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

The relationship between the El Niño/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 (ENSO τ ) 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 ENSO τ , with carbon exchange lagging the ENSO τ by two to six months. Further analysis by season and warm phase/cold phase shows the ENSO τ 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 ENSO τ warm phase. Total tropical land carbon flux anomalies of +0.59 GtC/year result from a typical (one standard deviation) warm ENSO τ event and are consistent with estimates of carbon loss via tropical fire.