Influences of central Pacific warming on synoptic-scale wave intensity over the northwest Pacific

The present study focuses on changes in synoptic-scale waves (SSWs) over the northwest Pacific (NWP) during boreal summer and compares them with intraseasonal oscillations (ISOs). The results show that the intensity of eddy kinetic energy (EKE)-based SSWs (2–10-day period) has greater climatological means and interannual variations than the 10–20-day and 30–60-day ISOs. Additionally, the distribution of the SSWs is similar to that of the 10–20-day ISO, which shows a greater amplitude in the monsoon trough region. Further investigation reveals that the EKE-based SSW intensity over the South China Sea-tropical NWP region, where the greatest SSW activity occurs, is positively correlated with central Pacific (CP) warming. The EKE-based SSW intensity tends to be higher in years of developing CP warming. The influences of the Indian Ocean sea surface temperature (SST) are limited and show a weak correlation with the EKE-based SSW intensity over the key region (5° N–20° N, 115° E–155° E). In turn, the SSWs have impacts on local SST anomalies, with active SSWs leading to SST cooling. CP warming induces a favorable environment for SSW origination over CP and SSW development in the monsoon trough region. Concurrent with the CP warming-associated easterly vertical wind shear, upward vertical motion and abundant moisture supply, significant synoptic-scale EKE conversion occurs over the key region, which indicates its dominant role in influencing the SSW intensity. The influences of CP warming on SSW intensity over the NWP demonstrate the potential of the El Niño Modoki index for predicting interannual variations in SSW intensity. Moreover, considering that influences related to SSWs bear a resemblance to the 10–20-day ISOs, future research should take into account the contributions of SSWs to avoid overestimating the impact from ISOs.