Response of the terrestrial carbon cycle to the El Niño - Southern Oscillation

Land plays a dominant role in the interannual variability of the global carbon cycle. The canonical warming and drying of the terrestrial tropics observed during El Niño events calls for the study of the role of precipitation and temperature on carbon cycle variability. Here we use a dynamic vegetation and terrestrial carbon model vegetationglobal-atmosphere-soil (VEGAS) to investigate the response of terrestrial carbon cycle to El Niño-Southern Oscillation (ENSO) for the period 1980–2004. The simulated global total land–atmosphere flux (Fta) by VEGAS agrees well with the atmospheric CO2 inversion modelling results on ENSO timescales and is dominated by the tropics. Analysis of composites of terrestrial responses and climate factors during ENSO events and lead–lag correlations have identified that in the tropics, anomalous precipitation lags ENSO by 1 month and temperature by 5–6 months, while simulated soil moisture lags by 5 months. Warmer and drier conditions there cause suppression of Net Primary Production (NPP) and enhancement of Heteotrophic Respiration (Rh) simultaneously, resulting in the lagging of tropical Fta by 6 months. Sensitivity simulations reveal that 2/3 of Fta change comes from NPP and 1/3 from Rh. In VEGAS, fire burning accounts for about 25% of total Fta anomalies. Precipitation during ENSO events contributes 56% of variation of Fta; temperature accounts for 44%, which includes 25% from the enhancement of Rh and 7% from the increase of the vegetation respiration. We identify the remaining 12% variation of Fta to be from an indirect effect of temperature through its effect on soil wetness, which in turn affects NPP. Such insight into the direct and indirect effects of climatic factors highlights the critical role of soil moisture in ecosystem and carbon cycle–a poorly constrained factor.