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Impact of historical climate change on the
Southern Ocean carbon cycle and
implications for future change
Richard Matear, and Andrew Lenton
Jan 2009
Wealth from Oceans Flagship and CSIRO Marine and
Atmospheric Research
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
Key Question: How will the Southern
Ocean Carbon Cycle respond to global
warming?
Background
 Importance of the Southern Ocean to the global carbon cycle
 Review a recent studies of the response of the Southern
Ocean carbon cycle to historical climate change
Simulations of how historical climate change influences
Southern Ocean carbon cycle
 Summarize modeling approach and the NCEP re-analysis
forcing fields used to drive the ocean carbon model
 Review how climate variability in heat and freshwater fluxes
and wind stress alter the SO carbon uptake (both the natural
carbon and anthropogenic CO2 uptake)
Discuss how to detect the changes in the Southern Ocean
carbon cycle
Southern Annular Mode (SAM)
Lenton & Matear 2007
SAM calculated from NCEP-R1 850hPa geopotential height
anomaly
A Positive SAM has stronger westerly winds
Robust feature of climate change projections (Fyfe 2007)
Southern Ocean overturning
circulation
Saturation of Southern Ocean carbon
uptake (LeQuere et al 2007)
Rising CO2 in the
atmosphere should drive
an increased oceanic
uptake of CO2 (red line)
Ocean uptake with
climate variability (blue
line)
Increasing oceanic uptake
Control:
Natural
Enhanced Eddy:
Natural
Anthropogenic
Anthropogenic
Zickfield etal 2007
Ocean General Circulation Model with
biogeochemical cycles
•Ocean model simulations based on the ocean
component of the Mk3.5 CSIRO climate model
•Approximately 1 x 2 degree north-south and eastwest resolution
•Light, mixed layer depth and phosphate formulation
for export production of carbon and constant rain
ratio for calcium carbon export (8%)
•Simulations started from climatological T, S, DIC,
Alkalinity and phosphate fields in year 1850
•Simulation run till 1948 after which the forcing fields
are allowed to vary
•Period of the 1940s used to diagnose the seasonal
freshwater flux into the ocean to account for potential
systematic errors in the NCEP freshwater fluxes
Model Experiments
Experiment
Forcing
Heat Flux
Freshwater
Flux
Winds
Total
Variable
Variable
Variable
1948
1948
1948
1948
Hflx
Variable
1948
1948
Fflx
1948
Variable
1948
Tau
1948
1948
Variable
Table . The daily forcing fields used to drive the model from year 1948 to 2002 for the
various experiments. Variable forcing refers to the use of forcing fields from years 1948
to 2002 while 1948 denotes the use of forcing fields from only year 1948. For cases where
the winds are allowed to vary (Total and Tau) the interannually varying wind speeds are
used in the calculation of the gas exchange coefficient for the air-sea CO2 fluxes
otherwise the 1948 year winds are used.
NCEP Atmospheric Forcing Changes
HF
FW
• SO is south of 40°S
• Increased heat and
freshwater fluxes into the SO
• Increased zonal windstress
Taux
Tauy
Zonal averaged windstress
Taux max
Latitude of Taux max
Stronger westerly winds which migrate south
Comparison between natural and
anthropogenic carbon uptake
Natural carbon
Natural carbon uptake
dominates the climate
variability response
Anthropogenic carbon
Opposite response of
natural and
anthropogenic carbon
uptake
Total
1948
Tau
Hflx
Fflx
Correlation with the Annual mean
South Ocean CO2 Fluxes
Experiment
Changes in SO anthropogenic carbon
fluxes versus natural carbon fluxes
Correlation
Regression
-0.52
-0.10
Hflx
-0.81
-0.12
Fflx
-0.96
-0.22
Tau
-0.91
-0.26
Total
1948
The correlation coefficient and regression value of the changes in the annual
mean SO anthropogenic carbon uptake versus the change in the annual mean
SO natural carbon fluxes. The changes in the carbon fluxes are determined by
subtracting the fluxes from the 1948 experiment. The regression value gives the
change in the anthropogenic carbon fluxes per unit change in the natural carbon
fluxes.
Changes in export production
Southern Ocean averaged
export production
Little simulated variability in
export production (< 10%)
Total
1948
Tau
Hflx
Fflx
Affect of wind changes on
export production for year
2002 - 1948 (total exp.)
Ventilation of the Southern Ocean
Annual averaged surface
density
Ocean ventilation based
on the outcrop area of
water denser than 27.1
(AAIW or denser)
Total
1948
Tau
Hflx
Fflx
Winds and freshwater
fluxes have the largest
impact on the ventilation
of the Southern Ocean
Relationship between carbon uptake
and ventilation
Natural carbon
FW
HF
Winds
Total
High correlation with ocean
ventilation variability
Experiment
Anthropogenic carbon
Ventilation Anomaly
Correlation with ocean ventilation
Natural
Anthropogeni
Total
-0.8165
0.1050
1948
-
-
Hflx
-0.8888
0.8507
Fflx
-0.9612
0.9542
Tau
-0.9617
0.9045
Summary
High negative correlation between SO changes in natural and
anthropogenic carbon uptake
 For recent past and for next several decades the natural response will
dominate the SO response
In the Southern Ocean, the response to the different forcing fields
is complex
 Model response is dominated by changes in ocean ventilation
 Not clear that the SO carbon uptake is declining
 Southern Ocean simulated reduction in oceanic uptake of carbon is
sensitive to the forcing fields used in the ocean simulation
(particularly the freshwater flux). Using sea surface salinity restoring
would give results consistent with LeQuere etal 2007
 LeQuere et al 2007 conclusion that the SO uptake is declining is premature
Detecting Change
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
Saturation of Southern Ocean carbon
uptake (LeQuere et al 2007)
Rising CO2 in the
atmosphere should drive
an increased oceanic
uptake of CO2 (red line)
Ocean uptake with
climate variability (blue
line)
Increasing oceanic uptake
Increased uptake
Trend in Southern Ocean Carbon
Uptake
Southern Oceanic uptake is increasing in contrast to Le
Quere et al 2007 decline
Law et al., Science 2008
What can the surface pCO2 measurements
tell us about the Southern Ocean Carbon
sink?
Trend over years 1980 to 2004
pCO2 Trend (ppm/yr): Total and Tau
experiments
Total
Tau
Dissolved Oxygen:
zonally-averaged Pacific (umol/kg)
DO minimum in the upwelled CPDW
CPDW is also associated with a Dissolved Inorganic Carbon
Maximum
Synthetic Inversion of Atmospheric O2
Simulated sea-air oxygen flux
(red line)
Inversion of simulated
atmospheric oxygen data (two
different networks)
The inversion can reproduce
the interannual variability and
trend
Dissolved oxygen changes in
the ocean interior would also
occur if there was an increase
in the ACC
Law and Matear In prep
Recent changes in SO stratification
Eddy resolving simulations do not produce an increase
In Antarctic Circumpolar Current and an increase in upwelling
Bonning et al., Nature Geosciences 2008
Simulated ACC Transport
Total
1948
Tau
Hflx
Fflx
Summary
High negative correlation between SO changes in natural and
anthropogenic carbon uptake
 For recent past and for next several decades the natural carbon cycle
response will dominate the SO response to climate variability
Southern Ocean carbon response to climate change is not clear
because:
 the response to the different forcing fields is complex
 Increase in Heat and Freshwater Fluxes into the ocean will counter act the increase in winds
 How one prescribes the freshwater flux does influence the past variability in SO carbon uptake
 Not clear how the SO will respond to a more positive SAM: - Greater Ekman
transport or energy channeled into increased eddy activity
Biogeochemical observations of CO2 and oxygen could be used to
detect change and determine how the SO responds to a more
positive SAM
Thank you! Questions?
Thank you, Questions?
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
Box Model Representation of the SO
1
FA / FN  (pCO2A(t) 280)/(pCO2Deep  280)
2
3
4
pCO2A equals 310 and 370 ppm.
deep water pCO2 equals 450 ppm
5
6
7
Ratio of change in anthropogenic to natural carbon flux
of -0.17 to -0.55
Metzl and Lenton in prep
Metzl and Lenton in prep
PF
SAZ
SAZ
AZ
Antarctic
Zone
SAF
STF
STF
Southern Ocean Thermohaline
Circulation
SAMW
AAIW
1000m
UCDW
2000m
AABW
NADW
3000m
LCDW
Antarctica
4000m
40°S
50°S
60°S
70°S
80°S
Modified from Speer et al 2000
CO2 Uptake: Response to SSS
restoring