VARIABILITY OF OCEAN CO2 PARTIAL PRESSURE AND AIR-SEA CO2 FLUXES

IN THE SUBANTARCTIC ZONE OF THE SOUTHERN OCEAN

J. Boutin(1), L. Merlivat(1) and K. Currie(2)

(1) LOCEAN, Paris, France(2) NIWA, New Zealand

Year 2001-2002 2003 2004-2005 2005Nb CARIOCA buoys 3 2 2 1

Duration of pCO2 meas. (Months) 9 10 31 2

Ship tracks used for building Takahashi climatology (from Li et al, 2005)+ Trajectories of 8 CARIOCA buoys deployed in the Southern Ocean,

CARIOCA drifters

• Hourly measurements (real time ARGOS transmission)

• Ocean measurements at 2m depth:–pCO2 (accuracy <3atm)–SST–SSS–Fluorescence

• Atm. measurements of:–Wind speed –Atm. Pressure

• Trajectory influenced by :–15m depth currents

Lifetime: up to 17 months

DIC deduced from pCO2, SST and SSS assuming Alk/SSS relationship (Jabaud et al., 2004)

Air-sea CO2 flux computation

FAir-sea CO2 flux

= K(U,sst)CO2 exchange

coefficient

. (pCO2 – pCO2atm)

CO2 partial pressure difference

(pCO2atm very homogeneous)

K derived from satellite or CARIOCA wind speed (U) via Wanninkhof (1992) K-U relationship pCO2

atm deduced from xCO2 and CARIOCA Patm

Sonic Anemometer on CARIOCA in 2004-2005 => 17 months of wind measurements on each buoy; precision CARIOCA / QSCAT satellite wind speeds: 1.3m/s

20

QS

CA

T w

ind

spe

ed (

m/s

)

20

0

10

0 10 20CARIOCA wind speed (m/s)

Large flux variability dominated by P variability (no clear seasonal cycle)

Large differences in flux and P after July 2004:

Mean fluxes: Northern buoy: -9.6 mmol m-2 day-1; Southern buoy: -5.0 mmol m-2 day-1

March/April 04 –June/Sept. 05

Southern buoy

Northern buoy

Comparison with Takahashi(2002) climatology

North-South/East-West gradient structure seen by the buoys qualitatively coherent with yearly p Takahashi mean in the western Pacific

(2004-2005)

p time and space colocation with Takahashi climatology

Mean p along CARIOCA trajectory:

NorthCarioca : -35atmSouthCarioca : -16atm

NorthTaka : -17atmSouthTaka : -13atm

Large scale spatial/temporal structures coherent but :-West and North p measured by CARIOCA lower than Takahashi : interannual variability? Oceanic trend different from atmospheric trend between 1995 and 2005?-small scale structures on CARIOCA data

pCO2

In Winter :-Southern buoy close to the SAF as defined by Belkin and Gordon (1996)-Southern buoy travels much faster than Northern buoy

=> Southern buoy probably in a jet on SAF

High pCO2 recorded by the Southern buoy close to the SAF front

Interpretation of North-South gradient

pCO2

DIC

SAF signature even clearer on DIC

pCO2 - SST relationship derived from CARIOCA in Winter and from ship data south of Tasmania and New Zealand

Atm. trend 1998-2005

Close to NZ coastal province Close to SAF

pCO2 anticorrelated with SST close to SAF => evidence of mixingSimilar slope as the ones detected from ship data south of Tasmania and New Zealand (Rangama et al., JGR, 2005)

Interpretation of CARIOCA data using ocean color

SAF

Satellite Chl images, a tracer of ocean circulation

Mixing also responsible for high Chl in Springas seen on satellite MODIS ocean color image (November month) ?

Chl (mg/m3)

One month later (December 2004) a bloom developed north of SAF => strong mesoscale variability on northern buoy

Interpretation of CARIOCA data using ocean color

December 2004, northern buoy, pCO2 and DIC decrease associated with the development of a Chl bloom

fCO2

292 304 328 340

Chloro-a

Chloro-a

DIC

20432034 20612052 mol/kg)atm)

21/12

29/12

SUMMARY-No clear seasonal cycle in the Subantarctic Zone (SAZ); variability dominated by spatial and small scale variability-Close to Subantarctic front: mixing is the dominant mechanism controlling pCO2 variability; agreement with Takahashi climatology-North/South-East/West structures similar to Takahashi climatology But lower pCO2 in the north and west part of the SAZ in the Pacific Ocean

SUMMARY-No clear seasonal cycle in the Subantarctic Zone (SAZ); variability dominated by spatial and small scale variability-Close to Subantarctic front: mixing is the dominant mechanism controlling pCO2 variability; agreement with Takahashi climatology-North/South-East/West structures similar to Takahashi climatology But lower pCO2 in the north and west part of the SAZ in the Pacific Ocean

NEXT STEPHow to reconcile air-sea fluxes estimated at regional scale by indirect methods

(model inversions) and from surface ocean observations in the Southern Ocean?pCO2 space/time extrapolation?

-Need to refine biogeochemical provinces first defined by Longhurst (1998) (northern and southern buoys under the influence of different processes although in the same Subantarctic water ring province)

SUMMARY-No clear seasonal cycle in the Subantarctic Zone (SAZ); variability dominated by spatial and small scale variability-Close to Subantarctic front: mixing is the dominant mechanism controlling pCO2 variability; agreement with Takahashi climatology-North/South-East/West structures similar to Takahashi climatology But lower pCO2 in the north and west part of the SAZ in the Pacific Ocean

NEXT STEPHow to reconcile air-sea fluxes estimated at regional scale by indirect methods

(model inversions) and from surface ocean observations in the Southern Ocean?pCO2 space/time extrapolation?

-Need to refine biogeochemical provinces first defined by Longhurst (1998) (northern and southern buoys under the influence of different processes although in the same Subantarctic water ring province)

Climatological Longhurst biogeochemical provinces

Subantarctic water ring

subtropical

NZ

SUMMARY-No clear seasonal cycle in the Subantarctic Zone (SAZ); variability dominated by spatial and small scale variability-Close to Subantarctic front: mixing is the dominant mechanism controlling pCO2 variability; agreement with Takahashi climatology-North/South-East/West structures similar to Takahashi climatology But lower pCO2 in the north and west part of the SAZ in the Pacific Ocean

NEXT STEPHow to reconcile air-sea fluxes estimated at regional scale by indirect methods

(model inversions) and from surface ocean observations in the Southern Ocean?pCO2 space/time extrapolation?

-Need to refine biogeochemical provinces first defined by Longhurst (1998) (northern and southern buoys under the influence of different processes although in the same Subantarctic water ring province)

-Acquire new data sets in the Southern Atlantic Ocean: deployment of 6 CARIOCA buoys in the frame of CARBOOCEAN (EU integrated FP6 project 2005-2010).