Air-sea CO<sub>2</sub> fluxes in the Atlantic as measured during the FICARAM cruises

Abstract

Abstract. A total of fourteen hydrographic cruises spanning from 2000 to 2008 were conducted during the spring and autumn seasons between Spain and the Southern Ocean, under the framework of the Spanish research project FICARAM. The performed underway measurements are processed and analysed to describe the meridional air-sea CO2 fluxes (FCO2) along the Atlantic Ocean. The data was organised into different biogeochemical oceanographic provinces, according mainly to the thermohaline characteristics. The obtained spatial and temporal distributions of FCO2 follow the generally expected patterns and annual trends. The Subtropical regions in both hemispheres alternated the CO2 source and sink nature from autumn to spring, respectively. On the other hand, Tropical waters and the Patagonian Sea clearly behaved as sinks of atmospheric CO2 like the waters of the Drake Passage during autumn. The obtained results during the cruises also revealed significant long-term trends, such as the warming of equatorial waters (0.11±0.03°C yr−1) and the decrease of surface salinity (−0.16±0.01 yr−1) in tropical waters caused by the influence of the Amazon River plume. This reduction in surface salinity appears to have a direct influence over the CO2 storage rates, fostering the uptake capacity of atmospheric CO2 (−0.09±0.03 mol m−2 yr−1). An analysis of the biogeochemical forcing on the CO2 fugacity (fCO2) variability performed from an empirical algorithm highlighted the major role of the Amazon River input in the tropical North Atlantic fluxes. In addition, it has provided a quantitative measure of the importance of the thermodynamic control of FCO2 at temperate latitudes.

Origin of CO₂ undersaturation in the western tropical Atlantic

“…Further north, lower sea surface salinities are observed in the North Equatorial Counter Current (NECC) that develops between approximately 2 • N and 10 • N. The climatology of Takahashi et al (2009) shows an fCO 2 gradient between these two current systems with higher values south of 2 • N and lower values in the NECC region but, on average, seawater fCO 2 is above the atmospheric level. This North-South gradient was also recently observed by Padin et al (2009) who reported near-equilibrium conditions between 1 • N and 8 • N around 28 • W during their FICARAM cruises while CO 2 supersaturation occurred further south between 1 • N and 15 • S. A few cruises have reported CO 2 concentrations below the atmospheric level in the north western tropical Atlantic and the origin of the CO 2 undersaturation was attributed to the discharge of the Amazon outflow (Ternon et al, 2000;Körtzinger, 2003). The Amazon is the largest river in the world representing about 20% of river supply to the world ocean, with a flow of 0.2 Sv (1 Sv = 10 6 m 3 s −1 ).…”

Increased CO2 outgassing in February‐May 2010 in the tropical Atlantic following the 2009 Pacific El Niño

“…In the tropical Atlantic, the variability of fCO 2 is affected by salinity changes [ Oudot et al ., ], and low fCO 2 values have been observed in the region of the NECC where high precipitation occurs because of the presence of the ITCZ [ Lefèvre et al ., ; Oudot et al ., ; Padin et al ., ]. This mechanism has also been observed in the equatorial Pacific [ Turk et al ., ] and is due to the chemical dilution taking place in these regions.…”