Modulation of Boreal Extended Summer Tropical Cyclogenesis over the Northwest Pacific by the Quasi‐Biweekly Oscillation under different El Niño‐Southern Oscillation Phases

The study of the interannual and intraseasonal modulation of tropical cyclogenesis (TCG) in the northwest Pacific (NWP) basin has received significant attention in the past. Most previous studies have focused on the individual impact of El Nino-southern oscillation (ENSO) or Madden-Julian Oscillation on TCG, whereas the combined effect of ENSO and another leading intraseasonal oscillation mode, quasi-biweekly oscillation (QBWO), on NWP TCG has rarely been studied. Therefore, this study systemically investigates the modulation of NWP TCG by the QBWO under different ENSO phases. A significant modulation of TCG by QBWO in the ENSO neutral years is observed, with significantly increased (decreased) TCG over the NWP in QBWO active (inactive) phases. Moreover, during El Nino years and La Nina years, the modulation of TCG by QBWO is not significant and no distinctive difference in the TCG frequency can be detected. Further analyses show that the modulation of TCG by the QBWO is mainly related to the changes in large-scale environment and energy that affect TCG associated with active or inactive phases of the QBWO under different ENSO phases. Large-scale environmental factors and energy are more favourable for TCG during ENSO neutral years compared with El Nino and La Nina years. These results are associated with QBWO cycle under the impact of background (e.g., boundary layer moisture and zonal wind shear) which is determined by ENSO conditions.