The impact of interdecadal modulations of ENSO amplitude on terrestrial ecosystems

The El Niño-Southern Oscillation (ENSO) phenomenon is the most important natural driver impacting the climate system and the carbon cycle on annual to multi-decadal time scales. Recent studies show (i) the existence of an asymmetry between warm equatorial El Niño and cold equatorial La Niña SST and precipitation patterns, and (ii) the existence of decadal modulations of ENSO amplitude. The extent to which the non-linearity and the decadal modulation affect the land carbon cycle response is currently unknown. Observationally-based records are too short to quantify and identify the degree of non-linearity. Here, we analyze output from a 1000-yr preindustrial control simulation with an IPCC-class Earth System Model. We show that averaged over times of large amplitude ENSO variability there is a mean perturbation release of carbon from the terrestrial biosphere to the atmosphere, mainly occurring in the tropics. This suggests that the mean state and the variability of the terrestrial carbon system are not independent. The processes that contribute to the asymmetry will be presented, and the role of ENSO modulations in past and future changes of the carbon cycle will be considered.