During El Niño events, atmospheric teleconnections with sea surface temperature (SST) anomalies in the equatorial Pacific cause higher temperatures and reduced rainfall in the Amazon, leading to increased CO2 emissions. While some of the temperature increase results directly from the SST-atmosphere teleconnection, drier soil resulting from reduced rainfall can also contribute to higher temperatures and resulting CO2 flux anomalies. Researchers from the University of California, Irvine and the Oak Ridge National Laboratory modified the Energy Exascale Earth System Model (E3SM) to decouple the direct effects of SST anomalies from the resulting soil moisture anomalies, in order to determine the relative importance of each of these drivers.
Soil moisture variability was found to amplify and extend the effects of SST forcing on eastern Amazon temperature and carbon fluxes in E3SM. During the wet season, the direct, circulation-driven effect of ENSO SST anomalies dominated temperature and carbon cycle variability throughout the Amazon. During the following dry season, after ENSO SST anomalies had dissipated, soil moisture variability became the dominant driver in the east, explaining 67% to 82% of the temperature difference between El Niño and La Niña years, and 85% to 91% of the difference in carbon fluxes. These results highlight the need to consider the interdependence between temperature and hydrology when attributing the relative contribution of these factors to interannual variability of the terrestrial carbon cycle. Specifically, when offline models are forced with observations or reanalysis, the contribution of temperature may be overestimated when its own variability is modulated by hydrology via land-atmosphere coupling.
We demonstrated that in E3SM, soil moisture anomalies resulting from SST variability extended and strengthened the temperature and CO2 flux anomalies associated with ENSO. This indicates the need to consider the interdependent relationship between temperature and the hydrologic cycle when attributing mechanisms to ENSO-driven variability in the tropical terrestrial carbon cycle.