Dynamics of air–sea CO<sub>2</sub> fluxes in the northwestern European shelf based on voluntary observing ship and satellite observations

Abstract

Abstract. From January 2011 to December 2013, we constructed a comprehensive pCO2 data set based on voluntary observing ship (VOS) measurements in the western English Channel (WEC). We subsequently estimated surface pCO2 and air–sea CO2 fluxes in northwestern European continental shelf waters using multiple linear regressions (MLRs) from remotely sensed sea surface temperature (SST), chlorophyll a concentration (Chl a), wind speed (WND), photosynthetically active radiation (PAR) and modeled mixed layer depth (MLD). We developed specific MLRs for the seasonally stratified northern WEC (nWEC) and the permanently well-mixed southern WEC (sWEC) and calculated surface pCO2 with uncertainties of 17 and 16 μatm, respectively. We extrapolated the relationships obtained for the WEC based on the 2011–2013 data set (1) temporally over a decade and (2) spatially in the adjacent Celtic and Irish seas (CS and IS), two regions which exhibit hydrographical and biogeochemical characteristics similar to those of WEC waters. We validated these extrapolations with pCO2 data from the SOCAT and LDEO databases and obtained good agreement between modeled and observed data. On an annual scale, seasonally stratified systems acted as a sink of CO2 from the atmosphere of −0.6 ± 0.3, −0.9 ± 0.3 and −0.5 ± 0.3 mol C m−2 yr−1 in the northern Celtic Sea, southern Celtic sea and nWEC, respectively, whereas permanently well-mixed systems acted as source of CO2 to the atmosphere of 0.2 ± 0.2 and 0.3 ± 0.2 mol C m−2 yr−1 in the sWEC and IS, respectively. Air–sea CO2 fluxes showed important inter-annual variability resulting in significant differences in the intensity and/or direction of annual fluxes. We scaled the mean annual fluxes over these provinces for the last decade and obtained the first annual average uptake of −1.11 ± 0.32 Tg C yr−1 for this part of the northwestern European continental shelf. Our study showed that combining VOS data with satellite observations can be a powerful tool to estimate and extrapolate air–sea CO2 fluxes in sparsely sampled area.

Decadal reanalysis of biogeochemical indicators and fluxes in the North West European shelf‐sea ecosystem

“…The interannual variability of the fluxes was more homogeneous and smaller than the interannual means, in general (Figure 12b). However, variability and means were comparable in the Irish Sea and English Channel, indicating that these areas can switch from being weak sinks [Kitidis et al, 2012] to weak sources of CO 2 in some years, as a consequence of interannual fluctuations of the ecosystem dynamics (e.g., primary production) and forcing (e.g., water temperature) [Marrec et al, 2015;Borges and Frankignoulle, 2003]. The uncertainty in the fluxes was in general lower than their average values, indicating that the reanalysis is suitable for assessing flux directions, i.e., defining sink or source zones, at a 90% confidence level.…”

A multi-decade record of high-quality <i>f</i>CO<sub>2</sub> data in version 3 of the Surface Ocean CO<sub>2</sub> Atlas (SOCAT)

“…Regional f CO 2 values from SOCAT are used in diverse environmental studies with topics ranging from ocean acidification to genomics, gas transfer velocity and evaluation of independent measurements (Blomquist et al, 2014;Larsen et al, 2014;Holding et al, 2015;Marrec et al, 2015;Bonou et al, 2016;Reum et al, 2016). Reum et al (2016) assess the co-variance between pCO 2 , pH and other environmental parameters with the aim to improve the design of future ocean acidification incubation experiments.…”

“…Larsen et al (2014) establish a significant correlation between gene expression for the relative turnover (synthesis or consumption) of CO 2 and surface ocean f CO 2 . SOCAT f CO 2 values are also used for evaluation of surface ocean f CO 2 estimates from eddy correlation (Blomquist et al, 2014) or from other carbonate parameters (Bonou et al, 2016) and for evaluation of regression parameterisations (Marrec et al, 2015;Xu et al, 2016).…”

A multi-decade record of high-quality fCO<sub>2</sub> data in version 3 of the Surface Ocean CO<sub>2</sub> Atlas (SOCAT)

“…-Reports on ocean observing systems include publications from OceanObs'09 (Borges et al, 2010;Monteiro et al, 2010), the Framework for Ocean Observing (FOO, 2012), the Tropical Pacific Observing System 2020 (Mathis et al, 2014) and the 2nd International Indian Ocean Expedition ).…”