eddies and ocean biogeochemistry

Eddies and Eddies and Ocean BiogeochemistryOcean Biogeochemistry

Andreas Oschlies

IFM-GEOMAR

The Biological Pump: The Biological Pump: Traditional 1D ViewTraditional 1D View

Sea surface

z(mix)~z(euph)

CO2, O2

organic matterinorganic nutrientsnutrients, CO2

z

``relatively constant´´ C:N:P:-O2

Biological Pump in 3DBiological Pump in 3D

Z(euphot. zone)

Z(winter mixed layer)

CO2, O2

(1)

(2)particulate and dissolved organic matter

low lats high lats

Z(euphot. zone)


CO2, O2

newly-remineralised dissolved inorganic matter

(3a)

(1)


low lats high lats


Z(euphot. zone)


CO2, O2

newly-remineralised dissolved inorganic matter

(3a)

(1)


low lats high lats

(3b)

newly-generated inorganic matter deficit

(Oschlies & Kähler, 2004)


Potential of the biological pumpPotential of the biological pumpPresent-day sea-surface nitrate concentrations

mmol/m3

Mean profile

(Conkright et al., 1994)

Controls are not fully understood

Potential of the biological pumpPotential of the biological pumpPresent-day sea-surface nitrate concentrations

mmol/m3

Mean profile

(Conkright et al., 1994)

Controls are not fully understood

Subtropical deserts

Surface ChlorophyllSurface Chlorophyll

Biogeographical Provinces

Nitrate distributionNitrate distributionSubtropical nitrate “bowl”

• Observed biological production requires net supply across NO3=const surfaces.

• Gauss’s theorem: mean advection cannot contribute to transport across mean iso-surface.

diapycnal mixing Ekman transport anomalies eddy stirring (isopycnal &

dia-nutrial)

Where have eddies come into play?Where have eddies come into play?

Apparent observational discrepancy in oligotrophic subtropical gyres:

• Large-scale biogeochemical estimates of export production >> local direct measurements

• Still not fully resolved despite several decades of research

Subtropical desert conundrumsSubtropical desert conundrumsSimulated NO3 supply to euphotic zone Eastern basin: ~factor 12

•high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1)



• low 15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1)




• low simulated NO3 supply (Oschlies, 2002: ~0.02 mol N m-2 yr-

1)





1)

Bermuda: ~ factor 4

•high rates of 3He supply (Jenkins, 1988 : ~0.6 mol N m-2 yr-1)

• lower sedimentation rates (Michaels et al.,1994:~0.15 mol N m-2 yr-1)





1)

Bermuda: ~ factor 4



•Substantial interannual variability (Lipschultz, 2001; Oschlies, 2001)





1)

Bermuda: ~ factor 4



•Substantial interannual variability (Lipschultz, 2001; Oschlies, 2001)

Discrepancy based on different tracers!Role of conversion factors?


• Large-scale biogeochemical estimates of export production >> local direct measurements– Trace metal contamination, sediment trap problems,…

=> underestimated local production rates?


• Large-scale biogeochemical estimates of export production >> local direct measurements– Trace metal contamination, sediment trap problems,…

=> underestimated local production rates?

– Unintended tradition of undersampling!

under-representation of episodic eddy events?

Evidence for episodic nutrient supplyEvidence for episodic nutrient supply

Section Azores – Cape Farewell (Strass, 1992)

Evidence for Evidence for episodic episodic

nutrient supply nutrient supply by eddiesby eddies

(McNeil et al., 1999)

Bermuda Testbed Mooring

Time series 4 months,

eddy time scale 15 days.

Eddy pumping concept (vertical one)Eddy pumping concept (vertical one)

(McGillicuddy et al., 1998)

On the relevance of eddy pumpingOn the relevance of eddy pumping

• “vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget”

• “Eddy pumping and wintertime convection are the two dominant mechanisms transporting new nutrients into the euphotic zone”

• Nutrient flux by eddy pumping “is more than an order of magnitude higher than the diapycnal diffusive flux as well as … vertical transport due to isopycnal mixing”.

First counterargument: StatisticsFirst counterargument: Statistics

• Should we have missed the important events?

Undersampling of episodic events

under-representation of episodic events?

• chance of under-representation = change of over-representation

Undersampling = under-representation?Undersampling = under-representation?

Hawaii Ocean Timeseries Site (Karl et al., 2003)

T, z = 0m

T, z = 200m

(Siegel et al., 1999)

SLA

>3 years BATS vs Topex-Poseidon

Undersampling = under-representation?Undersampling = under-representation?

Counter argument 2: high estimates Counter argument 2: high estimates based on modelsbased on models

• Falkowski et al. (1991), eddy off Hawaii:

infer ~< 20% enhancement of large-scale primary production by eddies.– based on direct fluorescence measurements of

primary production

Counter argument 2: high estimates Counter argument 2: high estimates based on modelsbased on models

• Falkowski et al. (1991), eddy off Hawaii:

infer ~< 20% enhancement of large-scale primary production by eddies.– based on direct fluorescence measurements of

primary production

• McGillicuddy et al. (1998), eddy off Bermuda: infer ~100% enhancement of large-scale nutrient supply.– based on nitrate-density relationship and

inconsistent model assumptions

Altimetry-based eddy-pumping estimateAltimetry-based eddy-pumping estimate

(Siegel et al., 1999)

SLA

zeuph

Estimated nitrate supply

Altimetry-based eddy-pumping estimateAltimetry-based eddy-pumping estimate

(Martin & Pondaven, 2003)

Based on mutually exclusive assumptions:

• All eddy events contribute to local nitrate flux (wave-like eddies)

• 100% of upwelled nutrient is taken up locally (slow growth at base of euphotic zone water must be trapped in moving eddy)

• Plausible efficiency more likely 20-25%

Model-based assessmentModel-based assessment

Spring bloom in eddy-resolving model (1/9x2/15 degrees)

ecosystem model,(Oschlies & Garcon, 1999)

(Oschlies, 2002)

Model statistics at BATSModel statistics at BATSmean NO3

rms vert. displ. of iso-NO3 surfaces Corr2(SSH,Z(NO3)) Corr2(,NO3))

BATS(1/9)o model

~ correct amplitude for lifting of iso-NO3 surfaces

Model-based assessment at BATSModel-based assessment at BATS

mg C

hl/m3

mm

ol NO

3 /m3

Chlorophyll

Nitrate

cumulative NO3 supply

SSH

0.05mol/m2 after spring (Lipschultz (2001):

0.07 in 1992,0.04 in 1993)

Siegel et al. (1999) method would predict 6 times too much NO3 supply associated with eddy event (1).

(Oschlies, 2002)

Counter argument 3: Recharging issuesCounter argument 3: Recharging issues

Eddy-pumping process

.

recharging

time


.

recharging

time

• Sinking is diapycnal transport



.

recharging

time


• Recharging of nutrients on shallow isopycnals matters.



.

recharging

time



• Recharging requires diapycnal nutrient transport (local or remote).



.

recharging

time



• Recharging requires diapycnal nutrient transport (local or remote).

• Bottleneck is diapycnal transport rather than isopycnal uplift!

(Oschlies, 2002)


What about eddy-resolving models?What about eddy-resolving models?

• Idealised models (frontal dynamics)

Idealised modelsIdealised models

(Levy et al., 2001)

Large local impacts

> 100% increase in regional production.

Often run in spin-up or spin-down mode.

Representative of steady-state large-scale mean?

““We can never do merely one thing”We can never do merely one thing”(Hardin, 1985)(Hardin, 1985)

SST New Production (Mahadevan & Archer, 2000)

New production increases with finer and finer resolution.No convergence seen, yet.

(Mahadevan & Archer, 2000)

0.36oC cooling over 120/2=60 days BATS: Hawaiifor MLD = 50m: 14 W/m2 18 W/m2

for MLD = 100m: 28 W/m2 36 W/m2

120 day mean

higher NO3 supplylower SST

Heat transport constraint on nutrient transport?

““We can never do merely one thing”We can never do merely one thing”(Hardin, 1985)(Hardin, 1985)

Basin-scale models (i)Basin-scale models (i)

Spring bloom in eddy-resolving model (1/9x2/15 degrees)

ecosystem model,(Oschlies & Garcon, 1999)

(Oschlies, 2002)

permitting

Surface heat “flux correction”Surface heat “flux correction”

1/9o model, forced by ECMWF 1989-93 ERA

1/3o model, forced by ECMWF 1989-93 ERA

About 25W/m2 additional heating required to reproduce observed SSTs,(little less at higher resolution: ML restratification by eddies)

(Oschlies, 2002)

Eddy-induced stratificationEddy-induced stratification

Heat flux required to balance eddy-induced stratification of ML.Eddies stratify and heat the surface ML in most areas.

(Oschlies, 2002)

Baroclinic instabilities in ML generate stratification

lightwarm

densecold

(Nurser & Zhang, 2000)

y

z

Sub-mesoscale Sub-mesoscale heatfluxheatflux

Inferred from altimetry: Positive everywhere!

(Fox-Kemper & Ferrari, 2008)

NH winter

SH winter

Simulated North Simulated North Atlantic spring Atlantic spring

bloombloom1/3 x 2/5 degrees

1/9 x 2/15 degrees

Eddy resolving looks “better”.Is there a significant net impact?

Surface chlorophyll (mg/m3)

(Oschlies, 2002)

Small difference in oligotrophic subtropical gyre.Some difference at gyre’s margins.

Simulated annual NOSimulated annual NO33

supply into upper supply into upper 126m (mol m126m (mol m-2 -2 yryr-1-1))

eddy resolving

eddy permitting

viscous

(Oschlies, 2002)

Eddy impacts on Eddy impacts on mean nutrient supplymean nutrient supply

total supply by eddies

Eddy supply by vertical excursions (includes “eddy pumping”)

Eddy supply by lateral stirring(exceeds vertical eddy contribution over large parts of subtropical gyre!)

(Oschlies, 2002)

Role of lateral stirringRole of lateral stirring• Supply by lateral stirring might reach larger distances

for organic nutrients with longer lifetimes/slower utilisation rates (Lee & Williams, 2000)

=3 months =1 year

Basin-scale models (ii)Basin-scale models (ii)

McGillicuddy et al. (2003):

• Nutrient transport model– z = 0-104m:

N uptake rate = min(QN,L)

– z > 104m:

Remineralisation:

1/ [ NO3obs(x,y,0) – NO3]

– Sensitivity experiments: = 10, 30, 60 days

(results only shown for = 10 days, though)

NO3

0

(McGillicuddy et al., 2003)

Basin-scale models (ii): NP pathwaysBasin-scale models (ii): NP pathways

1/10o degree model (McGillicuddy et al., 2003)

For = 10 days, vertical advection by eddies dominates nutrient supply!

Overestimated eddy pumping Overestimated eddy pumping by rapid restoringby rapid restoring

.

Recharging issue: •10 days realistic? •How different are results for = 30 days, 60 days?

time

Conclusions (i)Conclusions (i)

• Eddy pumping occurs, but mean impact has often been grossly overestimated because of inconsistent assumptions about time scales.



• Vertical eddy pumping cannot resolve observational discrepancy.




• Lateral stirring by eddies at least as important.




• Lateral stirring by eddies at least as important.

• Other processes overlooked previously?– Eddy/wind interactions– Submesoscale

Eddy/wind interactionsEddy/wind interactions

• Theory: Induces vertical circulation at eddy’s margin (Martin & Richards, 2001; Mahadevan et al., 2008, comment on McGillicuddy et al., 2007)

Simulated impact of eddy-wind Simulated impact of eddy-wind interactioninteraction

(Eden & Dietze, 2009)

Simulated new production (mmol C m-2 d-1)

without wind-current

interaction

with wind-current

interaction

Difference

~ 5% reduction

Also: reduced EKE (10-50%),

reduced energy input by wind

Submesoscale upwelling?Submesoscale upwelling?• Ubiquitous, large associated vertical velocities

(Martin & Richards, 2001) (Levy et al., 2001)

PRIME eddy,c.i. 5m/day

• Heat flux constraint on nutrient fluxes?

First results from steady-state First results from steady-state basin-scale simulationbasin-scale simulation

1° 1/3° 1/9° 1/27° 1/54°

SSS,yr 100

1/27o – 1/9o:Less phytoplankton,Less new production!

(courtesy Marina Levy)

PHY NP

Conclusions (ii)Conclusions (ii)

• Eddy/wind interactions have small (negative) impact on nutrient supply.

• Submesoscale variability may reduce nutrient supply (ML restratification).

• Can we pump nutrients without pumping heat?– Useful constraints from surface heat fluxes?

– Depends on correlations of T and NO3 in thermo-/nutricline.

Why should we still want to resolve Why should we still want to resolve eddies in biogeochemical models?eddies in biogeochemical models?

• Isopycnal stirring important to get oxygen minimum zones right (“shaddow zones”)

Ventilation of OMZsVentilation of OMZsKiso zonal

Kiso meridional

(Eden & Greatbatch, 2009)

Ventilation of OMZsVentilation of OMZsKiso zonal

Kiso meridional

(Eden & Greatbatch, 2009)

Dissolved O2 along 23oW, 4/3o model

Kiso=0 m2/s

Kiso=2000 m2/s

mol/l

<5

>50

Impacts on species composition?Impacts on species composition?• Lima et al. (2002): larger phytoplankton

favoured at higher eddy activity

• Hansen & Samuelsen (2009): more diatoms, less flagellates at finer resolution off Norway

(run for 1.5 years)

(run for 200 days)

Why should we still want to resolve Why should we still want to resolve eddies in biogeochemical models?eddies in biogeochemical models?

• Isopycnal stirring important to get oxygen minimum zones right (“shaddow zones”)

• Sensitivity of Southern Ocean CO2 uptake to past & future climate change.

• Eddies as means to structure marine ecosystems? Impact on species composition?

Thank you!Thank you!

Why should be want to resolve eddies?Why should be want to resolve eddies?

• Plots look so much nicer…• It’s very expensive!

– Computational efforts: 1 x (1/10)o ~ 1000 x 1o

– OK for process studies

– Impact on mean properties?

Idealised models (ii)Idealised models (ii)

PHY

ZOO

PP

(Spall & Richards, 2000)

Large local impacts

< 10% increase in regional production.

Equilibrium reached?

Simulated impact of eddy-wind Simulated impact of eddy-wind interactioninteraction

(Eden & Dietze, 2009)

Simulated EKE (cm2 s-2)

without wind-current

interaction

with wind-current

interaction

Difference

~ 10-50% reduction

Analogy with heat budgetAnalogy with heat budget

(Greatbatch et al., 2007)

Gauss’s theorem: mean advection cannot contribute to net transport acrossclosed mean-isosurface

Analogy with heat budgetAnalogy with heat budget

(Greatbatch et al., 2007)

Export of organic matter

Gauss’s theorem: mean advection cannot contribute to net transport acrossclosed mean isosurface

Sensitivity to isopycnal mixing Sensitivity to isopycnal mixing

Typical range in coarse-resolution models(has little effect on density and velocity fields)

Which diffusivity is “correct”, if any?

eddies and ocean biogeochemistry

Documents

no3 supply lewis

net supply

high rates of o2 consumption

euphotic zoneeastern

low 15no3 uptake

zonezwinter mixed layerco2

oligotrophic subtropical

o21low latshigh latszeuphot