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