Elevated uptake of CO 2 over Europe inferred from GOSAT X CO 2 retrievals: a real phenomenon or an artefact of the analysis?

Estimates of the natural CO2 flux over Europe inferred from in situ measurements of atmospheric CO2 mole fraction have been used previously to check top-down flux estimates inferred from space-borne dry-air CO2 column (XCO2 ) retrievals. Recent work has shown that CO2 fluxes inferred from XCO2 5 data from the Japanese Greenhouse gases Observing SATellite (GOSAT) have a larger seasonal amplitude and a more negative annual net CO2 balance than those inferred from the in situ data. The causes of this enhanced European CO2 uptake have since become the focus of recent studies. We show this elevated uptake over Europe could largely be explained by mis-fitting 10 data due to regional biases. We establish a reference in situ inversion that uses an Ensemble Kalman Filter (EnKF) to assimilate surface flask data and the XCO2 data from the surface-based Total Carbon Column Observing Network (TCCON). The same EnKF system is also used to assimilate two, independent versions of GOSAT XCO2 data. We find that the GOSAT-inferred European terrestrial biosphere uptake peaks 15 during the summer, similar to the reference inversion, but the net annual flux is 1.180.1GtCa-1 compared to a value of 0.56-0.1GtCa-1 for our control inversion that uses only in situ data. To reconcile these two estimates, we have performed a series of numerical experiments that assimilate observations with biases or assimilate synthetic observations for which part or all of the GOSAT XCO2 data are replaced with model 20 data. We find that 50-80% of the elevated European uptake in 2010 inferred from GOSAT data is due to retrievals outside the immediate European region, while most of the remainder can be explained by a sub-ppm retrieval bias over Europe. We have used data assimilation techniques to estimate monthly GOSAT XCO2 biases from the joint assimilation of in situ observations and GOSAT XCO2 retrievals. We find a monthly 25 varying bias of up to 0.5 ppm can explain an overestimate of the annual sink of up to 0.18 GtCa-1. Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication Details Feng, L. , Palmer, P. I. , Parker, R. J., Deutscher, N. M. , Feist, D. G. , Kivi, R. , Morino, I. & Sussmann, R. (2015). Elevated uptake of CO2 over Europe inferred from GOSAT XCO2 retrievals: a real phenomenon or an artefact of the analysis? Atmospheric Chemistry and Physics, 15 (3), 1989-2011. Authors L Feng, Paul I. Palmer, Robert J. Parker, Nicholas M. Deutscher, D Feist, Rigel Kivi, Isamu Morino, and Ralf Sussmann This journal article is available at Research Online: http://ro.uow. edu.au/smhpapers/3740 Edinburgh Research Explorer Elevated uptake of CO2 over Europe inferred from GOSAT XCO2 retrievals: a real phenomenon or an artefact of the analysis? Citation for published version: Feng, L, Palmer, PI, Parker, RJ, Deutscher, NM, Feist, DG, Kivi, R, Morino, I & Sussmann, R 2015, 'Elevated uptake of CO2 over Europe inferred from GOSAT XCO2 retrievals: a real phenomenon or an artefact of the analysis? ' Atmospheric Chemistry and Physics Discussions, vol 15, no. 2, pp. 1989-2011. , 10.5194/acpd-15-1989-2015 Digital Object Identifier (DOI): 10.5194/acpd-15-1989-2015 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Atmospheric Chemistry and Physics Discussions General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact openaccess@ed.ac. uk providing details, and we will remove access to the work immediately and investigate your claim. Download date: 23. May. 2016 ACPD 15, 1989–2011, 2015 Elevated uptake of CO2 over Europe inferred from GOSAT XCO2 retrievals L. Feng et al.