Estimating the storage of anthropogenic carbon in the subtropical Indian Ocean: a comparison of five different approaches

M. Álvarez (1) , Claire Lo Monaco (2) , Toste Tanhua (3) , A. Yool (4) , Andreas Oschlies (3) , John L. Bullister (5) , Catherine Goyet (6) , Nicolas Metzl (2) , Franck Touratier (6) , E. Mcdonagh (4) , H. L. Bryden (4)

1 IMEDEA - Institut Mediterrani d'Estudis Avancats
2 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
3 IFM-GEOMAR - Leibniz-Institut für Meereswissenschaften
4 NOC - National Oceanography Centre [Southampton]
5 NOAA - National Oceanic and Atmospheric Administration
6 IMAGES-Espace DEV - Institut de Modélisation et d'Analyses en géo-environnement et santé - Espace Développement

The subtropical Indian Ocean along 32° S was for the first time simultaneously sampled in 2002 for inorganic carbon and transient tracers. The vertical distribution and inventory of anthropogenic carbon (C_ANT) from five different methods: four data-base methods (ΔC*, TrOCA, TTD and IPSL) and a simulation from the OCCAM model are compared and discussed along with the observed CFC-12 and CCl₄ distributions. In the surface layer, where carbon-based methods are uncertain, TTD and OCCAM yield the same result (7±0.2 molC m−2), helping to specify the surface C_ANT inventory. Below the mixed-layer, the comparison suggests that C_ANT penetrates deeper and more uniformly into the Antarctic Intermediate Water layer limit than estimated from the much utilized ΔC* method. Significant CFC-12 and CCl₄ values are detected in bottom waters, associated with Antarctic Bottom Water. In this layer, except for ΔC* and OCCAM, the other methods detect significant C_ANT values. Consequently, the lowest inventory is calculated using the ΔC* method (24±2 molC m−2) or OCCAM (24.4±2.8 molC m−2) while TrOCA, TTD, and IPSL lead to higher inventories (28.1±2.2, 28.9±2.3 and 30.8±2.5 molC m−2 respectively). Overall and despite the uncertainties each method is evaluated using its relationship with tracers and the knowledge about water masses in the subtropical Indian Ocean. Along 32° S our best estimate for the mean C_ANT specific inventory is 28±2 molC m−2. Comparison exercises for data-based C_ANTmethods along with time-series or repeat sections analysis should help to identify strengths and caveats in the C_ANT methods and to better constrain model simulations.

Domaines: Géophysique [physics.geo-ph]

M. Álvarez, Claire Lo Monaco, Toste Tanhua, A. Yool, Andreas Oschlies, et al.. Estimating the storage of anthropogenic carbon in the subtropical Indian Ocean: a comparison of five different approaches. Biogeosciences, 2009, 6 (4), pp.681-703. ⟨10.5194/bg-6-681-2009⟩. ⟨hal-01205910⟩