Decadal variability and trends in the global ocean carbon sink
Peter Landschuetzer1, Nicolas Gruber2, Alexander Haumann2 and Dorothee C E Bakker3, (1)Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich Switzerland, Zurich, Switzerland, (2)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (3)University of East Anglia, Norwich, United Kingdom
Abstract:
While the mean CO2 uptake by the ocean is well established, we know relatively little about how it varies on inter-annual to decadal timescales. Here, we use the SOCAT underway pCO2 observations and employ the 2-step neural network data-interpolation method of Landschützer et al. (2013) to reconstruct the surface ocean pCO2 on a monthly timescale from 1982 through 2011. We do this on a 1°x1° grid for the open ocean, while we for the first time add an enhanced resolution 1/4°x1/4° grid for the coastal oceans. Employing standard parameterizations for the air-sea exchange of CO2, these gridded and extrapolated data permit the calculation of global monthly maps of the air-sea flux of CO2 over the past three decades. Our results reveal a long-term strengthening in the oceanic uptake of atmospheric CO2, as expected in response to the increasing concentration of atmospheric CO2, but with strong decadal variations in the annual mean uptake. Namely, starting from a mean uptake of about -1.4 Pg C yr-1 in the early 1980s, the global carbon sink and especially that of the Southern Ocean weakened to a minimum uptake of only -0.8 Pg C yr-1 reached in 1999. This decrease confirms the strong impact that the southward migration of the southern hemisphere westerly wind belt had on the Southern Ocean carbon uptake. But since the turn of the century, the global ocean carbon sink increased by more than 1 Pg C yr-1 to an uptake rate of -2.3 Pg C yr-1, with half of this increase stemming from the Southern Ocean. While we demonstrated earlier (Landschützer et al., 2015) that this flux increase in the Southern Ocean can be attributed to a recent trend towards zonally more asymmetric pressure and wind conditions, the carbon sink variability in the Pacific and the Atlantic are driven by the large-scale climate modes PDO and AMO. In particular, the Pacific Ocean shows a trend towards La Niña dominated ENSO conditions, with a strong ”Modoki” signal and strong outgassing in the tropics combined with cooling in the extratropics and a PDO driven increase in carbon uptake in the high latitude North Pacific. Overall, this new estimate suggests a much more variable ocean carbon sink on decadal timescales than previously recognized.
