Seasonal Dependence of TC–ISO Relationship and Its Potential Effect on the Intensification of TCs in the Western North Pacific since the Early 2000s

Abstract

Abstract This study reported that the intensification of tropical cyclones (TCs) to major TCs (MTCs) in the western North Pacific (WNP) region exhibited strong difference between boreal autumn (SON) and summer (JJA) since the early 2000s; the ratio of MTCs to the total number of TCs (MTC ratio) has continuously increased in SON but not in JJA. Due to this difference, more MTCs form and pass through the western flank of the WNP region in SON. The increase of the MTC ratio in SON was associated with interdecadal variability in TC activity and 30–60-day intraseasonal oscillations (ISOs) variability. The mean genesis location of TCs and ISOs accompanied by a negative outgoing longwave radiation anomaly shrunk and shifted westward simultaneously in SON since the early 2000s due to the westward extension of the WNP subtropical high. However, this change was not observed in JJA. This westward shift of ISO substantially modulated large-scale thermodynamic and dynamic conditions, which in turn enhanced the TC–ISO interaction and accelerated energy conversion between TC and ISO. The kinetic energy budget along the MTC track was further analyzed to understand the TC–ISO interaction. Both the lower-level barotropic energy conversion (CK) and upper-level baroclinic energy conversion (CE) contributed to the intensification of TCs. CK mainly resulted from the scale interaction between TCs and ISO, whereas CE resulted from TC-related perturbations. Significant Statement This study reported the seasonality of TC intensification in the WNP during the early 2000s. Here, we extended the previous work to present that the interdecadal increase of the ratio of TC developing to major TC (MTC; ≥category 3; referred to MTC ratio) exhibits strong seasonal dependence. That is, the MTC ratio stays stationary approximately in 30% for JJA, but it jumps from 40% to 50% in SON. Consequently, more MTCs form and pass through the western flank of the WNP region in SON. The possible physical processes behind the increase of MTC ratio were discussed. These results may advance our knowledge about the TC intensification and were helpful for TC prediction.