Decadal changes in the ocean CO2 sink driven by ocean circulation variability

The ocean represents the largest sink for anthropogenic CO2 emissions, having absorbed roughly 40% of anthropogenic CO2 emissions since the beginning of the industrial era. However, there are indications that the oceanic CO2 sink may be in decline in recent decades, due to changes in the ocean's overturning circulation and to warming and acidification of the surface ocean. It is unclear, however, if the changes detected are part of a long-term trend or due to natural variability in ocean circulation patterns. Here a global ocean circulation inverse model (OCIM) is used to characterize the mean state of the ocean circulation over three time periods: the 1970s-1980s, the 1990s, and the 2000s. A prognostic ocean carbon cycle model is embedded in the OCIM to estimate how changes in ocean circulation over these time periods have impacted the oceanic CO2 sink. The results show that the oceanic CO2 sink weakened significantly during the 1990s, primarily due to a weakened Southern Ocean CO2 sink driven by enhanced outgassing of natural CO2. This trend was reversed in the 2000s and the oceanic CO2 sink rebounded significantly, indicating that natural climate variability may be dominating the changes in the oceanic CO2 sink over the last several decades.