The role of background SST changes in the ENSO-driven rainfall variability revealed from the atmospheric model experiments in CMIP5/6

Abstract

The tropical Pacific rainfall variability driven by El Niño–Southern Oscillation (ENSO rainfall) is the crucial bridge across which ENSO exerts its influence on global climate. Previous modeling studies have documented a robust intensification and eastward shift of ENSO-driven rainfall under global warming despite large inter-model biases in ENSO-related sea surface temperature variability (ENSO SST), but the formation mechanisms of the robust ENSO rainfall changes are not well clarified. Here, we utilized a set of atmospheric model experiments—namely, “amip”, forced by observed SST; “amip-p4K”, forced by observed SST plus uniform 4-K warming, and “amip-future4K”, forced by observed SST plus spatially patterned SST warming, in CMIP5/CMIP6—to disentangle the role of the changes in mean-state SST (uniform and spatially patterned SST warming) with an unchanged ENSO SST. Relative to the weak ENSO rainfall changes in amip-p4K, those in amip-future4K forced by an El Niño-like SST warming pattern show an increase and eastward shift over the equatorial central–eastern Pacific. The results in amip-future4K without ENSO SST changes coincide well with previous results of the coupled models in CMIP5/3, suggesting a dominant role played by the mean-state SST warming pattern in ENSO rainfall changes. By diagnosing the moisture budget decomposition and moist static energy transport, we revealed that the structural and amplitude changes in ENSO rainfall are mainly controlled by the dynamic and thermodynamic components, respectively. In addition, the El Niño-like SST warming pattern in amip-future4K can increase low-level specific humidity with a remarkable spatial gradient, particularly in the equatorial central–eastern Pacific. As a result, the low-level specific humidity dominates the pattern of changes in the vertical transport of moist static energy, and subsequently the ENSO rainfall changes, in amip-future4K. This study further verified the importance of background SST changes to ENSO rainfall under global warming.