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.

The subsurface layer reference to calculate preformed alkalinity and air–sea CO2 disequilibrium in the Atlantic Ocean

“…The variability of the seasonal cycle in surface waters is quite large in terms of non-conservative tracers like C T or pCO 2 (Bates et al, 1996;Corbière et al, 2007), and even for conservative ones like potential temperature (), salinity (S), NO or PO (Broecker, 1974). Typical sea surface temperature (SST) oscillations can reach up to 10 ºC (Pond & Pickard, 1978) in temperate waters and up to 6 ºC in subpolar regions were WMF processes abound (Lab Sea Group, 1998).…”

“…Typical sea surface temperature (SST) oscillations can reach up to 10 ºC (Pond & Pickard, 1978) in temperate waters and up to 6 ºC in subpolar regions were WMF processes abound (Lab Sea Group, 1998). In the case of pCO 2 , the amplitude of variations ranges between 80 and 160 ppm that eventually translate into significant changes of surface C T (Bates et al, 1996;Lefévre et al, 2004;Lüger, 2004;Corbière et al, 2007). The seawater pCO 2 ( pCO 2 sw ) depends on thermodynamics, biological 7 activity, physical mixing and air-sea gas exchanges.…”

CO<sub>2</sub> maximum in the oxygen minimum zone (OMZ)

“…This sampling is based on 3 sites (in the North: Iquique, 21 Total dissolved inorganic CO 2 (DIC) samples were analysed by potentiometry (Bradshaw and Brewer, 1988) with the National CO 2 measurement Service (SNAPCO 2 , Paris, France) protocols (e.g., Corbière et al, 2007). From 15 certified Dickson standards (Batches #44 and #49, SCRIPPS, USA), an accuracy of ∼1.7 µmol kg −1 and, a reproducibility of ∼2.7 µmol kg −1 , based on 16 replicates, have been determined .…”

Monitoring and interpreting the ocean uptake of atmospheric CO 2

“…In both the North Atlantic and Pacific, commercial ships have been used in this way since the early 1990s for CO 2 observations. Long time series to date are in the Atlantic, on shipping routes between the UK and the Caribbean [25,26] and between Newfoundland and Iceland [27], where in each case data are available stretching back to the mid-1990s, though with substantial gaps before 2002.…”