Revisited estimates of anthropogenic CO2 throughout the

Atlantic Ocean:a novel approach

Franck Touratier & Catherine Goyet

EA 1947 BDSI, Université de Perpignan, France

UNIVERSITE DE PERPIGNAN

BIOPHYSIQUE ET DYNAMIQUE DES SYSTEMES INTEGRES

How can we estimate anthropogenic CO2 in the ocean?Since anthropogenic CO2 cannot be measured at sea, two major kinds of models have been used to separate the anthropogenic signal from the bulk of TCO2: Models using back calculation techniques:

• approaches of Brewer (1978), and Chen and Millero (1979),• the C* approach of Gruber et al. (1996),• the approach of Peng et al. (1998),• the MIX approach (Goyet et al. 1999).

Ocean circulation models: 2.5D model of Stoecker et al. (1994), IPSL model (e.g. Delecluse 1994), MPI model (e.g. Maier-Reimer 1993), Hadley model (Taylor 1995), Princeton model (e.g. Sarmiento et al. 1995), …

• the CAO approach ------------------------------------------->

1. Definition of CAO (Carbon, Alkalinity, Oxygen tracer):

106 162 3 3 4 2

2 3 3 4

Respiration,decomposition

Photosynthesis

( ) ( ) ( ) 138

106 16

CH O NH H PO O

CO HNO H PO

2 122 H O

2 2 2123 1 17165 2 165

TCO O TA O

2 2165 1

131.5 2CAO TCO TA O

2 21.2 0.6CAO TCO TA O

A novel (back calculation) approach: the CAO model

CARINA database

La méthode TrOCATracer combining Oxygen, iorganic Carbon, and total

AlkalinityTouratier et Goyet (a,b; 2004)

2 21.2 0.6TrOCA TCO TA O

0 00 02 21.2 0.6TrOCA TCO TA O

‘quasi-conservatif’

‘conservatif’

0 00

2 202 2

1.2ANT

TA TA TrOCA TrOCAC TCO TCO

O O

TrOCA0 is the unknown of the system

CAO0

The form of the equation CAO0= ƒ () is first determined by using the program of e.g. Lewis and Wallace (1998). The best fit is obtained with an exponential of the form a e-

/b (r2 ≈ 1).

2 21.2 0.6CAO TCO TA O

0 89.041505.04 eCAO

0

1.2ANTCAO CAOC

The coefficients a and b are then determined by fitting the exponential function to CAO0 values computed from recent TCO2, TA, and O2 data sampled in waters from the latitude belt 20°S-40°S, and characterized by ≤ 2°C.

3. Precision of the CAO model

Prior 1991 During /after 1991

TCO2: 15 µmol kg-1 2 µmol kg-1

TA: 15 µmol kg-1 4 µmol kg-1

O2: 1.1 µmol kg-1 1.1 µmol kg-1

Potential temperature (; °C)

0 5 10 15 20 25 300

2

4

6

16

17

18

0 5 10 15 20 25 30

Err

or

(µm

ol

kg

-1)

0

5

20CAO (prior 1991)

CAO (during / after 1991)

CAO0

CO2ant (prior 1991)

CO2ant (during /

after 1991)

4. Results for the 1990s

North-south section

East-west sections

5. Results for the 1980s

North-south section

6. Anthropogenic CO2 inventories for the 1980s

Latitude

90°S

80°S

70°S

60°S

50°S

40°S

30°S

20°S

10°S EQ

10°N

20°N

30°N

40°N

50°N

60°N

70°N

80°N

90°NA

nth

rop

og

enic

CO

2 in

ven

tory

(G

t C

)

0

1

2

3

4

5

6

7Gruber et al.Chen et al.Stoecker et al.Sarmiento et al.This study (1980s)

7. Inventories for the 1980s and the 1990s

Latitude

90°S

80°S

70°S

60°S

50°S

40°S

30°S

20°S

10°S EQ

10°N

20°N

30°N

40°N

50°N

60°N

70°N

80°N

90°N

An

thro

po

gen

ic C

O2

inve

nto

ry (

Gt

C)

0

1

2

3

4

5

6

7During the 1990sDuring the 1980s

60 60

43

64

5430

9

22 68 78

44

25

42

10

37

5469

47 103

43

3861

190

157

49

216

Ne

t in

cre

as

e (

Gt

C)

-2

-1

0

1

2

8. Net increase of anthropogenic CO2 for the period 1984-1995:

??

The annual rate:

0.47 ±0.02 Gt C yr-1 0.66 Gt C yr-1

Perspectives:

We are planning to use the CAO model to estimate the concentrations of anthropogenic CO2 in other oceanic regions (Indian Ocean, Pacific Ocean), Using exactly the same observations to estimate the anthropogenic CO2, it would be useful to compare the results of the different models. The paper of Coatanoan et al. (2001), that compares the MIX and the C* approaches using data from the WOCE section I1 (northern Indian Ocean), shows that information can be gained from this kind of inter-comparison exercise. The end