Anthropogenic Carbon Uptake in the Southern Ocean: Investigating the Spread Across Climate Models
Anthropogenic Carbon Uptake in the Southern Ocean: Investigating the Spread Across Climate Models
Abstract:
Model- and data-based analyses indicate that up to 50% of the anthropogenic CO2 ab-
sorbed by the ocean is taken up in the Southern Ocean south of 30°S. Recent syntheses based
on CMIP5 models show that much of these models' spread in anthropogenic CO2 uptake is
the result of disagreement within the Southern Ocean. The reasons for this disagreement
remain unclear. In this study, we hypothesize that the disagreement between CMIP5 models
is mainly due to ocean circulation, and more speci cally to the upwelling of deep waters un-
contaminated with anthropogenic CO2 at the Antarctic Divergence. To test this hypothesis,
we divided the Southern Ocean into four provinces, determined by the underlying mechanism
of anthropogenic CO2 uptake. The "upwelling" province is dominated by the upwelling of
deep water uncontaminated with anthropogenic CO2; the "WBC" province is dominated by
the cooling of subtropical waters transported southward by Western Boundary Currents; the
"mixed layer" province is dominated by a combination of the above two mechanisms and
coincides with winter deep mixed layers; finally the "convection" province is dominated by
open-ocean deep convection associated with polynyas. We explored the diff erent provinces
across a suite of climate models that consists of (1) a subset of CMIP5 models and (2) three
GFDL climate models which only diff er by the resolution of their ocean (1°, 0.25° and 0.1°).
The anthropogenic CO2 is calculated from the diff erence between an idealized climate change
simulation and a preindustrial control simulation. For each province, we compared the an-
thropogenic CO2 sink across the models. We found that the upwelling province consistently
shows the most intense sink across the models, and it is also where models tend to disagree the
most. We also found that increasing resolution decreases the uptake of anthropogenic CO2 in
the upwelling province.
sorbed by the ocean is taken up in the Southern Ocean south of 30°S. Recent syntheses based
on CMIP5 models show that much of these models' spread in anthropogenic CO2 uptake is
the result of disagreement within the Southern Ocean. The reasons for this disagreement
remain unclear. In this study, we hypothesize that the disagreement between CMIP5 models
is mainly due to ocean circulation, and more speci cally to the upwelling of deep waters un-
contaminated with anthropogenic CO2 at the Antarctic Divergence. To test this hypothesis,
we divided the Southern Ocean into four provinces, determined by the underlying mechanism
of anthropogenic CO2 uptake. The "upwelling" province is dominated by the upwelling of
deep water uncontaminated with anthropogenic CO2; the "WBC" province is dominated by
the cooling of subtropical waters transported southward by Western Boundary Currents; the
"mixed layer" province is dominated by a combination of the above two mechanisms and
coincides with winter deep mixed layers; finally the "convection" province is dominated by
open-ocean deep convection associated with polynyas. We explored the diff erent provinces
across a suite of climate models that consists of (1) a subset of CMIP5 models and (2) three
GFDL climate models which only diff er by the resolution of their ocean (1°, 0.25° and 0.1°).
The anthropogenic CO2 is calculated from the diff erence between an idealized climate change
simulation and a preindustrial control simulation. For each province, we compared the an-
thropogenic CO2 sink across the models. We found that the upwelling province consistently
shows the most intense sink across the models, and it is also where models tend to disagree the
most. We also found that increasing resolution decreases the uptake of anthropogenic CO2 in
the upwelling province.