John Huthnance

1
The European North Atlantic shelfOcean-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 40N
• depth lt 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
• 15-21 Feb 1990)
• warm, salt NAW ? slope current Iberia and Biscay
  to Norway

6
Flow to N at 56½N (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 (ctd)

• 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. 60N
• 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
• gt t/?f
• 6-12 Sep 1998

13
Tides

• mostly semi-diurnal
• currents on shelf generally gt 0.1 m/s, locally gt
  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 gt 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
• Eddiescross-front 0.6 0.6
• ??Total?? 5.6 2.2

20
Exchange q, m2s-1
21
www.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
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 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