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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 l.jpg

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 l.jpg

Motivation

  • Global cycles

    oceanic N  shelf  primary production

    0.50.2(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


Ne atlantic area l.jpg

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


Adjacent oceanic flow l.jpg

(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


Along slope currents l.jpg

Along-slope currents

(RSDAS, Plymouth Marine Lab

15-21 Feb 1990)

warm, salt NAW  slope current Iberia and Biscay to Norway


Flow to n at 56 n cm s w scotland souza l.jpg

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


Nordic seas currents l.jpg

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


Estimated transport past 62n l.jpg

Estimated transport past 62N

McClimans


Slope current ct d l.jpg

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)


  • Overshoot at goban spur pingree et al 1999 l.jpg

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


    Wind forced flow exchange m 2 s l.jpg

    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)


    Wind driven upwelling l.jpg

    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


    Tides l.jpg

    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)


    Consequences of tides l.jpg

    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)


    Slide15 l.jpg

    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)


    Cascading l.jpg

    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


    Celtic sea malin shelf l.jpg

    Celtic Sea↓ Malin shelf↓

    • winter cooling


    Water exchange estimates l.jpg

    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)


    Estimated exchange nw iberia l.jpg

    Estimated exchange (NW Iberia)

    Summer (filaments)WinterAverage

    • 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-welling30.6

      Slope current 2ndy11

      Internal solitons1

      Eddies+cross-front0.60.6

      ??Total??5.62.2


    Exchange q m 2 s 1 l.jpg

    Exchange q´, m2s-1


    Slide21 l.jpg

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


    The shelf sea carbon pump l.jpg

    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


    Observed north sea air sea co2 flux l.jpg

    Observed North Sea air-sea CO2 flux

    Thomas et al Science 2004: net CO2 drawdown in the North Sea


    Polcoms ersem atlantic margin model l.jpg

    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


    The amm simulation l.jpg

    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


    Carbon budget l.jpg

    Carbon Budget

    High production

    Low/Conv. transport

    Low air-sea flux

    High/Div. transport

    High air-sea flux


    The shelf wide carbon budget l.jpg

    The loss term

    The shelf wide Carbon budget

    In-organic

    Small

    Difference = burial

    Organic

    Acidification

    Equilibrium


    Carbon export l.jpg

    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.


    How to get the carbon off the shelf l.jpg

    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


    Volume fluxes above and below 150m l.jpg

    Volume fluxes: above and below 150m

    Above: 1.89Sv

    Below:-1.94Sv

    This is a downwelling shelf


    Conclusions 1 carbon cycle l.jpg

    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


    Conclusions 2 modelling l.jpg

    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


    Role of the slope current l.jpg

    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.


    Global contribution in perspective l.jpg

    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 ?


    Outline conclusions l.jpg

    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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