Identifying the biological control of the annual and multi-year variations in South Atlantic air–sea CO 2 flux

[摘要] The accumulation of anthropogenic CO 2 emissions inthe atmosphere has been buffered by the absorption of CO 2 by the globalocean, which acts as a net CO 2 sink. The CO 2 flux between theatmosphere and the ocean, which collectively results in the oceanic carbonsink, is spatially and temporally variable, and fully understanding thedriving mechanisms behind this flux is key to assessing how the sink maychange in the future. In this study a time series decomposition analysis wasapplied to satellite observations to determine the drivers that control thesea–air difference of CO 2 partial pressure ( Δ p CO 2 ) and theCO 2 flux on seasonal and inter-annual timescales in the South AtlanticOcean. Linear trends in Δ p CO 2 and the CO 2 flux werecalculated to identify key areas of change. Seasonally, changes in both the Δ p CO 2 and CO 2 flux weredominated by sea surface temperature (SST) in the subtropics (north of 40 ∘  S) and were correlated with biological processes in the subpolarregions (south of 40 ∘  S). In the equatorial Atlantic, analysis ofthe data indicated that biological processes are likely a key driver as aresponse to upwelling and riverine inputs. These results highlighted thatseasonally Δ p CO 2 can act as an indicator to identify drivers ofthe CO 2 flux. Inter-annually, the SST and biological contributions tothe CO 2 flux in the subtropics were correlated with the multivariateEl Niño–Southern Oscillation (ENSO) index (MEI), which leads to a weaker (stronger) CO 2 sink in ElNiño (La Niña) years. The 16-year time series identified significant trends in Δ p CO 2 and CO 2 flux; however, these trends were not alwaysconsistent in spatial extent. Therefore, predicting the oceanic response toclimate change requires the examination of CO 2 flux rather than Δ p CO 2 . Positive CO 2 flux trends (weakening sink for atmosphericCO 2 ) were identified within the Benguela upwelling system, consistentwith increased upwelling and wind speeds. Negative trends in the CO 2 flux (intensifying sink for atmospheric CO 2 ) offshore into the SouthAtlantic gyre were consistent with an increase in the export of nutrientsfrom mesoscale features, which drives the biological drawdown of CO 2 .These multi-year trends in the CO 2 flux indicate that the biologicalcontribution to changes in the air–sea CO 2 flux cannot be overlookedwhen scaling up to estimates of the global ocean carbon sink.

[学科分类] 大气科学