Key controls on the seasonal and interannual variations of the carbonate system and air‐sea CO₂ flux in the Northeast Atlantic (Bay of Biscay)

Abstract

[1] Biogeochemical variations of surface water in the Northeast Atlantic (Bay of Biscay) were examined using high-frequency underway measurements combined with monthly sampling of carbon-related variables. The mechanisms controlling seasonal CO 2 variability were investigated by distinguishing the contributions of biological and physical processes to the monthly changes in dissolved inorganic carbon (DIC) and partial pressure of CO 2 (pCO 2 ). The seasonality of DIC (47-81 mmol kg À1 ) had a single peak with a winter maximum primarily driven by vertical mixing and a summer minimum driven by spring biological removal. Non-Redfield C:N uptake was observed in the nutrient-depleted summer but not during the spring bloom. In the North Atlantic, pCO 2 seasonality shows a latitudinal transition: from the temperature-dominated oligotrophic subtropical gyre to the subpolar region where pCO 2 is dominated by changing concentrations of DIC. In the midlatitude Bay of Biscay, the annual cycle of pCO 2 (61-75 matm) showed a double-peak distribution. The summer pCO 2 peak was mainly driven by temperature increase, while the winter peak resulted from the dominant effect of entrainment of subsurface water. Interannual variations of DIC were more pronounced in winter and were driven by the changes in the strength of winter mixing. Higher wintertime concentrations and seasonal amplitudes of DIC were observed in cold years when the mixed-layer depths were deeper, which appears to be associated with negative phases of the North Atlantic Oscillation

The ocean carbon sink – impacts, vulnerabilities and challenges

“…While the global ocean carbon sink estimates may not too strongly depend on this choice (otherwise projections with simple two box models for the global ocean would not have worked at all; Oeschger et al, 1975), the projected local CO 2 concentration in ocean surface waters is highly influenced by the chosen gas transfer velocity values, also for appropriate regional validation of ocean models. The co-limitation of biological production by various factors is an established concept; however, crucial details are not uniformly established, such as the potential variation of carbon to nitrogen ratios in biogenic matter under different environmental conditions (Riebesell et al, 2007;Jiang et al, 2013). Marine particle fluxes and their dynamics are still poorly understood and not yet adequately quantified in a dynamic way in response to external drivers (Klaas and Archer, 2002;Gehlen et al, 2006).…”

Biogeochemical variations at the Porcupine Abyssal Plain sustained Observatory in the northeast Atlantic Ocean, from weekly to inter-annual timescales

“…Concentrations of nitrate and DIC exhibit seasonal variations opposite to temperature. The seasonal variation in nitrate and DIC concentrations is controlled by convective mixing (resulting in the winter maximum) and biological uptake during the spring bloom period (resulting in the summer minimum), which is similar to elsewhere in the North Atlantic (Jiang et al, 2013).…”

“…Jiang et al (2013) compared seasonal carbon variability between different sites in the North Atlantic and suggested a latitudinal change in p(CO 2 ) seasonality from the temperaturedominated oligotrophic subtropical gyre to the subpolar region where p(CO 2 ) is dominated by changing concentrations of DIC. Our p(CO 2 ) observations at the PAP-SO site show the subpolar-like seasonal pattern, which is similar to that of the ocean weather station M (Skjelvan et al, 2008).…”

“…However flux calculations from SOO data in early 2011 do not suggest an increase in CO 2 flux. Increases in productivity do not necessarily result in enhanced oceanic CO 2 uptake as the gas exchange is also affected by other factors such as temperature and wind speed (Jiang et al, 2013) …”

“…It is suggested that NAO plays an important role in modulating the inter-annual variability in the northeast Atlantic region by affecting the intensity of winter convection (Bennington et al, 2009;Jiang et al, 2013). The Gibraltar minus Iceland version of the NAO index is really most applicable to the winter half of the year.…”