A positive carbon feedback to ENSO and volcanic aerosols in the tropical terrestrial biosphere

The relationship between the El Nio/Southern Oscillation (ENSO), volcanic aerosol, and the carbon cycle has been characterized through analyses of atmospheric CO ₂, biogeochemical modeling and recently through inverse estimation. However, the studies to date contain weaknesses that make quantitative assessment of the relationship unreliable. Here we present a systematic quantification of the relationship between ENSO, volcanic aerosols and regional net carbon exchange using results from the TransCom Atmospheric CO ₂ Inversion Intercomparison and a simple 2-step regression method. A modified ENSO index (ENSOτ) is created by estimating the component of ENSO variability that is linearly uncorrelated to aerosol optical depth, and the relationships are estimated by performing correlation analysis with the TransCom 3 tropical terrestrial carbon flux estimates. Flux anomalies from the tropical land regions (Tropical America, Northern Africa, Tropical Asia) show statistically significant correlations with anomalies of ENSOτ, with carbon exchange lagging the ENSOτ by two to six months. Further analysis by season and warm phase/cold phase shows the ENSOτ warm phase explaining >70% of the variability in tropical net carbon exchange. Tropical Asia shows the largest response with positive carbon flux anomalies following two to three months behind the peak of the ENSOτ warm phase. Total tropical land carbon flux anomalies of +0.59 GtC/year result from a typical (one standard deviation) warm ENSOτ event and are consistent with estimates of carbon loss via tropical fire.