1) The document reconstructs sparse dissolved inorganic carbon (DIC) observations in the Southern Ocean using a neural network mapping method. 2) The mean amplitude of the surface seasonal cycle in DIC is estimated to be 13 μmol/kg, with maxima occurring in austral spring and minima in austral autumn. 3) Estimates of net community production in summer indicate a drawdown of 1.3±0.9 petagrams of carbon, with most removal occurring north of the Antarctic Circumpolar Current. 4) Preliminary results suggest inter-annual variability in DIC is dominated by an anthropogenic trend, strongest in ocean uptake regions between 35-55°S.

1. Lydia Keppler
(she/her)
lydia.keppler@mpimet.mpg.de
@lydikeppler
Co-Authors: P. Landschützer, S. K. Lauvset, N. Gruber, & J. D. Müller
Reconstructing sub-surface
dissolved inorganic carbon from observations
in the Southern Ocean
The research leading to these results has received funding from the EU Horizon 2020 research and innovation program
under grant agreement no 821003 (4C project). The content of this presentation reflects only the author’s view. The
European Commission is not responsible for any use that may be made of the information it contains.

2. The seasonal cycle of inorganic carbon
in the Southern Ocean
Figure adapted from Mongwe et al. (2018)
Sea- air CO2 flux in observational estimates and
10 CMIP5 models between 1995 and 2005
• The seasonal cycle is the largest signal in the
natural carbon cycle in the ocean
• The seasonal cycle of carbonate system
parameters differs greatly among models,
especially in the Southern Ocean (Mongwe et
al., 2018)
1

3. The seasonal cycle of inorganic carbon
in the Southern Ocean
Figure adapted from Mongwe et al. (2018)
Sea- air CO2 flux in observational estimates and
10 CMIP5 models between 1995 and 2005
• The seasonal cycle is the largest signal in the
natural carbon cycle in the ocean
• The seasonal cycle of carbonate system
parameters differs greatly among models,
especially in the Southern Ocean (Mongwe et
al., 2018)
• A better constrained seasonal cycle of dissolved
inorganic carbon (DIC) is important for a better
understanding of the global carbon cycle and a
better representation of the carbon cycle in
Earth System Models (Nevison et al., 2016)
1

4. Image credit: Nicolas Metzl, LOCEAN/IPSL Laboratory 2

5. Observations in the Southern Ocean
SOCATv2019 (at the surface,
from 2004 through 2017)
GLODAPv2.2019 (at 10 m,
from 2004 through 2017)
SOCCOM floats (at 10 m,
from 2014 through 2017)
SOCCOMGLODAPSOCAT
autonomous floats
Figures adapted from Keppler (2020) 3

6. Gap-filling sparse observations
• Observations are sparse, heterogenously distributed
in time and space
• Different regions of outgassing and uptake in the
Southern Ocean, regions of hot spots
(e.g., Rintoul et al., 2018)
4Figures adapted from Keppler et al. (under review at GBC)

7. Gap-filling sparse observations
• Observations are sparse, heterogenously distributed
in time and space
• Different regions of outgassing and uptake in the
Southern Ocean, regions of hot spots
(e.g., Rintoul et al., 2018)
→ We need gap-filled mapped data products for
this regional study
mapping
4Figures adapted from Keppler et al. (under review at GBC)

8. SOM-FFN mapping of DIC
Adaptation of the SOM-FFN approach by Landschützer et al., 2014;
Figures adapted from Keppler et al. (under review at GBC)
5

9. SOM-FFN mapping of DIC
123456
clusternumber
SOM
Adaptation of the SOM-FFN approach by Landschützer et al., 2014;
Figures adapted from Keppler et al. (under review at GBC)
5

10. SOM-FFN mapping of DIC
123456
clusternumber
SOM
FFN
Adaptation of the SOM-FFN approach by Landschützer et al., 2014;
Figures adapted from Keppler et al. (under review at GBC)
5
(temporal mean at 10 m)

11. Temporal mean Southern Ocean DIC fields
10 m 100 m 300 m 500 m
The black lines mark the Subtropical Front (~40°S) and the Polar Front (55~°S)
from Orsi et al. (1995)
6Figures adapted from Keppler et al. (under review at GBC)

12. Surface seasonal cycle of DIC in the Southern Ocean
Surface DIC maxima in austral
spring (Aug‒Oct), minima in
austral autumn (Mar‒May)
Figures adapted from Keppler et al. (under review at GBC) 7

13. Surface seasonal cycle of DIC in the Southern Ocean
Mean: 13 μmol kg-1
(global mean: 16 μmol kg-1)
Surface DIC maxima in austral
spring (Aug‒Oct), minima in
austral autumn (Mar‒May)
Figures adapted from Keppler et al. (under review at GBC) 7
Surface DIC maxima

14. Comparison with SOCCOM floats
and at the Drake Passage
8
surface seasonal cycle
RMSE = 23 µmol/kg
(bias = 16 µmol/kg)
Figures adapted from Keppler et al. (under review at GBC)

15. Comparison with SOCCOM floats
and at the Drake Passage
8
surface seasonal cycle
RMSE = 23 µmol/kg
(bias = 16 µmol/kg)
RMSE = 30 µmol/kg
Figures adapted from Keppler et al. (under review at GBC)

16. • We can now estimate the biological draw-down over summer,
i.e., the summer net community production (summer NCP)
• Summer NCP =
Biological draw-down of DIC during summer
Figures adapted from Keppler et al. (under review at GBC) 9

17. Biological draw-down of DIC during summer
• We can now estimate the biological draw-down over summer,
i.e., the summer net community production (summer NCP)
• Summer NCP =
• Most biological draw-down of carbon in the Southern Ocean
occurs north of the ACC (an area of mean carbon uptake)
• Integrated over the Southern Ocean, we estimate a summer
NCP of 1.3±0.9 PgC per summer
(global: 8.2±5.6 PgC per summer)
Figures adapted from Keppler et al. (under review at GBC) 9

18. Outlook: Inter-annual variability of DIC (prelim.)
(seasonal cycle removed)
• Strongest signal: Anthropogenic trend
Figures adapted from Keppler et al. (in prep.) 10

19. Outlook: Inter-annual variability of DIC (prelim.)
(seasonal cycle removed)
• Strongest signal: Anthropogenic trend
• Most dominant in uptake region (35° to STF)
Figures adapted from Keppler et al. (in prep.) 10

20. Summary
11
• Reconstructed sparse DIC observations (surface & interior,
monthly climatology) with an adaptation of the SOM-FFN
method (gap-filling)

21. Summary
11
• Reconstructed sparse DIC observations (surface & interior,
monthly climatology) with an adaptation of the SOM-FFN
method (gap-filling)
• Mean amplitude of surface seasonal cycle in the Southern
Ocean: 13 μmol/kg
• Surface DIC maxima tend to be in austral spring (Aug‒Oct) and
surface DIC minima in austral autumn (Mar‒May)

22. Summary
11
• Reconstructed sparse DIC observations (surface & interior,
monthly climatology) with an adaptation of the SOM-FFN
method (gap-filling)
• Mean amplitude of surface seasonal cycle in the Southern
Ocean: 13 μmol/kg
• Surface DIC maxima tend to be in austral spring (Aug‒Oct) and
surface DIC minima in austral autumn (Mar‒May)
• Summer NCP of 1.3±0.9PgC in the Southern Ocean

23. Summary
11
• Reconstructed sparse DIC observations (surface & interior,
monthly climatology) with an adaptation of the SOM-FFN
method (gap-filling)
• Mean amplitude of surface seasonal cycle in the Southern
Ocean: 13 μmol/kg
• Surface DIC maxima tend to be in austral spring (Aug‒Oct) and
surface DIC minima in austral autumn (Mar‒May)
• Summer NCP of 1.3±0.9PgC in the Southern Ocean
• Inter-annual variability: Anthropogenic signal dominates,
strongest in uptake regions (prelim. results)

24. Summary
Thank you 11
For more details look out for our upcoming paper on a global scale:
Seasonal carbon dynamics in the global ocean based on a neural-network mapping of observations
(under review at Global Biogeochemical Cycles)
• Reconstructed sparse DIC observations (surface & interior,
monthly climatology) with an adaptation of the SOM-FFN
method (gap-filling)
• Mean amplitude of surface seasonal cycle in the Southern
Ocean: 13 μmol/kg
• Surface DIC maxima tend to be in austral spring (Aug‒Oct) and
surface DIC minima in austral autumn (Mar‒May)
• Summer NCP of 1.3±0.9PgC in the Southern Ocean
• Inter-annual variability: Anthropogenic signal dominates,
strongest in uptake regions (prelim. results)

25. Extra: SOM-FFN mapping of sparse DIC measurements
• Target variable: interior ocean DIC
(GLODAPv2.2019)
• SOM-input:
• Temperature and salinity (Argo)
• DIC annual-mean climatology (Lauvset)
• FFN-predictors:
• Temperature and salinity (Argo)
• Dissolved oxygen, silicate, nitrate
(World Ocean Atlas 2018)
target
FFN-predictor
Figures (target) plotted with GLODAP data (Olsen et al., 2016);
(SOM-clustering) adapted from Keppler et al. (under review at GBC);
(FFN-predictor) plotted with Argo data product (Roemmich & Gilson, 2009);
SOM-clustering

26. Extra: Zonal mean processes in the Southern Ocean
Schematic adapted from Keppler (2020), inspired by Talley et al. (2011)