1. Hotspot of CO2 fluxes along the Polar Front
Link with the Southern Ocean circulation
Landshutzer figure show variability of the carbon fluxes in the Southern Ocean, chnagtes from sink to source and back to sink of carbon to ocean circulation changes.
Overturning circulation but also the circumpolar current
Landshutzer et al., 2015
Ocean & Atmosphere – Climate Science CENTER
Clothilde Langlais, Andrew Lenton, Paula Condepardo, Judith Hauck, JB Sallee, Andreas Klocker, Joan Llort, Richard Matear, Steve Rintoul

2. What do we know about Carbon fluxes? What are the limitations?
Lenton et al., 2013
Air-sea CO2 fluxes year 2000
44oS
outgazing
58oS
Southern Ocean is responsible for a carbon uptake of 0.27to 0.43 PgC/yr south of 44∘S , and ~1PgC/yr south of 30oS.
Limitations:
Lack of observations
Poor Understanding of high latitude carbon cycle
Poor Understanding ocean circulation
OBSERVATIONS
uptake
The Southern Ocean south of 44∘S is responsible for a carbon uptake of 0.27to 0.43 PgC yr-1, which includes approximately 25–30% of the global ocean uptake of anthropogenic carbon [Lenton et al., 2013]. But accurately quantifying this sink and understanding the processes behind it remains challenging.
Most of sink happening between S – compensating effect south 58S and unsure if outgassing or uptake.
1.Lack of observation:
The net ocean-atmosphere CO2 flux of the high-latitude, seasonally ice-covered Southern Ocean is especially difficult to quantify due to a scarcity in observational data [Bakker et al., 2014].
2.Poor understanding of the high-latitude carbon cycle:
*the role of the coastal ocean and sea-ice zone, that have a high variability in biological production, is not well captured.
*complex interplay of physical and biological processes that drive sea–air CO2 exchange.
fig.2 Takahashi et al., 2002: competition between seasonal temperature effects and biological drawdown . In some areas the temperature effects exceed that of the biological utilization of CO2; where for others, the biological effect dominates the seasonal changes in surface-water pCO2.
3.Poor understanding of the upwelling of CDW

3. Overturning circulation and CO2 fluxes
Surfacing of CDW favours outgassing
Changes in the overturning has been linked with
changes in strength of the carbon sink
Interannual-decadal scale
[Lenton and Matear , 2007; Lovenduski et al. , 2008]
[Landshutzer et al. 2015]
Glacial-interglacial scale
[Barker et al., 2009 ; Anderson et al., 2009]
Deep-ocean:
main reservoir of carbon
The deep ocean is the main reservoir of carbon in the earth system
The Southern Ocean overturning circulation, connecting ocean interior with surface layers and sustaining the global carbon cycle .
CDW move upwards along the sloping isopycnals that rise to the south in the vicinity of the Antarctic Circumpolar Current (ACC), eventually outcropping towards its central and southern areas.
Modelling studies suggest that the upwelling of CDW is central to understand future response of the carbon sink levels (Lenton and Matear , 2007; Lovenduski et al. , 2008). CDW are relatively warm and salty waters rich in natural DIC, overlaid by colder and fresher surface waters (Antarctic Surface Waters ASW). When CDW reach the surface and are exposed to the atmosphere, their relative temperature and natural DIC content favour CO2 outgassing. [The upwelling also favour heat, oxygen and Cant uptake by the ocean because the upwelled waters are constantly transported away from the uptake zone through Ekman transport (Ito et al., 2010 ; Armour et al. 2016) .] At large scale, there has been a strengthening of zonal winds south of 45∘S, which is projected to increase in future (Le Quéré et al., 2007). A consequence will be an enhanced upwelling of nutrient and carbon in the Southern Ocean, slowing the increase in the carbon sink expected with rising atmospheric CO2 levels (Lenton and Matear , 2007; Lovenduski et al. , 2008).
But how well do we understand the upwelling of CDW?
Rintoul, 2000

4. What do we know about CDW pathways?
The upward pathway and the underlying physical mechanisms remain poorly understood.
Outcropping of CDW : in-between South ACC front (sACCf) and ACC Southern Boundary (sBy) [Orsi et al. 1995]
Transport and transformation of CDW
Mid-depth : along-isopycnals flow + wind-induced divergence + air-sea fluxes
Deeper : diapycnal mixing accounts for most of the transport
How well do we understand the upwelling fo CDW?
The upward pathway of these waters and the underlying physical mechanisms remain poorly understood.
Outcropping between sAAf and Southern Byd [Orsi et al., 1995]
It is believed that the return flow of the mid-depth cell, formed in the North Atlantic, is primarily along density
surfaces all the way to the Southern Ocean, where the dense waters come to the surface and are transformed back to light
waters due to air-sea fluxes [e.g., Marshall and Speer, 2012, review]. Nikurashin and Ferrari 2013
mixing drives a small fraction of transport in the upper ocean,
but accounts for most transport in the abyss [e.g., Marshall
and Speer, 2012, review].
[Marshall and Speer, 2012, Nikurashin and Ferrari 2013 ]

5. 1/10O Ocean Forecasting Australia Model (OFAM3) Zhang et al., 2016
MOM4 ocean model
50 z-level
WOMBAT BGC model
20 yr Spin-up with 1979
historical run :
JRA-55 surface forcing (bulk formulae)
State-independent restoring below 2000m
OBSERVATIONS
1/10o
ACC jets structure matters, how it influences Co2 air-sea fluxes.

6. Observations Takahashi et al., 2009 nominal year 2000
Air-sea CO2 fluxes
uptake
uptake
outgassing
outgassing
Observations Takahashi et al., 2009 nominal year 2000
Historical
OFAM3 JRA55

7. CO2 outgassing, ACC jets and CDW outcrop
Lenton et al. 2013: 0.27to 0.43 PgC/yr
44oS
44oS
0.33 Pg/yr
0.32 Pg/yr
PF-south
58oS
-0.01 Pg/yr
Upwelling of CDW near the divergence zone

8. Anomaly of ocean pCO2 along the PF-south
LONGITUDE
the gas transfer coefficient and the
difference between the partial pressures of seawater and the Atmosphere
oceanic pCO2 is highly variable in time and space
two major oceanic mechanisms that can explain the difference in pCO2 along the front: (i) differnet intensity
of biological production through changes in DIC and ALK; and/or
(ii) changes in ocean physics that can lead
to changes in SST, SSS, DIC and ALK in the upper ocean. dx dx dx dx dx

9. Anomaly of ocean pCO2 along the PF-south
DIC
DIC
DIC-T
DIC-T
DIC-T
DIC-T
DIC+ALK T S
oceanic pCO2 is highly variable in time and space
two major oceanic mechanisms that can explain the difference in pCO2 along the front: (i) differnet intensity
of biological production through changes in DIC and ALK; and/or
(ii) changes in ocean physics that can lead
to changes in SST, SSS, DIC and ALK in the upper ocean.
DIC
ALK
DIC+ALK

10. Supply of DIC in the mixed layer
Anomaly of ocean pCO2 along the PF-south
DIC in mixed layer
LONGITUDE

11. Less outgassing section at 110E September Ekman pumping CO2 flux sBy
sACCf PF
PF-south
natural
CDW
ASW

12. Stronger Outgassing Section at 80E September Ekman pumping CO2 flux
sBy PF
Merging of ACC jets at the Fawn Trough
PF-south
sACCf
natural
CDW
ASW

13. CDW surfacing is influenced by bathymetry
Take home message:
CDW surfacing is influenced by bathymetry
Surfacing of CDW favours outgassing
Changes in the overturning has been linked with
changes in strength of the carbon sink
Interannual-decadal scale
[Lenton and Matear , 2007; Lovenduski et al. , 2008]
[Landshutzer et al. 2015]
Glacial-interglacial scale
[Barker et al., 2009 ; Anderson et al., 2009]
Deep-ocean:
main reservoir of carbon
The deep ocean is the main reservoir of carbon in the earth system
The Southern Ocean overturning circulation, connecting ocean interior with surface layers and sustaining the global carbon cycle .
CDW move upwards along the sloping isopycnals that rise to the south in the vicinity of the Antarctic Circumpolar Current (ACC), eventually outcropping towards its central and southern areas.
Modelling studies suggest that the upwelling of CDW is central to understand future response of the carbon sink levels (Lenton and Matear , 2007; Lovenduski et al. , 2008). CDW are relatively warm and salty waters rich in natural DIC, overlaid by colder and fresher surface waters (Antarctic Surface Waters ASW). When CDW reach the surface and are exposed to the atmosphere, their relative temperature and natural DIC content favour CO2 outgassing. [The upwelling also favour heat, oxygen and Cant uptake by the ocean because the upwelled waters are constantly transported away from the uptake zone through Ekman transport (Ito et al., 2010 ; Armour et al. 2016) .] At large scale, there has been a strengthening of zonal winds south of 45∘S, which is projected to increase in future (Le Quéré et al., 2007). A consequence will be an enhanced upwelling of nutrient and carbon in the Southern Ocean, slowing the increase in the carbon sink expected with rising atmospheric CO2 levels (Lenton and Matear , 2007; Lovenduski et al. , 2008).
But how well do we understand the upwelling of CDW?
Perspective:
Interannual variability
Seasonal cycle of CO2 fluxes
Rintoul, 2000

14. Which resolution for this problem?
Observations
1/10o
1/4o
Here we will Follow the Cant pathways to ocean interior in 2 models.
Both MOM and WOMBAT.
Big difference between high low res model is in the spin up
Ocean model driven by Japanese atm reanalysis
Blended forcing
Do not represent the ACC in the same way : sharper jets, standing meanders. DOES it matter for Cant subduction?
PF and SAF 1o
Future of C in Southern Ocean| C. Langlais

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16. Fig. Supp 1 : Stationary Rossby waves: location and impact of resolution . a. AVISO time-mean geostrophic velocities and SSH contours and b. 1/10 ° model topography. Time-mean model horizontal velocities average over the top 200m and SSH contours (left), and time-mean model vertical velocities at 200m (right), for 3 different resolution: 1/10° (c. and d.), 1/4 ° (e. and f.) and 1 ° (g. and h.). Black contours show SSH contours with SAF and PF in bold.

17. Anthropogenic Carbon (Cant ) in Southern Ocean
Southern Ocean contributes to
30% of sink of anthropogenic carbon
dioxide
Inventory of 25+/-5 Pg C
in Southern Hemisphere
The Southern Ocean makes a substantial contribution to the oceanic carbon sink: more than 30% of the anthropogenic carbondioxide enter the ocean south of 40 S.
High uptake of anthropogenic carbon peaks at the Antarctic polar front (55S). however very little is stored there, high inventory north of 55 S.
This apparent difference between the implies that a large fraction of ACO2 absorbed into the Southern Ocean is subsequently exported to the northern basins. efficient uptake and storage of Cant by the Southern Ocean is a result of the vigorous overturning circulation
Inventory of Cant in southern hemisphere from GLODAP is Pg C in SE for 26 to 27.8 kg
Ito et al. 2010
Future of C in Southern Ocean| C. Langlais

20. Takahashi et al., 2002

21. Presentation title | Presenter name