Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008 Impact of historical climate change on...
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Transcript of Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008 Impact of historical climate change on...
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
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
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)
SAM calculated from NCEP-R1 850hPa geopotential height anomaly
A Positive SAM has stronger westerly winds
Robust feature of climate change projections (Fyfe 2007)
Lenton & Matear 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
Zickfield etal 2007
Enhanced Eddy:Natural
Anthropogenic
Control:Natural
Anthropogenic
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 east-west 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
• SO is south of 40°S
• Increased heat and freshwater fluxes into the SO
• Increased zonal windstress
HF FW
Taux
Tauy
Zonal averaged windstress
Stronger westerly winds which migrate south
Taux max Latitude of Taux max
Comparison between natural and anthropogenic carbon uptake
Natural carbon uptake dominates the climate variability response
Opposite response of natural and anthropogenic carbon uptakeAnthropogenic carbon
Natural carbon
Total1948TauHflxFflx
Correlation with the Annual mean South Ocean CO2 Fluxes
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.
Experiment Changes in SO anthropogenic carbon fluxes versus natural carbon fluxes
Correlation Regression
Total -0.52 -0.10
1948
Hflx -0.81 -0.12
Fflx -0.96 -0.22
Tau -0.91 -0.26
Changes in export production
Southern Ocean averaged export production
Little simulated variability in export production (< 10%)
Affect of wind changes on export production for year 2002 - 1948 (total exp.)
Total1948TauHflxFflx
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)
Winds and freshwater fluxes have the largest impact on the ventilation of the Southern Ocean
Total1948TauHflxFflx
Relationship between carbon uptake and ventilation
High correlation with ocean ventilation variability
Experiment Correlation with ocean ventilation
Natural Anthropogenic
Total -0.8165 0.1050
1948 - -
Hflx -0.8888 0.8507
Fflx -0.9612 0.9542
Tau -0.9617 0.9045
Anthropogenic carbon
Natural carbon
FWHFWindsTotal
Ventilation Anomaly
SummaryHigh 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 pre-mature
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
Detecting Change
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
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
Incr
ease
d u
pta
ke
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 CPDWCPDW 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
Bonning et al., Nature Geosciences 2008
Eddy resolving simulations do not produce an increaseIn Antarctic Circumpolar Current and an increase in upwelling
Simulated ACC Transport
Total1948TauHflxFflx
SummaryHigh 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?
Law et al 2008; Matear & Lenton 2008; McNeil & Matear 2008
Thank you, Questions?
Box Model Representation of the SO
ΔFA/ΔFN=−(pCO2A(t)−280)/(pCO2Deep−280) 1
2
pCO2A equals 310 and 370 ppm. 3
deep water pCO2 equals 450 ppm 4
5
Ratio of change in anthropogenic to natural carbon flux 6
of -0.17 to -0.55 7
Metzl and Lenton in prep
Metzl and Lenton in prep
Southern Ocean Thermohaline Circulation
Modified from Speer et al 2000
Antarctic ZoneSAZSTF
PF
CO2 Uptake: Response to SSS restoring