Observed small spatial scale and seasonal variability of the CO₂-system in the Southern Ocean

Laure Resplandy (1) , Jacqueline Boutin (1) , Liliane Merlivat (1)

1 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques

The considerable uncertainties in the carbon budget of the Southern Ocean are largely attributed to unresolved variability, in particular at seasonal time scale and small spatial scale (~ 100 km). In this study, the variability of surface pCO₂ and DIC at seasonal and small-spatial scales is examined using a dataset of surface drifters including ~ 80 000 measurements at high spatio-temporal resolution. On spatial scales of 100 km, we find gradients ranging from 5 to 50 μ atm for pCO₂ and 2 to 30 μ mol kg^-1 for DIC, with highest values in energetic and frontal regions. This result is supported by a second estimate obtained with SST satellite images and local DIC/SST relationships derived from drifters observations. We find that dynamical processes drives the variability of DIC at small spatial scale in most regions of the Southern Ocean, the cascade of large-scale gradients down to small spatial scales leading to gradients up to 15 μ mol kg^-1 over 100 km. Although the role of biological activity is more localized, it enhances the variability up to 30 μ mol kg^-1 over 100 km. The seasonal cycle of surface DIC is reconstructed following Mahadevan et al. (2011), using an annual climatology of DIC and a monthly climatology of mixed layer depth. This method is evaluated using drifters observations and proves to be a reasonable first-order estimate of the seasonality in the Southern Ocean, which could be used to validate models simulations. We find that small spatial scales structures are a non negligible source of variability for DIC, with amplitudes of about a third of the variations associated with the seasonality and up to 10 times the magnitude of large-scale gradients. The amplitude of small-scale variability reported here should be kept in mind when inferring temporal changes (seasonality, inter-annual variability, decadal trends) of the carbon budget from low resolution observations and models.

Domaines: Géophysique [physics.geo-ph]; Géophysique [physics.geo-ph]

Laure Resplandy, Jacqueline Boutin, Liliane Merlivat. Observed small spatial scale and seasonal variability of the CO₂-system in the Southern Ocean. Biogeosciences Discussions, 2013, 10, pp.13855-13895. ⟨10.5194/BGD-10-13855-2013⟩. ⟨hal-00873740⟩