How ocean CO 2 fluxes are estimated/measured Colm Sweeney [ ] Princeton University and Lamont-Doherty Earth Observatory

Outline IV. Improving our estimates of air-sea fluxes - Time-space distribution of pCO 2 - Parameterization of gas transfer velocity III. Surface measurements: -Measurements of surface pCO 2 -Methods for interpolation II. The air-sea flux measurement -Covariance -Gradient technique I.Concept -Ocean carbon chemistry primer -The air-sea flux

Ocean Carbon Chemistry Primer CO 2(gas) CO 2 + H 2 O  H 2 CO 3 H 3 CO 2  H + + HCO 3 - HCO 3 -  H + + CO 3 2- Carbonic acid Bicarbonate Carbonate CO 2 + CO 3 2-  2 HCO 3 - TCO 2

Ocean Carbon Chemistry Primer CO 2(gas) CO 2 + H 2 O  H 2 CO 3 H 3 CO 2  H + + HCO 3 - HCO 3 -  H + + CO 3 2- Carbonic acid Bicarbonate Carbonate CO 2 + CO 3 2-  2 HCO  atm560  atm 8  mol kg  mol kg  mol kg  mol kg  mol kg  mol kg  mol kg  mol kg %  pCO 2  8%  TCO 2 TCO 2 Taken from Feely et al. (2001)

Concept k =f(u * ) Sc -n u * – frictional velocity s – solubility Sc – schmit number (v/D) n – 0.4 – 0.67 (high slope…low slope) Net air-sea gas flux: F gas =ks(pCO 2w -pCO 2a ) I=ks(pCO 2a ) River input: 0.6 PgC yr -1  pCO 2 ~2  atm Keeling et al. E=ks(pCO 2w )

Bomb 14 C Broecker and Peng (1994) Transfer velocity k av = 22 cm/hr u * = 7.4 m/s Semi-infinite Half space

Early estimates air-sea CO 2 exchange Natural 14 CO 2 / 12 CO 2 in gassing 14 CO 2 / 12 CO 2 out gassing n+ 14 N  14 C Decay: 14 C  14 N + e - Pre-industrial assumption: 14 CO 2 in = 14 CO 2 out + Decay Solve for I mol m -2 yr -1 uatm -1 =21.4 cm hr -1

Early estimates air-sea CO 2 exchange Natural 14 CO 2 / 12 CO 2 in gassing 14 CO 2 / 12 CO 2 out gassing n+ 14 N  14 C Decay: 14 C  14 N + e - Pre-industrial assumption: 14 CO 2 in = 14 CO 2 out + Decay Solve for I mol m -2 yr -1 uatm -1 =21.4 cm hr Rn  218 Po + 4 He [Rn] mixed layer Rn [Rn] no loss Rn + gas exchange 226 Ra aq  222 Rn gas + 4 He Outgassing of Radon = mol m -2 yr -1 uatm -1 =21.9 cm hr -1 [Rn]

Flux Measurements in the Atmosphere

Direct covariance technique

Covariance flux of H 2 O and CO 2 F air-sea = 3-D Sonic Anemometers IR Detector (Sample) H 2 O/CO 2 samples IR Detector (Motion Detection) Std Res Pump

Gradient Flux Technique Frictional velocity Measured Gradient (3-13m) Gradient Function - empirically determined based on Monin Obukhov (MO) similarity theory McGillis et al. (2001) Covariance intake

GasEx-98 Comparison -estimates of transfer velocity GasEx-2001

Estimates of gas transfer velocity Rayleigh Distribution For ocean wind speeds P(u) k- short term Bomb 14 C k av =22 cm /hr

Estimates of CO 2 fluxes from measurements of  pCO 2 1. Shipboard measurements of atmospheric and surface ocean pCO 2 2. The ocean pCO 2 climatology 3. Flux calculations using the climatology

Shipboard measurements of atmospheric and surface ocean pCO 2

Equilibration of air sample IR Detector Air flow Re-circulation Drain

Takahashi pCO 2 database 1,183,000 measurements - Since ~1968

Monthly distribution of pCO 2

The climatology 1.Exclude all El-Nino years. -dramatic change in annual fluxes have been observed El-Nino periods based on SIO<-1.5 and SST changes. 2. Normalize pCO 2 single reference year (1995) -In warm waters (lat. <45)  pCO 2 remains constant 3. Interpolate data on to 4 o x 5 o x 365 day grid -finite differencing algorithm is used with a 2-D transport model from Toggwieler et al. (1989) to propagate the influence of observed data at one day time steps. Distribution is solved iteratively Time pCO 2

The pCO 2 Climatology

Global CO2 flux

Test of interpolation  pCO 2  T 0.28 C ~0.8 PgC =3.5%

Sampling resolution 250K samples (Takahashi ’97) 500K samples (Takahashi ’99) 940K samples (Takahashi ’02)

PgC yr -1 Change in fluxes with increases in samples

Gas Transfer Velocity and Fluxes

Estimates using different gas exchange- wind speed relationships Feely et al., 2001

Long vs. short term winds PgC yr -1 NCEP(1995) 41 Year average Monthly

Sources of uncertainty Seasonal distribution of pCO 2 (0.8 PgC) Estimate of skin temperature (-0.6 to –0.1 PgC) Estimates of the transfer velocity (20-40%) Estimates of windspeed (2 m/s)

How can we do better?

Factors influencing CO 2 flux estimates Wind k  pCO 2 Air-Sea CO 2 Flux SST Transport Biology Wind Waves Bubbles Surface Film Near Surface Turbulence Bock et al. (1999)

Better spatial-temporal coverage 2. Predictions using synoptic data sets: 1. Deployment of ships and moorings:

time space 1 m 2 1 km 2 Globe Ocean Basin Regional (10 6 km 2 ) centuries decadal Inter-annual seasonal daily Remote sensing Space and time coverage of ocean carbon observing networks hourly Process Studies Repeat Trans-basin Sections VOS surface pCO 2 Shipboard Time-Series Moored Time-Series

Factors influencing surface water pCO 2 Temperature (C)-2 –30 (  ln pCO 2 /  T) = o C % VariableRangeRelationEffect TCO 2 (  mol kg -1 ) (  ln pCO 2 /  Tln TCO 2 ) = % Alkalinity(  mol kg -1 ) (  ln pCO 2 /  Tln TALK) = % Salinity(  mol kg -1 ) (  ln pCO 2 /  Tln S) = 0.94 ~10% Alkalinity and salinity are proportional and can be accounted for

SummerFall Winter Spring Stephens et al., 1996 Temperature correlations

Prediction of  pCO 2

~ Bermuda Courtesy of Nick Bates ~100 uatm ~9.5 C 4.23% C uatm Due to temperature TCO 2 =33  mol/kg

Temp vs. Biology Takahashi et al. (2002)

Temp. (C) CO 2 +H 2 O  O 2 +CH 2 O Upwelling Palmer Sta.

MODIS May 2001 Sea Surface Temperature May 2001 Chlorophyll PAR December 2000 Derived from GSFC Data Assimilation Office 3 hr retrievals.

Predicting pCO2 NPP SST Z mix

Estimates of gas transfer velocity Wind k Waves Bubbles Surface Film Near Surface Turbulence

k(600) [cm·h ] R n [mm·h ] k(600)   0.679R n  R n 2 Gas exchange vs. rain rate (MP distribution) Ho et al. 1997

Summary III. Improving our estimates of air-sea fluxes - Time-space distribution of pCO 2 - Deployment of ships and buoys - Use of satellite measurements to calculate change in TCO 2 - Parameterization of gas transfer velocity - micro-scale measurements II. Estimates using surface  pCO 2 : - Provide us with estimates of fluxes on a monthly basis based climatology adjusted for a single non-El Nino year - Errors in flux estimates occur due to lack of direct pCO 2, wind speed and understanding of the gas transfer velocity I. The air-sea flux measurement - Provide true short-term (~1 hr) measurements of flux which can be associated with wind speeds measured on that same time scale. - Are limited to areas of high  pCO 2

Inventory methods Estimates of integrated change in carbon inventory 1) Time series approach – Comparing measurements made between two time intervals –Compare residuals of multiple parameter regressions using T, S, TALK and nutrients 2) C * Method –Estimate of the total inventory of anthropogenic carbon in any given region

Hydrographic samplisg stations

C * Method (Gruber et al.) C*C* 170O 2 116CO 2 Soft tissue [O 2 ] sat -O 2 [O 2 ] meas =0 TT 170 O 2 16 NO 3 2- Carbonate pCO 2(i) =280 CaCO 3 Ca 2+ +CO 3 2- C ant = C m – ∆C bio – C eq280 – C diseq = ∆C* - ∆C diseq ∆C bio =r C:O  O 2 + ½(r N:O  O 2 +  CO 3 2- ) C diseq C eq280 ∆C bio

Anthropogenic CO 2 (  mol kg -1 )

Pre-industrial CO 2

International CLIVAR/CO 2 Lines (including US) CO 2 Clivar Repeat Hydro.