Role of the circulation on the anthropogenic CO2 inventory in the North-East Atlantic: A climatological analysis

Abstract

Climatology-based storage rate of anthropogenic CO2 (Cant, referred to year 2000) in the North-East Atlantic (53 ± 9 kmol s−1, 0.020 ± 0.003 Pg-C yr−1) is described on annual mean terms. Cant advection (32 ± 14 kmol s−1) occurs mostly in the upper 1800 m and contributes to 60% of the Cant storage rate. The Azores and Portugal Currents act as ‘Cant streams’ importing 389 ± 90 kmol s−1, most of which recirculates southwards with the Canary Current (−214 ± 34 kmol s−1). The Azores Counter Current (−79 ± 36 kmol s−1) and the northward-flowing Mediterranean Water advective branch (−31 ± 12 kmol s−1) comprise secondary Cant export routes. By means of Cant transport decomposition, we find horizontal circulation to represent 11% of the Cant storage rate, while overturning circulation is the main driver (48% of the Cant storage rate). Within the domain of this study, overturning circulation is a key mechanism by which Cant in the upper layer (0–500 dbar) is drawdown (74 ± 14 kmol s−1) to intermediate levels (500–2000 dbar), and entrained (37 ± 7 kmol s−1) into the Mediterranean Outflow Water to form Mediterranean Water. This newly formed water mass partly exports Cant to the North Atlantic at a rate of −39 ± 9 kmol s−1 and partly contributes to the Cant storage in the North-East Atlantic (with up to 0.015 ± 0.006 Pg-C yr−1). Closing the Cant budget, 40% of the Cant storage in the North-East Atlantic is attributable to anthropogenic CO2 uptake from the atmosphere (21 ± 10 kmol s−1).