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The European North Atlantic shelf [Ocean-Shelf Exchange, internal waves]

The European North Atlantic shelf [Ocean-Shelf Exchange, internal waves]. John Huthnance Proudman Oceanographic Laboratory Liverpool, UK Motivation Context Processes and currents Estimating exchange / models Maybe more about carbon cycling. Motivation. Global cycles

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The European North Atlantic shelf [Ocean-Shelf Exchange, internal waves]

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  1. The European North Atlantic shelf[Ocean-Shelf Exchange, internal waves] John Huthnance Proudman Oceanographic Laboratory Liverpool, UK Motivation Context Processes and currents Estimating exchange / models Maybe more about carbon cycling

  2. Motivation • Global cycles oceanic N  shelf  primary production 0.5 0.2 (Gt/y) (Walsh, 1991) (Wollast, 1993) • OC budget uncertainty ~ 1 Gt/y ~ shelf export • CO2 release by upwelling, respiration vs draw-down • JGOFS-LOICZ Continental Margin TaskTeam [Maybe more about this later] • Physical interests [including exchange; emphasis for now] • special slope processes • shelf influence on ocean and vice versa • e.g. contribution to ocean mixing

  3. NE Atlantic area Shelf has • Varied orientation • width mostly 100-500 km • narrower S of 40°N • depth < 200 m (~ break) • except off Norway • Canyons • Irregular coast with gaps • Fjords (north from ~ 55°) • ~ Small river input

  4. (Van Aken in Huthnance et al 2002) Upper ~ 500 m flows to S from Biscay Saline Mediterranean outflow at 500 – 1500 m, against slope to N winter cooling  deep convection in Nordic seas and N Biscay ( dense bottom layer) Adjacent Oceanic flow

  5. Along-slope currents (RSDAS, Plymouth Marine Lab 15-21 Feb 1990) warm, salt NAW  slope current Iberia and Biscay to Norway

  6. Flow to N at 56½°N(cm/s; W Scotland; Souza)

  7. Nordic Seas currents Upper ~ 500 m flows to N in Rockall Trough & further north NAW  Nordic seas round Faroes, Iceland Moderate rivers & coastal currents Baltic→NCC largest

  8. Estimated transport past 62N McClimans

  9. Slope current (ct’d) • Bottom Ekman layer takes exchange transport gHs/8f of order 1 m2/s where s is steric slope H-1y, typically 10-7 (down-slope bottom flow for poleward slope current) • Instabilities • - Eddies: Faroe-Shetland Channel • - “SWODDIES” from slope current off northern Spain • (Pingree and LeCann, 1992) • Capes, canyons, varied shelf width • - local up-/down-welling, cross-slope exchange • e.g. Cape São Vicente & Goban Spur "overshoot” O(1 Sv)

  10. “Overshoot” at Goban Spur(Pingree et al. 1999)

  11. Wind-forced flow / exchange, m2/s • Irish-Norwegian shelf & westerlies  downwelling (but not consistently) • strong prevailing westerlies, max. ~ 60°N • storm surges • cross-slope exchange estimate ~ Ekman transport NOCS wind speeds, Josey et al. (1998; 2002) directions, standard deviations from Isemer & Hasse (1995)

  12. Wind-driven upwelling NE “trade” winds → Summer upwelling off W Spain, Portugal, ↔ coast direction (Finisterre; less off Algarve) Filaments each → Exchange ~ 0.6Sv > τ/ρf 6-12 Sep 1998

  13. Tides • mostly semi-diurnal • currents on shelf generally > 0.1 m/s, locally > 0.5 m/s • much water  shelf within 12.4 hours • comparable internal tidal currents generated locally over steep slopes (Celtic Sea (Pingree), W Scotland, W-T ridge)

  14. Consequences of tides • water carried by internal solitons (up to 1 m2/s) • local along- or cross-slope rectified flow • contribution to long-term displacements • shear dispersion K ~ tDU2 because tidal current varies with depth (friction) tD ~ 103s (Prandle, 1984) • small effect unless U > 0.5 m/s • Energy dissipation, mixing (barotropic & internal tide)

  15. Faroe-Shetland Channel, internal tide energy flux M2 shown, ambiguity in baroclinic flux, slope super-critical Flux in non-linear hf waves comparable with dissipation Slope sub-critical; energy has nowhere else to go, dissipates Very variable through time (slope current, eddies)

  16. Cascading Winter cooling or evaporation helped by lack of freshwater on shelf  dense water  down-slope flow under gravity typical cascading fluxes locally 0.5 – 1.6 m2s-1 • significant where present • eg. Celtic Sea, Malin, Hebrides shelves

  17. Celtic Sea↓ Malin shelf↓ • winter cooling

  18. Water exchange estimates From drifters: • Cross-slope dispersion estimates • north of Scotland ~ 360 m2s-1 (Burrows and Thorpe, 1999) ~ 700 m2s-1 (Booth, 1988) • Current variance estimates ~ 0.01 m2s-2 north of Scotland 0.01-0.02 m2s-2 off Norway (Poulain et al., 1996)

  19. Estimated exchange (NW Iberia) Summer (filaments) Winter Average • Drifters dispersion (Des Barton) ~ 870 m2s-1 ~ 190 m2s-1 ~ 560 m2s-1 • salinity & along-slope flow (Daniualt et al. 1994)500 m2s-1  Exchange flux across 200m depth contour 3.8 m2s-1(assume 26 km offshore scale; ~ replace shelf water in 10 days) • observed rms. U cross-slope 19 mm/s in 200 m ≡ 3.8 m2s-1 ! . . . . . . . above 200 m → 3.1 m2s-1 • Contributing processes (m2s-1) Up-/down-welling 3 0.6 Slope current 2ndy 1 1 Internal solitons 1 Eddies+cross-front 0.60.6 ??Total?? 5.6 2.2

  20. Exchange q´, m2s-1

  21. www.metoffice.gov.uk/research/hadleycentre/models/carbon_cycle/intro_global.htmlwww.metoffice.gov.uk/research/hadleycentre/models/carbon_cycle/intro_global.html

  22. The shelf-sea carbon pump Sea surface Photosynthesis Thermocline Shelf sea Respiration Mixing Deep Ocean Vertical asymmetry in P-R drives air-sea CO2 difference. But these seas are well mixed in winter so need to remove C laterally Section Sea bed

  23. Observed North Sea air-sea CO2 flux Thomas et al Science 2004: net CO2 drawdown in the North Sea

  24. Phytoplankton Pelagic Si N u t r i e n t s Dino-f Pico-f Diatoms NO3 Flagell -ates Particulates DIC NH4 Bacteria PO4 Dissolved Hetero- trophs Micro- Meso- Consumers Suspension Feeders D e t r i t u s Oxygenated Layer N u t r I e n t s Aerobic Bacteria Meio- benthos Deposit Feeders Redox Discontinuity Layer Anaerobic Bacteria Reduced Layer Benthic POLCOMS-ERSEM: Atlantic Margin Model 3D coupled hydrodynamic ecosystem model

  25. The AMM simulation • Developed from the NCOF operational model • POLCOM-ERSEM • ~12 km resolution, 42 s-levels • 1987 spin-up, 1988 to 2005 – 18 years • ERA40 + Operational ECMWF Surface forcing • ~300 river flows • 15 tidal constituents • Time varying (spatially constant) atmos pCo2 • Mean annual cycle for • Ocean boundaries • EO SPM/CDOM Attenuation • River nutrient and DIC • Recent developments: Run10 • 34 to 42 s-levels • COARE v3 surface forcing • GOTM turbulence model

  26. Carbon Budget High production Low/Conv. transport Low air-sea flux High/Div. transport High air-sea flux

  27. The loss term The shelf wide Carbon budget In-organic Small Difference = burial Organic Acidification Equilibrium

  28. Carbon export • Horizontal advection is the dominant loss term • Net advective loss of carbon (subtracting rivers): 0.9x1012mol C yr-1 • Net burial: 0.02x1012mol C yr-1 • But to be an effective sink must leave the shelf to DEEP water • Otherwise may re-equilibrate with atmosphere.

  29. How to get the Carbon off the shelf ? • The main current out of the north sea is a surface current • Shelf-edge: ‘frictional’ processes: e.g. Ekman draining; coastal downwelling After Turrell et al 1994

  30. Volume fluxes: above and below 150m Above: 1.89Sv Below:-1.94Sv This is a downwelling shelf

  31. Conclusions 1: Carbon Cycle The NW European shelf is a net sink of atmospheric CO2 • Shelf edge regions tend to be strong sinks • Open stratified regions are neutral or weaker sinks. • Coastal regions are either sources or sinks The circulation is vital in maintaining the shelf sea pump • Tidally active shelf seas lack 'export production' or burial • Regions of weak or convergent DIC transport have very weak air-sea fluxes There is no simple relation between productivity and air-sea CO2 flux

  32. Conclusions 2: Modelling • Modelling the air-sea CO2 flux in shelf seas requires accurate • Circulation • Mixing • Chemistry • Biology Currently under-estimate the shelf sea air-sea flux • The balance between ocean and shelf primary production is not yet well represented in these simulations • The near coastal region is particularly important: can act as either sink or source - but also the most challenging • Complex optics • Needs increased horizontal resolution • Land-sea fluxes uncertain

  33. Role of the slope current • Acts to replenish on-shelf nutrients (positive correlation with summer organic carbon) • Acts to remove DIC (negative correlation with summer inorganic carbon) • Together it helps drive the continental shelf carbon pump.

  34. Global contribution (in perspective) • 0.01 pg Cyr-1 of ~2 pg Cyr-1 Biological pump • 1.5 pg Cyr-1 of ~90 pg Cyr-1 Downwelling flux How does this up-scale to shelf seas globally ?

  35. Outline / conclusions • Prevalent along-slope flow poleward • not uniform, maybe not “continuous” • maybe covered by different surface flow • Strong wind forcing • up- and down-welling • filaments increase exchange • Strong tidal currents and mixing on wide shelves • Relatively small exchange in eddies • Moderate freshwater and stratification • except Norwegian Coastal Current • Local rectified tides, solitons, cascading • Overall exchange 2-3 m2s-1

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