TIME SERIES OF FRESHWATER FLUX AT 26°N IN THE ATLANTIC OCEAN Elaine McDonagh1 ([email protected])Ÿ Brian King 1 Ÿ Harry Bryden1 Ÿ Peggy Courtois1 Ÿ Stuart Cunningham2 Ÿ Zoltan Szuts3 Ÿ Chris Atkinson4 1. NaSonal Oceanography Centre, Southampton Ÿ 2. ScoVsh AssociaSon for Marine Science Ÿ 3. MPI Meteorology Ÿ 4. UK Met Office OCEANIC FRESHWATER FLUX AT 26°N APRIL 2004 TO AUGUST 2010 Florida Straits Ekman Western boundary wedge Sum of components Interior deeper than 2000db. Interior shallower than 2000db. •  Constrain the salinity flux at each Sme step to be -­‐26 Svpsu (the salinity flux at Bering Strait) by adding a volume flux at the secSon average salinity. •  The net volume flux across the secSon is the freshwater flux. Uncertainty (Sv psu) Uncertainty is esSmated by considering 2.6 Florida Straits the transport and salinity uncertainty in Interior U
pper 1.01 each element of the calculaSon. TOTAL 0.72 Wedge uncertainty = 3.01 Sv psu in salinity flux or Ekman 0.57 0.09 Sv in equivalent freshwater for each 0.54 Bering Strait ten day esSmate. The uncertainty is Interior Deep 0.30 dominated by uncertainty in the volume 0.21 Barotropic compensaSon flux through Florida Straits. TOTAL 3.01 Mean = -­‐1.13 Sv, σ = 0.23 Sv COMPONENTS OF THE FRESHWATER FLUX The salinity flux across 26°N is split into overturning and horizontal components. Each component is associated with zero net mass flux. The Ekman flux is separated from the overturning flux by compensaSng it with the secSon average salinity. The overturning and Ekman fluxes both transport salinity northwards (the upper level northward flow is more saline than the lower level southward flow). The horizontal circulaSon transports salinity southwards (the northward flow in the Florida Straits is less saline that the southward flow in the subtropical gyre interior). Salinity fluxes are converted to equivalent freshwater fluxes by dividing by the secSon average salinity. These equivalent freshwater flux components sum to the freshwater divergence between 26°N and Bering Strait. The throughflow component represents the change in the secSon average salinity between Bering Strait and 26°N. 0.29 ± 0.04 Sv 0.06 ± 0.00 Sv -­‐0.32 ± 0.22 Sv -­‐0.67 ± 0.23 Sv COMPARISON WITH OTHER ESTIMATES ev
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The mean from our esSmate of the freshwater divergence (-­‐0.33 Sv) is compared to integrated surface flux climatologies and reanalyses (conSnuous lines) and esSmates from hydrographic secSons (symbols). BERING STRAIT CONSIDERATIONS We use the mean transport through Bering Strait as our constraint. What effect does the seasonal variability in the Bering Strait flow have? Bering Strait volume transport
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Bering Strait salinity
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throughflow component only. Bering St. observaSons from Woodgate et al GRL, 2005 delta fw flux (Sv)
THE CALCULATION At each Sme step (every ten days) calculate a salinity flux for each segment of the 26°N secSon (shown above) using •  salinity and transports from the 26°N mooring array •  salinity and density (for geostrophic calculaSon) from Argo data opSmally interpolated on density using Hydrobase climatology. E<P decrease in freshwater storage What is it? •  Mass flux across the secSon. •  Based on salinity conservaSon between Bering Strait and 26°N. Uses Bering Strait salinity flux of 26 Svpsu. Pacific to ArcSc flux of 0.8 Sv at 32.5 psu. •  Difference between -­‐0.8 Sv and freshwater flux represents integrated surface freshwater fluxes and change in the oceanic freshwater storage between Bering Strait and 26°N. •  Mean divergence implies 0.33 Sv of freshwater added to the ocean between Bering Strait and 26°N (or decreased oceanic freshwater storage). delta equivalent fw flux (Sv)
E>P Increase in freshwater storage transport (Sv)
salinity
• 
Ra*onale We have generated a conSnuous Sme series of oceanic freshwater flux across 26°N. This Sme series gives the integrated freshwater exchange (with atmosphere, cryosphere and terrestrial hydrosphere) and changes in oceanic freshwater storage over a region including the ArcSc and the deep water formaSon sites of the North AtlanSc. The AtlanSc is the most saline ocean, this high salinity is thought to result in the formaSon of deep water in the North AtlanSc. Further the formaSon of North AtlanSc Deep water is thought to be sensiSve to changes in freshwater inputs to the North AtlanSc. salinity flux (Sv psu)
Acknowledgements This work was funded by MONACO, a NERC RAPID-­‐WATCH project. Data from the RAPID-­‐WATCH/MOCHA project are funded by the Natural Environment Research Council (NERC) and NaSonal Science FoundaSon (NSF) and are freely available from www.noc.soton.ac.uk/rapidmoc. Florida Current transports esSmates are funded by the NaSonal Oceanic and Atmospheric AdministraSon (NOAA), and are available from www.aoml.noaa.gov/phod/floridacurrent. Argo data were collected and made freely available by the InternaSonal Argo Project and the naSonal programmes that contribute to it (h6p://www.argo.ucsd.edu, hkp://argo.jcommops.org). The HydroBase climatology, funded by NSF, is made available by Ruth Curry, WHOI.