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 change AASW WW Upper CDW STMW SAMW STSW -5±4 µmol/kg No change Total Carbon (µmol/kg) 35°S 50°S Extended Multi-Linear Regression (eMLR) technique Only conservative tracers are used (Salinity, Temperature, NO) 50°S35°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) CO 2 scenario: Pre-industrial run keeping atmospheric CO2 constant at 278 ppm Anthropogenic run using the observed atmospheric CO2 values II. Model-Data comparison: m odel description Ocean Carbon Model NEMO2

DATA/MODEL: CANT II. Model-Data comparison: anthropogenic carbon ANTHROPOGENIC CARBON DISTRIBUTION (µmol/kg) MODEL (in 2000)DATA (late 1990’s) 500m

DATA/MODEL changes (1) DATA MODEL Anthropogenic Carbon change (µmol/kg) DATA Total Carbon change (µmol/kg) MODEL 8 II. Model-Data comparison: carbon changes -2 0

DATA/MODEL nat C (1) Natural Carbon change (µmol/kg) MODEL (CO2atm = 278ppm) DATA (  TCO 2 -  Cant) 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) DATA (  TCO 2 -  Cant) II. Model-Data comparison: carbon changes m

Summary Mode Waters transport anthropogenic CO 2 from the surface down to ~1000m STMW: The invasion of anthropogenic CO 2 explains most of the TCO 2 increase.  TCO 2 increased by ~8 µmol/kg over 15 year SAMW: The invasion of anthropogenic CO 2 is compensated for by an equal decrease in natural carbon.  No change in TCO 2. South of the Polar Front: no significant change in anthropogenic carbon upper CDW: TCO 2 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 (~ m), 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…

 C TOT +48 GtC  C ANTH +46 GtC  C NAT +2 GtC  C TOT +21 GtC  C ANTH +25 GtC  C NAT -4 GtC  C TOT +3 GtC  C ANTH +18 GtC  C NAT -15 GtC  C TOT +24 GtC  C ANTH +3 GtC  C NAT +21 GtC Perspectives (1) Perspectives Changes in ocean carbon inventories (Pg of Carbon) 500m-2000mbelow 2000m Full water column0-500m

CO 2 Total Carbon CLIMATE OCEANIC CARBON CYCLE OCEAN PROCESSES (photosynthesis, dynamics,...) CO2 emissions Anthropogenic CO 2 Scientific context Total Carbon Scientific context

Water masses Potential temperature [°C] STMW SAMW Upper CDW WW AASWSTSW 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 (2) SAMW Eastern STMW STMW Subpolar MW STMW Anthropogenic Carbon distribution [µmol/kg] STMW SAMW Upper CDW WW I. Observations: Mode Waters

OISO 1 to 3 (1998) Cant 500m (1) WOCE ( ) 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 (TCO 2, TA, O 2 and conservative tracers)

OISO 1 to 3 (1998) Cant 500m (2) WOCE ( ) Comparison of recent observations ( ) with historical measurements (1985) to evaluate the decadal change in ocean carbon Anthropogenic Carbon in Mode Waters Anthropogenic carbon calculated from observations (TCO 2, TA, O 2 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 A nthropogenic 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°S35°S +6±3 µmol/kg no change +15±5 µmol/kgNo change AASW 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, O 2, 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 change AASW WW Upper CDW STMW SAMW STSW -5±4 µmol/kg No change Total Carbon (µmol/kg) 35°S 50°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 t 1 and t 2. t 1 : INDIGO (1985)  Total Carbon (t 1 ) = a 1.A 1 + b 1.B 1 + c 1.C 1 + d 1 t 2 : OISO ( )  Total Carbon (t 2 ) = a 2.A 2 + b 2.B 2 + c 2.C 2 + d 2 Coefficients determined for time t 1 and t 2 are then applied to the same set of tracers to evaluate the change in Total Carbon between t 1 and t 2 : Total Carbon change (t 2 - t 1 ) = (a 2 -a 1 ).A 2 + (b 2 -b 1 ).B 2 + (c 2 -c 1 ).C 2 + (d 2 -d 1 )

eMLR methods 1 & 2 Total Carbon changes (  TCO2) [µmol/kg] eMLR method 1: physical and BGC tracers   TCO2 = eMRL (S, T, O 2, Nut, Alk) STMW SAMW WW Upper CDW eMLR method 2: using only conservative tracers   TCO 2 = 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:  C T (eMLR1) –  Canth (eMLR2) STMW SAMW WW Upper CDW

DATA/MODEL changes (2) DATA Anthropogenic Carbon change (µmol/kg) DATA Total Carbon change (µmol/kg) MODEL II. Model-Data comparison: carbon changes MODEL

DATA/MODEL OLD: CANT DATA DATA / MODEL : Total and Anthropogenic Carbon changes MODEL Anthropogenic Carbon change (µmol/kg) MODELDATA Total Carbon change (µmol/kg)

MODEL: Natural Carbon changes at 700m [µmol/kg] DATA/MODEL OLD: Nat C Natural Carbon change (µmol/kg) MODEL (CO2atm = 278ppm) DATA / MODEL : ‘Natural’ Carbon changes DATA (  TCO 2 -  Cant) 0 700m Carbon decrease associated with Warming Oxygen increase Nutrients decrease

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