BVOC emission flux response to the El Niño-Southern Oscillation

Isoprene and monoterpene emissions from the terrestrial biosphere play a significant role in major atmospheric processes. Emissions depend on the vegetation's response to atmospheric conditions (primarily temperature and light), as well as other stresses e.g. from droughts and herbivory. It has been well documented that biogenic volatile organic compound (BVOC) emissions are sensitive to climatic influences. The El Niño-Southern Oscillation (ENSO) is a natural cycle, arising from sea surface temperature (SST) anomalies in the tropical Pacific, which perturbs the natural seasonality of weather systems on both global and regional scales. Several studies evaluated the sensitivity of BVOC fluxes during ENSO events using historical transient simulations. While this approach employs realistic scenarios, it is difficult to assess the individual impact of ENSO given multiple forcing on the climate system e.g. from anthropogenic emissions of CO2 and aerosol. In this study, a global atmospheric chemistry-climate model with enabled interactive vegetation was used to conduct two sets of simulations: 1) isolated ENSO event simulations, in which a single ENSO event is used to perturb otherwise baseline conditions, and 2) sustained ENSO simulations, in which the same ENSO conditions are reproduced for an extended period of time. From the isolated ENSO events, we present global and regional BVOC emission changes resulting from the immediate vegetation response to atmospheric states. More focus is given to the sustained ENSO simulations which have the benefit of reducing the internal variability for more robust statistics when linking atmospheric and vegetation variables with BVOC flux anomalies. Additionally, these simulations explore long-term changes in the biosphere with potential shifts in vegetation in this possible climate mode, accounting for the prospect of increased intensity and frequency of ENSO with climate change. Our results show that strong El Niño events increase global isoprene emission fluxes by 2.9 % and that one single ENSO event perturbs the Earth system to the point where BVOC emission fluxes do not return to baseline emissions within several years after the event. We show that persistent ENSO conditions shift the vegetation to a new quasi-equilibrium state, leading to an amplification of BVOC emission changes with up to 19 % increase in isoprene fluxes over the Amazon. We provide evidence that BVOC-induced changes in plant phenology such as the leaf area index (LAI), have a significant influence on BVOC emissions in the sustained ENSO climate mode. Under review for Atmospheric Chemistry and Physics (ACP)