Impact of MJO Seasonality on the Intensification of Intraseasonal PNA During El Niño and La Niña Winters

This study mainly examined the distinct early-to-late winter variations in the intraseasonal Pacific–North American teleconnection mode (ISPNA) under the two El Niño–Southern Oscillation phases. Results showed that these variations were controlled by the Madden–Julian Oscillation (MJO) seasonality. Compared to El Niño winters, the ISPNA exhibits a stronger early-to-late winter enhancement during La Niña winters. Furthermore, the MJO and the ISPNA exhibit a phase-locked relationship, which is more pronounced and reliable in late winter. The Rossby wave activity associated with the MJO that propagates northward from the tropics toward the subtropical action center of the ISPNA indicates that increased MJO activity contributes to greater ISPNA intensity. The MJO also exhibits clear seasonality, exhibiting enhancement from early winter to late winter, which is particularly evident during La Niña winters and results in intensified ISPNA during these periods.