Abstract
Abstract. The considerable uncertainties in the carbon budget of the Southern Ocean are largely attributed to unresolved variability, in particular at a seasonal timescale and small spatial scale (~ 100 km). In this study, the variability of surface pCO2 and dissolved inorganic carbon (DIC) at seasonal and small spatial scales is examined using a data set of surface drifters including ~ 80 000 measurements at high spatiotemporal resolution. On spatial scales of 100 km, we find gradients ranging from 5 to 50 μatm for pCO2 and 2 to 30 μmol kg−1 for DIC, with highest values in energetic and frontal regions. This result is supported by a second estimate obtained with sea surface temperature (SST) satellite images and local DIC–SST relationships derived from drifter observations. We find that dynamical processes drive the variability of DIC at small spatial scale in most regions of the Southern Ocean and the cascade of large-scale gradients down to small spatial scales, leading to gradients up to 15 μmol kg−1 over 100 km. Although the role of biological activity is more localized, it enhances the variability up to 30 μmol 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 drifter observations and proves to be a reasonable first-order estimate of the seasonality in the Southern Ocean that could be used to validate model simulations. We find that small spatial-scale 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, interannual variability, decadal trends) of the carbon budget from low-resolution observations and models.
Highlights
The Southern Ocean is a key region for the global carbon cycle and the climate system
After drifting along the Subantarctic Front (SAF), it crossed the Southwest Indian Ridge in January 2007, where the Polar Front (PF) and SAF combine in what is the major pathway of the Antarctic Circumpolar Current (ACC) across the ridge (35◦ E–48◦ S, Fig. 2)
The pCO2 and dissolved inorganic carbon (DIC) variability at a small spatial scale in the Southern Ocean is estimated using an extensive data set of surface drifter measurements
Summary
The Southern Ocean is a key region for the global carbon cycle and the climate system. The Southern Ocean remains the region where uncertainties regarding the air–sea CO2 flux and the carbon budget are the highest (e.g., Gruber et al, 2009). Quantifying this subtle balance and its sensitivity to dynamical and biological changes is one of the most challenging issues in assessing today’s global carbon budget. The mean of all models agree surprisingly well with observations to an uptake of ∼ 0.34 ± 0.2 PgC yr−1 in both Gruber et al (2009) and Lenton et al (2013), individual model results are spread over a wide range, between 0.15 and 0.72 PgC yr−1 in Gruber et al (2009) and between −0.3 (i.e., out-gazing) and 0.3 PgC yr−1 in Lenton et al (2013)
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