Unaccounted role of Mediterranean Water in the drawdown of anthropogenic carbon

Abstract

A three‐sided box, MedBox, comprising the Strait of Gibraltar, 24°–41°N east of 22°W, was constructed using hydrographic and chemical data from three World Ocean Circulation Experiment cruises in 1997–1998. An extended optimum multiparameter (OMP) analysis resolves the water mass structure in the MedBox. The mass transport is estimated from data and models warranting mass conservation and no deep water formation within the region. Combining the mass transport, the OMP analysis, and the spatial distribution of anthropogenic carbon (CANT), some relevant questions about the Mediterranean Water (MW) formation, transport, and role in the global CO2 cycle are resolved. MW is produced at a rate of 2.7 ± 0.2 Sv in the eastern North Atlantic. It entrains 2.3 Sv of central waters during its formation, including also remnants of Antarctic Intermediate Water. The main advection route of MW is northward, close to the Iberian margin (2.2 Sv), while about 0.5 Sv are exported into the subtropical eastern North Atlantic. The eastern North Atlantic within the MedBox is a strong region of air‐sea CANT uptake, 0.7 ± 0.7 mmol m−2 d−1, where CANT is accumulated at a rate of 66 ± 21 kmol s−1 (0.025 GtC yr−1), 17% are taken up through the air‐sea interface, and the rest is advected into the area by the circulation. The entrainment of central waters to form MW drives a drawdown of 151 ± 14 kmol s−1 (0.06 GtC yr−1) from the surface to intermediate levels, and 88 ± 8 kmol s−1 (0.03 GtC yr−1) are exported with MW into the North Atlantic, mainly by the horizontal/eddy circulation.