A response in the ENSO cycle to an extratropical forcing mechanism during the El Niño to La Niña transition

Current El Niño-Southern oscillation (ENSO) theory emphasizes that the forcing that drives the cycle mainly exists within tropical regions. However, these ideas are quite limited in explaining completely the occurrence of ENSO. Here, we examine whether extratropical forcing can affect ENSO cycle, specifically the transition from El Niño to La Niña. Although dispersion of the Okhotsk-Japan (OKJ) atmospheric wave train across the mid-latitude North Pacific during June terminates in the subtropics, the associated regime of southward surface wind anomalies could reach Eastern Equatorial Pacific (EEP). The OKJ wave train plays a substantial role in generating a similar underlying sea surface temperature (SST) wave train through a barotropic process in air–sea interactions and after September, it is negatively correlated strongly with the SST around EEP. Strong OKJ propagation in the positive (negative) phase during June is more (less) significantly associated with a subsequent La Niña (El Niño) episode that is matured after October. Negative SST anomalies at the southern end of the SST wave train with strong overlying OKJ propagation in the positive phase during June and the associated southward surface wind anomalies retained its strength by the further infusion of energy and gradual southward displacement joining the negative SST anomalies around EEP after the October when La Niña usually matured in-situ. Strong OKJ propagation in the positive phase during June tends to occur during a quick summer and fall transition period from El Niño to La Niña. This study strongly suggests that extratropical forcing plays an ignored role in affecting ENSO cycle especially in the formation of La Niña, which was not included in current ENSO theory.