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Abstract

Trends and variability in the ocean-atmosphere CO2 flux and the uptake of anthropogenic CO2 are simulated for the period 1948–2003, using a biogeochemical carbon cycle model (HAMOCC5) coupled online to a global ocean general circulation model (MPI-OM). The model is forced by daily National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1948 to 2003. We find a global interannual variability of ±0.50 PgC yr−1 (2σ) which is largely dominated by ocean dynamics in the equatorial Pacific. On decadal scale, two patterns are of global importance. First, the regime shift that occurs in 1975–1977 when the modeled interannual variability of the equatorial Pacific changes from ±0.32 PgC yr−1 to ±0.23 PgC yr−1 and the mean outgasing of CO2 decreases from 0.70 PgC yr−1 to 0.58 PgC yr−1. Second, the trend that occurs in the Southern Ocean, where we find an increase of the CO2 fluxes over the full 56-year simulation period. The flux results from stronger upwelling and deeper mixed layers which are caused by increasing wind velocities. We estimate an average CO2 flux into the ocean of 1.49 PgC yr−1 for 1980–1989 and 1.74 PgC yr−1 for 1990–1999, with extremes of 1.20 PgC yr−1 at the La Niña event in 1996 and 2.10 PgC yr−1 during the El Niño events in 1993 and 1998. Because of the rising buffer factor, the uptake of anthropogenic CO2 is slowing down in regions of shallow mixing toward the end of the simulation. Overall, about 124 Pg of anthropogenic carbon has accumulated in the model ocean until the end of 2003