Variability of North Atlantic CO 2 fluxes for the 2000–2017 period estimated from atmospheric inverse analyses

Abstract. We present new estimates of the regional North Atlantic (15–80 ∘  N) CO 2 flux for the 2000–2017 period using atmospheric CO 2 measurements from the NOAA long-term surface site network in combination with an atmospheric carbon cycle data assimilation system (GEOS-Chem–LETKF, Local Ensemble Transform Kalman Filter). We assess the sensitivity of flux estimates to alternative ocean CO 2 prior flux distributions and to the specification of uncertainties associated with ocean fluxes. We present a new scheme to characterize uncertainty in ocean prior fluxes, derived from a set of eight surface p CO 2 -based ocean flux products, and which reflects uncertainties associated with measurement density and p CO 2 -interpolation methods. This scheme provides improved model performance in comparison to fixed prior uncertainty schemes, based on metrics of model–observation differences at the network of surface sites. Long-term average posterior flux estimates for the 2000–2017 period from our GEOS-Chem–LETKF analyses are − 0.255  ±  0.037 PgC yr −1 for the subtropical basin (15–50 ∘  N) and − 0.203  ±  0.037 PgC yr −1 for the subpolar region (50–80 ∘  N, eastern boundary at 20 ∘  E). Our basin-scale estimates of interannual variability (IAV) are 0.036  ±  0.006 and 0.034  ±  0.009 PgC yr −1 for subtropical and subpolar regions, respectively. We find statistically significant trends in carbon uptake for the subtropical and subpolar North Atlantic of − 0.064  ±  0.007 and − 0.063  ±  0.008 PgC yr −1  decade −1 ; these trends are of comparable magnitude to estimates from surface ocean p CO 2 -based flux products, but they are larger, by a factor of 3–4, than trends estimated from global ocean biogeochemistry models.