Why does there occur spring predictability barrier for eastern Pacific El Niño but summer predictability barrier for central Pacific El Niño?

Predictability barriers (PBs) significantly limit the prediction skill of the El Niño-Southern Oscillation (ENSO). The EP-El Niño predictions often encounter the impact of the spring PB, while the CP-El Niño predictions usually have PB phenomenon in summer. Using a data analysis approach for predictability dynamics and the pre-industrial control runs from the CCSM4 model outputs, the study explores the PB mechanism for EP- and CP-El Niño by tracing the evolution of initial errors. It is found that the significant spring PB for EP-El Niño and the notable summer PB for CP-El Niño often occur during the transition period from warm to cold phase of the initial sea surface temperature (SST) error evolution in the east-central tropical Pacific; however, there still exist respective initial errors that have specific spatial patterns leading to summer PB for EP-El Niño and spring PB for CP-El Niño. A comparison is made among the initial errors causing PBs for the two types of El Niño, and the results show that the initial errors resulting in the summer PB of CP-El Niño events tend to have more positive SST errors than those causing the spring PB for EP-El Niño in both the tropics and the Victoria mode (VM) region in the Northeast Pacific. These particular positive characteristics of the initial errors causing the summer PB pose a role that hinders the dissipation of the tropical Pacific positive SST errors and thus delay the spring transition phase of SST error, consequently triggering the occurrence of the summer PB for CP-El Niño. The role of the VM mode on SST in the tropical Pacific is inextricably linked to CP-El Niño formation and also sheds light on why the CP-El Niño tends to occur PB in summer. These findings provide a theoretical basis for advancing forecasting skills of El Niño diversity.