Observed small spatial scale and seasonal variability of the COsub2/sub system in the Southern Ocean

摘要

The considerable uncertainties in the carbon budget of the SouthernOcean are largely attributed to unresolved variability, in particularat a seasonal timescale and small spatial scale(~ 100 km). In this study, the variability of surfacepCO₂ and dissolved inorganic carbon (DIC) at seasonal and small spatial scales isexamined using a data set of surface drifters including ~ 80 000measurements at high spatiotemporal resolution. On spatial scales of100 km, we find gradients ranging from 5 to 50 μatmfor pCO₂ and 2 to 30 μmol kg?1 for DIC,with highest values in energetic and frontal regions. This result issupported by a second estimate obtained with sea surface temperature (SST) satellite images andlocal DIC–SST relationships derived from drifter observations. Wefind that dynamical processes drive the variability of DIC at smallspatial scale in most regions of the Southern Ocean and the cascade oflarge-scale gradients down to small spatial scales, leading togradients up to 15 μmol kg?1 over100 km. Although the role of biological activity is morelocalized, it enhances the variability up to 30 μmol kg?1over 100 km. The seasonal cycle ofsurface DIC is reconstructed following Mahadevan et al. (2011), using anannual climatology of DIC and a monthly climatology of mixed layerdepth. This method is evaluated using drifter observations and provesto be a reasonable first-order estimate of the seasonality in theSouthern Ocean that could be used to validate model simulations. Wefind that small spatial-scale structures are a non-negligible sourceof variability for DIC, with amplitudes of about a third of thevariations associated with the seasonality and up to 10 times themagnitude of large-scale gradients. The amplitude of small-scalevariability reported here should be kept in mind when inferringtemporal changes (seasonality, interannual variability, decadaltrends) of the carbon budget from low-resolution observations and models.