www.pol.ac.uk

John Huthnance

Proudman Oceanographic LaboratoryLiverpool, UK

•Motivation

•Context

•Processes and currents

•Estimating exchange / models

•Maybe more about carbon cycling

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

Motivation

• Global cyclesoceanic 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

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)

0 4 00 8 00 1 200 16 00 2 000

dis tanc e fro m 4 0 N s ectio n (k m)

30 00

25 00

20 00

15 00

10 00

5 00

0

pre

ssure

(dbar)

O ME X II

Along-slope currents

(RSDAS, Plymouth Marine Lab15-21 Feb 1990)

warm, salt NAW slope current Iberia and Biscay to Norway

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

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

1980 1985 1990 1995 2000

5

6

7

8

9

10

11

12

Sv

McClimans

Slope current (ct’d)

• Bottom Ekman layer takes exchange transport gHs/8f of order 1

m2/swhere s is steric slope H-1y,

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 exchangee.g. Cape São Vicente & Goban Spur "overshoot” O(1 Sv)

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

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)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

S Bisc E Bisc Celtic W Irish W Scot N Scot Norw

Wind-driven upwellingNE “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

• 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

• 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)

Faroe-Shetland Channel, internal tide energy flux

M2 shown, ambiguity in baroclinic flux, slope super-critical

Flux in non-linear hf waves comparable with dissipationSlope sub-critical; energy has nowhere else to go, dissipatesVery variable through time (slope current, eddies)

Cascading

typical cascading fluxes locally 0.5 – 1.6 m2s-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

Celtic Sea↓ Malin shelf↓

• winter cooling

0 50 100 150 200 250 D istance,km

-400

-300

-200

-100

0

Dep

th,m

47°16'N / 07°22 'W 49°54 'N / 05°55 'W

T-scale

9.5

10.5

11.0

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)

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 0.6Slope current 2ndy 1 1Internal solitons 1Eddies+cross-front 0.6 0.6??Total?? 5.6 2.2

Exchange q´, m2s-1

0

0.5

1

1.5

2

2.5

3

S Bisc E Bisc Celtic W Irish W Scot N Scot Norw

q'

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

The shelf-sea carbon pump

Sea surface

Thermocline

Sea bed

Section

Deep Ocean

Shelf sea

Vertical asymmetry in P-R drives air-sea CO2 difference.But these seas are well mixed in winter so need to remove C laterally

Photosynthesis

RespirationMixing

Observed North Sea air-sea CO2 flux

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

Hetero-trophs

Bacteria

Meso-Micro-

Particulates

Dissolved

Phytoplankton

Consumers

Pico-fDiatoms

Flagell-ates

NO3

PO4

NH4

Si

DIC

Nutrients

Dino-f

Meio-benthos

AnaerobicBacteria

AerobicBacteria

DepositFeeders

SuspensionFeeders

Detritus

NutrIents

OxygenatedLayer

Reduced Layer

RedoxDiscontinuity

Layer

Pelagic

Benthic

POLCOMS-ERSEM: Atlantic Margin Model

3D coupled hydrodynamic ecosystem model

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

High productionLow/Conv. transportLow air-sea flux

High/Div. transportHigh air-sea flux

The shelf wide Carbon budget

The loss term

Difference = burial

In-organic

Organic

Acidification

Small

Equilibrium

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 ?

• 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

Above: 1.89SvBelow:-1.94Sv

This is a downwelling shelf

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

• Modelling the air-sea CO2 flux in shelf seas requires accurate– Circulation– Mixing– Chemistry– BiologyCurrently 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

• 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)

• 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•Prevalent along-slope flow poleward

not uniform, maybe not “continuous”

maybe covered by different surface flow

•Strong wind forcingup- and down-wellingfilaments 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