Disentangling equatorial Atlantic’s influence on ENSO since 1980

The influence of equatorial Atlantic SST on the El Niño Southern Oscillation (ENSO) has remained robust over the 40 years, despite the strong weakening of Atlantic Niño variability post 2000. To investigate the nature of these interactions through pacemaker experiments with the Norwegian Earth System Model (NorESM). In these experiments tropical Atlantic sea surface temperatures (SST) are restored to observations over the period 1980 to 2020. We perform two experiments, one with long-term warming included and one with it removed linearly. Each experiment consists of 20 ensemble members, sampling internal variability, model uncertainty (NorESM1/NorESM2), and nudging approach (anomaly vs full field restoring). Our results show first that equatorial Atlantic SST variations in the west determine the impact on the ENSO, rather than those in the east. And second, that the long-term warming of the tropical Atlantic SST has opposed this interaction. While the first effect has maintained the robust connection during the last 40 years, we expect the second effect to dominate in the long-term, leading to weaker Atlantic Niño impacts on the Pacific.