ENSO-induced regime shift from energy to water limitation in tropical ecosystems?

Tropical forests take up a significant fraction of human carbon emissions, thereby contributing to slowing down global warming. In the context of El Niño-Southern Oscillation (ENSO), the El Niño and La Niña phenomena can cause anomalously weather patterns across tropical regions. However, it is largely unknown to what extent ENSO-induced climatic anomalies can alter the response of ecosystem function, for example, by inducing water-limited conditions as a consequence of dry and warm weather.

To address this question, we (i) quantify the degree of vegetation water limitation using the ecosystem limitation index (ELI), and (ii) correlate the local ELI with prior sea surface temperature in the Niño3.4 region, which represents the strength of ENSO. Gridded observation-based evapotranspiration data and near-infrared reflectance of vegetation are used to represent vegetation functioning in the ELI calculation. First results show that there are significant ELI-El Niño relationships in northern Amazon forests, while for La Niña conditions we find such relationships more widespread across the Amazon, Africa, Southeast Asia, and northern Australia. Despite these relationships, in most tropical ecosystems energy is still the main driver of vegetation functioning, while actual water limitation only occurs in a few regions including eastern South America and northern Australia, likely due to decreased precipitation. Thereby, water limitation as a consequence of ENSO impacts could be more prominent during the dry season in the tropics. Finally, for near-neutral ENSO conditions we find generally weak impacts on ELI in the tropics. Understanding the ENSO-induced shift in ecosystem limitation is crucial for better understanding the interannual variability of the land carbon sink, as well as because ENSO variability is expected to increase with more frequent extreme El Niño and subsequently more occurrences of consecutive La Niña, likely enhancing its influence on ecosystems.