Since the industrial revolution, anthropogenic CO2 emissions have increased unabated and continue to raise atmospheric CO2 concentrations which contribute to global warming.
The global oceans have buffered this rise by acting as a sink for atmospheric CO2 and the strength of the ocean as a sink for CO2 appears to be increasing with time. However, this can vary regionally, and the ocean can oscillate between being a source and sink of atmospheric CO2. Understanding the driving mechanisms at play is therefore crucial to understanding and predicting how this sink may change in the future.
This study utilises a time series analysis of satellite observations to determine the drivers that control the sea–air difference of CO2 partial pressure and the CO2 flux on seasonal and inter-annual timescales in the South Atlantic Ocean.
Seasonally, changes in CO2 flux were dominated by sea surface temperature in the subtropics (north of 40°S) and were correlated with biological processes in the subpolar regions (south of 40°S). In the equatorial Atlantic, analysis of the data indicated that biological processes are likely a key driver as a response to upwelling and riverine inputs.
Inter-annually, the SST and biological contributions to the CO2 flux in the subtropics were correlated with the multivariate El Niño–Southern Oscillation (ENSO) index, which leads to a weaker CO2 sink in El Niño years.
The 16-year time series also identified spatial trends in the flux of CO2. A weakening sink for atmospheric CO2 was identified within the Benguela upwelling system, which was consistent with increased upwelling and wind speeds. Conversely an intensifying sink for atmospheric CO2 was found offshore into the South Atlantic gyre where an increase in the export of nutrients from mesoscale features drives the biological drawdown of CO2.
These multi-year trends in the CO2 flux indicate that the biological contribution to changes in the air–sea CO2 flux cannot be overlooked when scaling up to estimates of the global ocean carbon sink.