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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

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
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
ne atlantic area
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
(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
Along-slope currents

(RSDAS, Plymouth Marine Lab

15-21 Feb 1990)

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

nordic seas currents
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

slope current ct d
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)
wind forced flow exchange m 2 s
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
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
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
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

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
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
water exchange estimates
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
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

slide21

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

the shelf sea carbon pump
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
Observed North Sea air-sea CO2 flux

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

polcoms ersem atlantic margin model

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
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
Carbon Budget

High production

Low/Conv. transport

Low air-sea flux

High/Div. transport

High air-sea flux

the shelf wide carbon budget

The loss term

The shelf wide Carbon budget

In-organic

Small

Difference = burial

Organic

Acidification

Equilibrium

carbon export
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
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
Volume fluxes: above and below 150m

Above: 1.89Sv

Below:-1.94Sv

This is a downwelling shelf

conclusions 1 carbon cycle
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
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
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
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
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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