, evaporation and runoff, an approach hindered by sparsity of measurements, and by the ice and ocean budget method, where the net surface freshwater flux within a defined boundary is calculated from the rate of dilution of salinity, comparing ocean inflows with ice and ocean outflows. Here a third method is introduced, the geochemical method, as a modification of the budget method. A standard approach uses geochemical tracers (salinity, oxygen isotopes, inorganic nutrients) to compute "source fractions" that quantify a water parcel's constituent proportions of seawater, freshwater of meteoric origin, and either sea ice melt or brine (from the freezing-out of sea ice). The geochemical method combines the source fractions with the boundary velocity field of the budget method to quantify the net flux derived from each source. Here it is shown that the geochemical method generates an Arctic Ocean surface freshwater flux, which is also the meteoric source flux, of 200±44 mSv  (1 Sv = 10^6 m^3 s^-1), statistically indistinguishable from the budget method's 187±44 mSv, so that two different approaches to surface freshwater flux calculation are reconciled. The freshwater export rate of sea ice (40±14 mSv) is similar to the brine export flux, due to the "freshwater deficit" left by the freezing-out of sea ice (60±50 mSv). Inorganic nutrients are used to define Atlantic and Pacific seawater categories, and the results show significant non-conservation, whereby Atlantic seawater is effectively "converted" into Pacific seawater. This is hypothesized to be a consequence of denitrification within the Arctic Ocean, a process likely becoming more important with seasonal sea ice retreat. While inorganic nutrients may now be delivering ambiguous results on seawater origins, they may prove useful to quantify the Arctic Ocean's net denitrification rate. End point degeneracy is also discussed: multiple property definitions that lie along the same "mixing line" generate confused results.

1994-0416
2111-2131
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e
Bacon, Sheldon
1e7aa6e3-4fb4-4230-8ba7-90837304a9a7
Tsubouchi, Takamasa
9b139671-b3dd-49cb-a37b-25faea839c06
Torres-Valdés, Sinhué
d66ec9fe-9bb3-48cd-97e0-246950f5eca1
Garabato, Alberto C.Naveira
97c0e923-f076-4b38-b89b-938e11cea7a6
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e
Bacon, Sheldon
1e7aa6e3-4fb4-4230-8ba7-90837304a9a7
Tsubouchi, Takamasa
9b139671-b3dd-49cb-a37b-25faea839c06
Torres-Valdés, Sinhué
d66ec9fe-9bb3-48cd-97e0-246950f5eca1
Garabato, Alberto C.Naveira
97c0e923-f076-4b38-b89b-938e11cea7a6

Forryan, Alexander, Bacon, Sheldon, Tsubouchi, Takamasa, Torres-Valdés, Sinhué and Garabato, Alberto C.Naveira (2019) Arctic freshwater fluxes: sources, tracer budgets and inconsistencies. Cryosphere, 13 (8), 2111-2131. (doi:10.5194/tc-13-2111-2019).

Record type: Article

Abstract

The net rate of freshwater input to the Arctic Ocean has been calculated in the past by two methods: directly, as the sum of precipitation, evaporation and runoff, an approach hindered by sparsity of measurements, and by the ice and ocean budget method, where the net surface freshwater flux within a defined boundary is calculated from the rate of dilution of salinity, comparing ocean inflows with ice and ocean outflows. Here a third method is introduced, the geochemical method, as a modification of the budget method. A standard approach uses geochemical tracers (salinity, oxygen isotopes, inorganic nutrients) to compute "source fractions" that quantify a water parcel's constituent proportions of seawater, freshwater of meteoric origin, and either sea ice melt or brine (from the freezing-out of sea ice). The geochemical method combines the source fractions with the boundary velocity field of the budget method to quantify the net flux derived from each source. Here it is shown that the geochemical method generates an Arctic Ocean surface freshwater flux, which is also the meteoric source flux, of 200±44 mSv  (1 Sv = 10^6 m^3 s^-1), statistically indistinguishable from the budget method's 187±44 mSv, so that two different approaches to surface freshwater flux calculation are reconciled. The freshwater export rate of sea ice (40±14 mSv) is similar to the brine export flux, due to the "freshwater deficit" left by the freezing-out of sea ice (60±50 mSv). Inorganic nutrients are used to define Atlantic and Pacific seawater categories, and the results show significant non-conservation, whereby Atlantic seawater is effectively "converted" into Pacific seawater. This is hypothesized to be a consequence of denitrification within the Arctic Ocean, a process likely becoming more important with seasonal sea ice retreat. While inorganic nutrients may now be delivering ambiguous results on seawater origins, they may prove useful to quantify the Arctic Ocean's net denitrification rate. End point degeneracy is also discussed: multiple property definitions that lie along the same "mixing line" generate confused results.

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Accepted/In Press date: 21 July 2019
e-pub ahead of print date: 14 August 2019
Published date: August 2019

Identifiers

Local EPrints ID: 433703
URI: http://eprints.soton.ac.uk/id/eprint/433703
ISSN: 1994-0416
PURE UUID: d1ccdcd9-3203-4cc9-b43a-8bf1252e2bba

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Date deposited: 02 Sep 2019 16:30
Last modified: 07 Oct 2020 00:24

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