Simulation and assimilation of global ocean pCO2 and air-sea CO2 fluxes using ship observations of surface ocean pCO2 in a simplified biogeochemical offline model

We used an offline tracer transport model, driven by reanalysis ocean currents and coupled to a simple biogeochemical model, to synthesize the surface ocean pCO(2) and air-sea CO2 flux of the global ocean from 1996 to 2004, using a variational assimilation method. This oceanic CO2 flux analysis system was developed at the National Institute for Environmental Studies (NIES), Japan, as part of a project that provides prior fluxes for atmospheric inversions using CO2 measurements made from an on-board instrument attached to the Greenhouse gas Observing SATellite (GOSAT). Nearly 250 000 pCO(2) observations from the database of Takahashi et al. (2007) have been assimilated into the model with a strong constraint provide by ship-track observations while maintaining a weak constraint of 20% on global averages of monthly mean pCO(2) in regions where observations are limited. The synthesized global air-sea CO2 flux shows a net sink of 1.48 PgC yr(-1). The Southern Ocean air-sea CO2 flux is a sink of 0.41 PgC yr(-1). The interannual variability of synthesized CO2 flux from the El Nino region suggests a weaker source (by an amplitude of 0.4 PgC yr(-1)) during the El Nino events in 1997/1998 and 2003/2004. The assimilated air-sea CO2 flux shows remarkable correlations with the CO2 fluxes obtained from atmospheric inversions on interannual time-scales.