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Www.pol.ac.uk John Huthnance Proudman Oceanographic Laboratory Liverpool, UK Motivation Context Processes and currents Estimating exchange / models Maybe.

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Presentation on theme: "Www.pol.ac.uk John Huthnance Proudman Oceanographic Laboratory Liverpool, UK Motivation Context Processes and currents Estimating exchange / models Maybe."— Presentation transcript:

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

2 Motivation Global cycles oceanic N  shelf  primary production (Gt/y) (Walsh, 1991)(Wollast, 1993) –OC budget uncertainty ~ 1 Gt/y ~ shelf export –CO 2 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 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 Adjacent Oceanic flow (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)

5 Along-slope currents (RSDAS, Plymouth Marine Lab 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 m 2 /s where s is steric slope H  -1  y, typically (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, m 2 /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 m 2 /s) local along- or cross-slope rectified flow –contribution to long-term displacements shear dispersion K ~ t D U 2 because tidal current varies with depth (friction) t D ~ 10 3 s (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 typical cascading fluxes locally 0.5 – 1.6 m 2 s -1 –significant where present –eg. Celtic Sea, Malin, Hebrides shelves Winter cooling or evaporation helped by lack of freshwater on shelf  dense water  down-slope flow under gravity

17 Celtic Sea↓ Malin shelf↓ winter cooling

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

19 Estimated exchange (NW Iberia) Summer (filaments)WinterAverage Drifters dispersion (Des Barton) ~ 870 m 2 s -1 ~ 190 m 2 s -1 ~ 560 m 2 s -1 salinity & along-slope flow (Daniualt et al. 1994) 500 m 2 s -1  Exchange flux across 200m depth contour 3.8 m 2 s -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 m 2 s -1 ! above 200 m → 3.1 m 2 s -1 Contributing processes (m 2 s -1 ) Up-/down-welling30.6 Slope current 2 ndy 11 Internal solitons1 Eddies+cross-front ??Total??5.62.2

20 Exchange q´, m 2 s -1

21

22 The shelf-sea carbon pump Sea surface Thermocline Sea bed Section Deep Ocean Shelf sea Vertical asymmetry in P-R drives air-sea CO 2 difference. But these seas are well mixed in winter so need to remove C laterally Photosynthesis Respiration Mixing

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

24 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 pCo 2 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 shelf wide Carbon budget The loss term Difference = burial In-organic Organic Acidification Small Equilibrium

28 Carbon export Horizontal advection is the dominant loss term Net advective loss of carbon (subtracting rivers): 0.9x10 12 mol C yr -1 Net burial: 0.02x10 12 mol 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 m 2 s -1


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