1

Claire Lo Monaco, Andrew Lenton and Nicolas Metzl

(LOCEAN-IPSL, Paris)

Natural and Anthropogenic Carbon Changes in the South Indian Ocean

Plan

•Ocean acidification

•Feedback on climate change

Carbon Budget 2007 (Global Carbon Project, 2008)

Conclusions:

“Anthropogenic CO2 emissions have been growing about four times faster since 2000 than during the previous decade, […]

Natural CO2 sinks are growing, but more slowly than atmospheric CO2, […]

All of these changes characterize a carbon cycle that is generating stronger climate forcing and sooner than expected.”

Scientific context

Plan

•Ocean acidification

•Feedback on climate change

I. Observations

Can we separate between anthropogenic and ‘natural’ carbon changes?

II. Model-Data comparison

Do coarse resolution Ocean-Carbon Models reproduce the observed changes?

III. On-going/future work

What drives DIC changes in the model? Would it be coherent with observations?

Scientific context

Measurements of Total Carbon and the associated parameters (temperature, salinity, oxygen, alkalinity, nutrients, …)

One cruise conducted in 1985

Six cruises conducted between 1998 and 2001(interannual variability)

Data (1)

OISO 1-6 :

INDIGO1 :

Mean position of hydrological fronts after Belkin and Gordon (1996)

I. Observations: sampling sites

Kerguelen Plateau

Data (2)

Large interannual variability can occur in the top 1000m of the ocean

- in the frontal region (associated with movement of the SAF)

- north of the SAF (mesoscale feature created by the Madagascar Current and the Agulhas Return Current

Mean position of hydrological fronts after Belkin and Gordon (1996)

I. Observations: sampling sites

Kerguelen Plateau

Rodgers et al. (subm!)

STMW

SAMW

AASW WW

Upper CDW

STSW

Water masses (1)

Mean position of hydrological fronts after Belkin and Gordon (1996)

I. Observations: water masses

Total carbon distribution (µmol/kg)

Kerguelen Plateau

STMW

SAMW

AASW WW

Upper CDW

STSW

Water masses (2)

Mean position of hydrological fronts after Belkin and Gordon (1996)

I. Observations: water masses

Total carbon distribution (µmol/kg)

Kerguelen Plateau

Deep winter mixing (down to ~500m)

Mode Waters formation

Cant

I. Observations : carbon changes

STMW

SAMW

WW

Upper CDW

Extended Multi-Linear Regression (eMLR) technique, Friis et al. (2005, DSR.I)

Anthropogenic carbon increased by 5-10 µmol/kg in Mode Waters (over 15 years)

~1/6 of the total accumulation of anthropogenic carbon (over 200 years)

Anthropogenic Carbon change over 15 years [µmol/kg]

STMW

SAMW

WW

Upper CDW

Anthropogenic Carbon distribution [µmol/kg]

Back-calculation (preformed carbon method), Lo Monaco et al. (2005, JGR)

TCO2 changes

I. Observation : carbon changes

STMW

SAMW

WW

Upper CDW

Total Carbon change over 15 years [µmol/kg]

+6±3 µmol/kg

no change

+15±5 µmol/kgNo changeAASW WW

Upper CDW

STMW

SAMW

STSW

-5±4 µmol/kg

No change

Total Carbon (µmol/kg) Total Carbon (µmol/kg)

35°S50°S

Extended Multi-Linear Regression (eMLR) technique

Only conservative tracers are used (Salinity, Temperature, NO)

50°S 35°S

eMLR results (1)

Total Carbon changes (TCO2) [µmol/kg]

STMW

SAMW

WW

Upper CDW

I. Observation : carbon changes

Anthropogenic Carbon changes (Canth) [µmol/kg]

STMW

SAMW

WW

Upper CDW

eMLR results (2)

I. Observation : carbon changes

Natural Carbon change (Cnat) [µmol/kg]

STMW

SAMW

WW

Upper CDW

STMW

SAMW

WW

Upper CDW

Anthropogenic Carbon changes (Canth) [µmol/kg]

STMW

SAMW

WW

Upper CDW

=

Total Carbon changes (TCO2) [µmol/kg]

+

MODEL description

Components:

Ocean Model OPA9 (GM90 and TKE mixed layer scheme),

Biogeochemical Model PISCES (NPZD model),

Ice Model LIM2

Resolution:

2°x 2° resolution (enhanced at the equator)

31 non -regular vertical levels (19 levels in the upper 500 meters)

Forgings:

ERA40 heat fluxes and winds

CORE freshwater fluxes (SST and SSS  restored to Reynolds SST and Levitus SSS using bulk  formulas)

CO2 scenario:

Pre-industrial run keeping atmospheric CO2 constant at 278 ppm

Anthropogenic run using the observed atmospheric CO2 values

II. Model-Data comparison: model description

Ocean Carbon Model NEMO2

DATA/MODEL: CANTII. Model-Data comparison: anthropogenic carbon

ANTHROPOGENIC CARBON DISTRIBUTION (µmol/kg)

MODEL (in 2000)DATA (late 1990’s)

500m

4030

2020

40

6030

DATA/MODEL changes (1)

DATA MODEL

Anthropogenic Carbon change (µmol/kg)

6

4

3

8

10

12

DATA

Total Carbon change (µmol/kg)

8 10

642

MODEL

8

II. Model-Data comparison: carbon changes

-20

DATA/MODEL nat C (1)

Natural Carbon change (µmol/kg)

MODEL (CO2atm = 278ppm)

-2

-4 -2

-4

DATA (TCO2-Cant)

2

-2

0

II. Model-Data comparison: carbon changes

-5

When the anthropogenic signal is removed, the remaining pattern shows

- a decrease in upper CDW,

but no significant change in sub-surface Antarctic waters

- a smaller decrease in SAMW,

but no significant change in STMW

DATA/MODEL nat C (2)

Natural Carbon change (µmol/kg)

MODEL (CO2atm = 278ppm)

-2

-4 -2

-4

DATA (TCO2-Cant)

2

-2

0

II. Model-Data comparison: carbon changes

-5

1000m

SummarySummary

Mode Waters transport anthropogenic CO2 from the surface down to ~1000m

STMW: The invasion of anthropogenic CO2 explains most of the TCO2 increase.

TCO2 increased by ~8 µmol/kg over 15 year

SAMW: The invasion of anthropogenic CO2 is compensated for by an equal decrease in natural carbon.

No change in TCO2.

South of the Polar Front: no significant change in anthropogenic carbon

upper CDW: TCO2 decreased by 9 (± 6) µmol/kg

II. MODEL

I. OBSERVATIONS

The observed changes are reasonably well reproduced in the model

The model suggests that the decrease in ocean carbon observed at mid-depths (~500-2000m), is a large scale feature representative of other regions in the

South Atlantic and South Pacific

Perspectives (2)Perspectives

CARINA+GLODAP merged dataset: Southern Indian sector

Ongoing/future work

What are the mechanisms driving the change in natural carbon in the model?Would it be coherent with observations?

XXXXXX END XXXXXX

To be continued…

CTOT +48 GtCCANTH +46 GtCCNAT +2 GtC

CTOT +21 GtCCANTH +25 GtCCNAT -4 GtC

CTOT +3 GtCCANTH +18 GtCCNAT -15 GtC

CTOT +24 GtCCANTH +3 GtCCNAT +21 GtC

Perspectives (1)PerspectivesChanges in ocean carbon inventories (Pg of Carbon)

500m-2000m below 2000m

Full water column 0-500m

CO2

Total Carbon

CLIMATE

OCEANIC CARBON CYCLE

OCEAN PROCESSE

S (photosynthe

sis, dynamics, ...)

CO2 emissio

ns

Anthropogenic CO2

Scientific context

Total Carbon

Scientific context

Water masses

Potential temperature [°C]

STMW

SAMWUpper CDW

WWAASW STSW

Mean position of hydrological fronts after Belkin and Gordon (1996)

I. Observations: water masses

Mode waters (1)

Eastern Subtropical Mode Waters

Subtropical Mode Waters

I. Observations: Mode waters

SAMW

Eastern STMW

STMW

Subpolar MW

STMW

Mode Waters distribution after Hanawa and Talley, 2000)

Mode Waters distribution after Hanawa and Talley, 2000)

Mode waters (2)

SAMW

Eastern STMW

STMW

Subpolar MW

STMW

Anthropogenic Carbon distribution [µmol/kg]

STMW

SAMWUpper CDW

WW

I. Observations: Mode Waters

OISO 1 to 3 (1998)

Cant 500m (1)

WOCE (1995-1996)

Large accumulation of anthropogenic carbon at mid-latitudes (20-40°S) in recently formed Mode Waters

Anthropogenic Carbon in Mode Waters

Observations :

Anthropogenic carbon distribution at 500m in the late 1990’s[µmol/kg]

500m

Anthropogenic carbon calculated from observations (TCO2, TA, O2 and

conservative tracers)

OISO 1 to 3 (1998)

Cant 500m (2)

WOCE (1995-1996)

Comparison of recent observations (1998-2001) with historical measurements (1985)

to evaluate the decadal change in ocean carbon

Anthropogenic Carbon in Mode Waters

Anthropogenic carbon calculated from observations (TCO2, TA, O2 and

conservative tracers)

Anthropogenic carbon distribution at 500m in the late 1990’s

Observations :

INDIGO 1 (1985)

[µmol/kg]

500m

Cant TCO2 distributions Total Carbon and Anthropogenic Carbon distributions

Anthropogenic Carbon distribution [µmol/kg]

STMW

SAMW

WW

Upper CDW

Total carbon distribution [µmol/kg]

STMW

SAMW

AASW WW

Upper CDW

STSW

Total Carbon changes (µmol/kg)

50°S 35°S

+6±3 µmol/kg

no change

+15±5 µmol/kgNo changeAASW WW

Upper CDW

STMW

SAMW

STSW

-5±4 µmol/kg

No change

Direct method

I. Observations

Anthropogenic carbon distribution [µmol/kg]

STMW

SAMW

AASW WW

Upper CDW

STSW

Total Carbon changes (µmol/kg)

TCO2 changes (2)

I. Observation : carbon changes

Physical and biogeochemical tracers (S, T, O2, Nut, Alk) are used to remove the effect of ocean variability (dynamics, biological activity).

Anthropogenic Carbon change over 15 years [µmol/kg]

STMW

SAMW

WW

Upper CDW

+6±3 µmol/kg

no change

+15±5 µmol/kgNo changeAASW WW

Upper CDW

STMW

SAMW

STSW

-5±4 µmol/kg

No change

Total Carbon (µmol/kg) Total Carbon (µmol/kg)

35°S50°S

eMLR method

Carbon changes : eMLR method

Extended Multi-Linear Regression (eMLR) technique

Friis et al. (2005, DSR.I)

Multi-Linear regressions of Total Carbon against selected tracers (A, B, C…) determined using observations collected at time t1 and t2.

t1 : INDIGO (1985) Total Carbon (t1) = a1.A1 + b1.B1 + c1.C1 + d1

t2 : OISO (1998-2001) Total Carbon (t2) = a2.A2 + b2.B2 + c2.C2 + d2

Coefficients determined for time t1 and t2 are then applied to the same set of tracers to evaluate the change in Total Carbon between t1 and t2:

Total Carbon change (t2 - t1) = (a2-a1).A2 + (b2-b1).B2 + (c2-c1).C2 + (d2-d1)

eMLR methods 1 & 2

Total Carbon changes (TCO2) [µmol/kg]

eMLR method 1: physical and BGC tracers TCO2 = eMRL (S, T, O2, Nut, Alk)

STMW

SAMW

WW

Upper CDW

eMLR method 2: using only conservative tracers TCO2 = eMRL (S, T, NO)

STMW

SAMW

WW

Upper CDW

I. Observation : carbon changes

Anthropogenic Carbon changes (Canth) [µmol/kg]

‘Natural’ C changes

Natural Carbon change (Cnat) [µmol/kg]

When the anthropogenic signal is removed from the total carbon change, the remaining pattern shows

- a large decrease in Upper Circumpolar Deep Water (6-12 µmol/kg), but no change in sub-surface Antarctic waters

- a small decrease in Subantarctic Mode Waters (4-6 µmol/kg), but no significant change in STMW (small increase in newly formed STMW?)

I. Observation : carbon changes

Difference between the 2 results:

CT (eMLR1) – Canth (eMLR2)

STMW

SAMW

WW

Upper CDW

DATA/MODEL changes (2)

DATA

Anthropogenic Carbon change (µmol/kg)

64

3

8

10

12

2

DATA

Total Carbon change (µmol/kg)

MODEL

II. Model-Data comparison: carbon changes

MODEL

DATA/MODEL OLD: CANT

DATA

11

9753

DATA / MODEL : Total and Anthropogenic Carbon changes

MODEL

Anthropogenic Carbon change (µmol/kg)

MODELDATA

Total Carbon change (µmol/kg)

-5

-3

3

5

7

9

11

MODEL: Natural Carbon changes at 700m [µmol/kg]

DATA/MODEL OLD: Nat C

Natural Carbon change (µmol/kg)

MODEL (CO2atm = 278ppm)

-11 -9-7

-5

-3

-7

-1

DATA / MODEL : ‘Natural’ Carbon changes

DATA (TCO2-Cant)

0

700m

Carbon decrease associated with

• Warming

• Oxygen increase

• Nutrients decrease

Natural Carbon changes at 700m [µmol/kg]

MODEL OLD: large scaleMODEL : Large scale changes

700m

Carbon decrease associated with

Nutrients decrease

Oxygen increase

Nitrate changes at 700m [µmol/kg]

Oxygen changes at 700m [µmol/kg]

Carbon decrease associated with

• Warming

• Oxygen increase

• Nutrients decrease