The Influence of El Nino/Southern Oscillation and Volcanic Aerosols on Tropical Terrestrial Carbon Flux

The relationship between the El Nino/Southern Oscillation and the carbon cycle has been explored in analysis of atmospheric CO2, biogeochemical modeling studies and recently through inverse estimation. The influence of volcanic aerosols on the relationship has been noted as interrupting the strong correlation between ENSO and net carbon exchange. This paper is a first attempt at a systematic quantification of the relationship between ENSO, volcanic aerosols and net carbon exchange using the results of the TransCom inverse estimate intercomparison. A new index (ENSO-τa) is created by combining aerosol optical depth with ENSO and performing correlation analysis with the aggregated and regional terrestrial carbon flux estimates from the TransCom experiment. Northern Africa and Tropical Asia show statistically significant positive lagged correlations (r=0.61, p<0.02; r=0.55, p<0.05, respectively) with the ENSO-τa index (flux follows ENSO-τa at 1 month and 4 months, respectively) and these relationships are dominated by the El Nino phase. Northern Africa and Tropical Asia respond most strongly to Jan-Mar ENSO-τa anomalies while Tropical Asia also responds to June-Aug ENSO- τa anomalies. A significant relationship is also found with the Temperate Asia region and this relationship is stronger without inclusion of the volcanic aerosol optical depth and is related to the La Nina phase. The relationship suggests that anomalies of 0.37 GtC/year result from a typical (one standard deviation) El Nino event across the tropical land regions and anomalies of 0.17 due to La Nina in the Temperate Asia region.The relationship between the El Nino/Southern Oscillation and the carbon cycle has been explored in analysis of atmospheric CO2, biogeochemical modeling studies and recently through inverse estimation. The influence of volcanic aerosols on the relationship has been noted as interrupting the strong correlation between ENSO and net carbon exchange. This paper is a first attempt at a systematic quantification of the relationship between ENSO, volcanic aerosols and net carbon exchange using the results of the TransCom inverse estimate intercomparison. A new index (ENSO-τa) is created by combining aerosol optical depth with ENSO and performing correlation analysis with the aggregated and regional terrestrial carbon flux estimates from the TransCom experiment. Northern Africa and Tropical Asia show statistically significant positive lagged correlations (r=0.61, p<0.02; r=0.55, p<0.05, respectively) with the ENSO-τa index (flux follows ENSO-τa at 1 month and 4 months, respectively) and these relationships are dominated by the El Nino phase. Northern Africa and Tropical Asia respond most strongly to Jan-Mar ENSO-τa anomalies while Tropical Asia also responds to June-Aug ENSO- τa anomalies. A significant relationship is also found with the Temperate Asia region and this relationship is stronger without inclusion of the volcanic aerosol optical depth and is related to the La Nina phase. The relationship suggests that anomalies of 0.37 GtC/year result from a typical (one standard deviation) El Nino event across the tropical land regions and anomalies of 0.17 due to La Nina in the Temperate Asia region.