Transport and storage of anthropogenic C across the Greenland-Portugal OVIDE section: Observations vs NEMO-PISCES

Authors: Racapé V., Zunino P., Lherminier P., Bopp L., Mercier, H. and Gehlen M.

At present, the ocean takes up approximately one-third of total anthropogenic carbon dioxide emissions. The North Atlantic Ocean is a major sink region for anthropogenic carbon (C_ANT) and a major contributor to its storage and transport. While evidence is growing that storage and transport are variable on interannual to decadal timescales, their evolution in recent decades and in the future remains uncertain. The variability is thought to be controlled in large part by the intensity of the meridional overturning circulation (MOC), which is suggested to slow down over the 21^st century. This study aims to investigate the relationship between C_ANT transport across the Greenland-Portugal OVIDE section and Cant storage in the North Atlantic basin, as well as their combined evolution over the past 40 years. It relies on the combination of a pluriannual data set and output from the global biogeochemical ocean general circulation model NEMO/PISCES at 1/2° spatial resolution forced by the atmospheric reanalysis DFS4.2. Based on an observation-model comparison, the C_ANT advective transport is largely underestimated by the model (5 times) compared to the data-based estimate. However, the vertical gradient of C_ANT in the water column is well reproduced by NEMO/PISCES, supporting the use of a novel indicator computed by Zunino et al. (2014) to express the variability of C_ANT transport (T°CANT). Following the approach by Zunino et al. (2014), the estimator, T°CANT, was first derived in the model from MOCσ and DELTA-C_ANT (the vertical gradient of C_ANT between the upper and the lower branch of MOCσ) averaged over the month of June. Next, the representativity of T°CANT derived from a single sampling event per year was evaluated against estimates computed from annual mean values of MOCσ and DELTA-C_ANT. After evaluation of the model skill over the period covered by observations, the model is used to extrapolate backward and forward in time. Here the approach was applied to the analyses of the interannual variability of C_ANT transport and storage over the past 40 years.