Interannual and decadal variability of the oceanic carbon sink in the North Atlantic subpolar gyre

ABSTRACT

The evaluation of interannual and decadal variations of air-sea CO₂ fluxes represents important step for understanding the changes in the global carbon cycle. In this study we analyse the variations of sea surface dissolved inorganic carbon (DIC) and total alkalinity (TA) in the North Atlantic over the period 1993–2003 (SURATLANT Program). The analysis focuses on the subpolar gyre (53°N–62°N/45°W–20°W). Large interannual variability of DIC and air-sea CO₂ fluxes is observed mostly during summer. In the extreme case, this region was a CO₂ source in 2003 explained by a dramatic warming and the absence of late-summer bloom. At the decadal scale, DIC and TA concentrations appeared stable indicating a complex balance between primary production, vertical mixing, horizontal transport and anthropogenic CO₂. We also found that winter fCO₂ has increased at a rate of +2.8 μatm yr⁻¹ between 1993 and 2003, due to strong surface warming (1.5 °C over 10 yr) particularly since winter 1995 when the North Atlantic Oscillation index moved into a negative phase. This resulted in a decrease of carbon uptake in the North Atlantic subpolar gyre, a trend also suggested for the period 1972–1989 but not captured by current class atmospheric inverse models.