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

Abstract. 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_ANT methods 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.