Mode Waters provides a privileged pathway for the transport of heat, salt and anthropogenic CO₂ into the ocean interior. The carbon cycle decadal variability in response to environmental changes is investigated using historical and recent data collected during the INDIGO (1985-1987) and OISO (1998-2003) oceanographic campaigns conducted in the South West Indian Ocean, an important zone for Mode Waters formation. The observed change in dissolved inorganic carbon over the 15-year period was 8 µmol/kg in Subantarctic Mode Water (500-800m), which is less than the anthropogenic carbon increase alone (13 µmol/kg). This difference may be explained by natural or climate-induced changes in ocean processes. Predictions from a global ocean-carbon model (OPA-PISCES) are used as a means to help interpret changes in the controlling processes: ocean dynamics, biological activity and air-sea interactions.

We investigated the interannual variability of the partial pressure of carbon dioxide (pCO₂) in the surface waters of the western subarctic gyre (155°E to 165°E, 48°N to 53°N) and the Alaska Gyre (AG, 195°E to 210°E, 45°N to 52°N) for a period of 9 years. We used automated measurements of pCO₂ in the surface water (pCO_2sw) and the air (pCO_2air) as well as sea surface temperature (SST) and salinity (S) obtained from the Japanese-Canadian joint Volunteer Observing Ship (VOS) program. We observed annual trends in the pCO_2sw and based on simple least square fit to observed data, the surface waters in the WSG showed a gradual pCO_2sw increase of 0.4 ppm yr^-1 which was three times larger than in the AG (1.8 ppm yr^-1) for the 9-year period. In the WSG, this was about half of the estimated atmospheric pCO₂ increase for the whole period (10 ppm or 1.2 ppm yr^-1), whereas gas exchange explained much of the increase in the AG (pCO_2air increased 1.6 ppm yr^-1). Interestingly, the two gyres showed opposite annual trends in the SST and salinity and in the WSG we observed a salinity and SST increase of 0.018 yr^-1 and 0.07°C yr^-1 (0.56°C for the whole study period), respectively, whereas we observed a small freshening of 0.015 yr^-1 and a cooling trend of about 0.11°C yr^-1 in the AG. We examine the possible mechanisms to explain the annual trends in pCO₂, based on the observed changes in SST and salinity as well as observations made by other investigators.

A combined study of micrometeorological flux measurement of carbon dioxide (CO₂) and methane (CH₄) and measurement of their stable carbon isotopes at a paddy field indicated that CH₄ production can affect not only greenhouse gas budget of wetland ecosystem but also isotopic signature of respired CO₂.

The Orbiting Carbon Observatory (OCO) mission will deliver space-based observations of atmospheric CO₂ with the potential to resolve many of the uncertainties in the spatial and temporal variability of carbon sources and sinks.  Our assessments of the measurement requirements for space-based remote sensing of atmospheric CO₂ conclude that the data must support retrievals of the column-averaged CO₂ dry air mole fraction, X_CO2, with precisions of 3 to 4 ppm to resolve the annually averaged gradients between the Northern and Southern hemispheres, but higher precision (1 to 2 ppm) will be needed to resolve East-West gradients and questions like the location and spatial extent of the Northern Hemisphere terrestrial carbon sink.  These conclusions are derived from the results of observational system simulation experiments (OSSEs) and synthesis inversion models [Rayner and O’Brien, 2001; O’Brien and Rayner, 2002; Rayner et al., 2002]. The X_CO2 precision requirements also considered the OCO mission design, the amplitude of X_CO2 spatial and temporal gradients, and the relationship between X_CO2 data precision and regional scale surface CO₂ flux uncertainties inferred from X_CO2 data.

Accurate estimates of the spatial distribution of pre-industrial and anthropogenic air-sea carbon fluxes are crucial to understanding the processes driving ocean carbon uptake. We present regional anthropogenic and pre-industrial air-sea fluxes estimated separately from their reconstructed concentrations and Ocean General Circulation Models (OGCM). The ocean interior carbon transports required to explain these fluxes are calculated and their implications for the global carbon cycle are discussed. 

Physical processes in the Southern Ocean are known to profoundly impact the global carbon cycle, but this region is one of the most difficult to simulate consistently in ocean general circulation models (OGCMs). Here we show that Southern Hemisphere winds, by altering the volume of light, actively-ventilated ocean water as well as the relative contribution to this volume from Ekman transport, exert strong control over both the magnitude and distribution of anthropogenic carbon uptake in an OGCM. These results are provocative in suggesting that climate warming, by increasing the magnitude of the wind stress at high southern latitudes, may act as a negative feedback on the global carbon cycle.

We report the interannual variations of winter CO₂ partial pressure in surface waters (pCO₂^sea) and overlying air (pCO₂^air) and air-sea CO₂ flux in the extensive area (3-34°N) from subtropical to equatorial along 137°E during the period of 1983-2003. The pCO₂^sea varied largely in the equatorial region of 3-6°N, depending on the variations of the oceanographic conditions related to the El Niño-Southern Oscillation (ENSO) events. The pCO₂^sea variations in the subtropical gyre north of 23°N were small due to highly counteracting effects between anti-correlated sea surface temperature (SST) and dissolved inorganic carbon (DIC) anomalies through the entrainment process, irrespective of large variations of SST. By contrast, it was found that there occurred a low negative correlation between SST and DIC in the region restricted around 15-18°N in the North Equatorial Current, which resulted in a large amplitude of variations of pCO₂^sea and hence CO₂ influx. The interannual variations of CO₂ flux depended predominantly on those of the difference between pCO₂^sea and pCO₂^air (ΔpCO₂) south of 18°N but on those of wind speed in the northern region. 

Inverse modeling methods are now commonly used for estimating surface fluxes of carbon dioxide, using atmospheric mass fraction measurements combined with a numerical atmospheric transport model. Michalak et al. [2004] recently developed a geostatistical approach to flux estimation that takes advantage of the spatial and/or temporal correlation in fluxes and does not require prior flux estimates. In this work, a geostatistical implementation of a fixed-lag Kalman smoother is developed and applied to the recovery of gridscale carbon dioxide fluxes for 1997 – 2001 using data from the NOAA-CMDL Cooperative Air Sampling Network.

The decadal variability of air-sea CO₂ fluxes is poorly known in the southern hemisphere. To evaluate the changes or stability of these fluxes over several years, we compare seasonal observations obtained in 1991 and 2000 the Southern Indian Ocean. For summer and winter, we observed a significant increase of ocean fugacity (fCO₂) in subtropical waters (20°-35°S), about the same rate as in the atmosphere. In polar waters south of 40°S where meso-scale biological activity is high in summer, the rising of oceanic fCO₂ is only well detected when comparing austral winter data. The decadal evolution of fCO₂ observed in the cold waters certainly results from anthropogenic CO₂ emissions, but is also probably modulated by variations of primary production.