ENSO-driven reverse coupling in interannual variability of pantropical water availability and global atmospheric CO2 growth rate

Abstract

The large interannual variability of global atmospheric CO2 growth rate originates primarily from variation in carbon dioxide (CO2) uptake of pantropical terrestrial ecosystems, which covaries with the El Niño–Southern Oscillation (ENSO) modulated climate fluctuations of water availability and temperature change. However, the role of ENSO in modulating the contributions of regional to overall water availability interannual variability, and the phase and strength of water availability-CO2 coupling remain poorly constrained across functionally diverse pantropical terrestrial ecosystems. Using satellite microwave and ground water availability and well-mixed global atmospheric CO2 concentration observations, the coupling in interannual variability of water availability-CO2 and their relationship with ENSO was investigated from 1998 to 2016. The results demonstrated causal sequence of ENSO, water availability, and global atmospheric CO2 growth rate, the phase and magnitude of water availability-CO2 coupling was primarily determined by phase and strength of correlation between ENSO and water availability, revealing ENSO-driven robust and reverse coupling of water availability-CO2. Moreover, tropical rainforests, savannas, and shrublands dominated the pantropical water availability variations and showed stronger coupling strength. Therefore, the strong interannual variability of atmospheric CO2 growth rate originates from ENSO-driven frequent variations of water availability and the subsequently concurrent carbon uptake over pantropical rainforests, savannas, and shrublands. The findings provided new insights to understand and predict interannual variability of water availability and CO2 growth rate based on ENSO and its predictability.