Role of the Climatological North Pacific High in the North Tropical Atlantic-ENSO Connection

Observational and climate model analysis showed that the anomalous sea surface temperature in the north tropical Atlantic (NTA) in boreal spring can trigger El Nino-Southern Oscillation (ENSO) in the subsequent winter. Similarly, the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are known to reasonably simulate the NTA effect. Nevertheless, the strengths of the NTA effect on ENSO among the climate models are also diverse. In this light, we revisited the possible causes that contributed to the different NTA effects on ENSO in the CMIP5 climate models. We found that the strength of the NTA triggering ENSO in the climate model tended to be proportional to the intensity of the climatological subtropical North Pacific high system in boreal spring. The stronger climatological subtropical North Pacific high accompanied enhanced trade wind, precipitation reduction, and cold sea surface temperature over the subtropics. Under these conditions, the moist static energy feedback process, also known as the moist enthalpy advection mechanism, effectively operated around the Pacific intertropical convergence zone. That is, the NTA-induced signals in the subtropical North Pacific readily intruded into the deep tropical Pacific with the aid of the feedback processes, leading to an ENSO event. Consistent with the CMIP5 analysis results, the observed NTA effect on ENSO became stronger during the decades when the climatological North Pacific subtropical high intensified, underpinning the importance of climatology in the subtropical North Pacific in the NTA-ENSO connection.

Automated summary

This study examined the impact of anomalous sea surface temperature in the north tropical Atlantic (NTA) on El Nino-Southern Oscillation (ENSO) and the diversity of this impact in climate models. The analysis found that the strength of NTA's effect on ENSO in the climate models was proportional to the intensity of the climatological subtropical North Pacific high system in boreal spring. The observed NTA effect on ENSO also became stronger during periods when the climatological subtropical North Pacific high intensified, indicating the importance of climatology in the NTA-ENSO connection.