Out-of-phase relationship between tropical cyclones generated locally in the South China Sea and non-locally from the Northwest Pacific Ocean

Abstract

Tropical cyclones (TCs) in the South China Sea (SCS) are either generated locally or formed non-locally in the Northwest Pacific Ocean (NWP) and entered into the SCS. Here, it is found that local TCs vary out-of-phase with non-local TCs in summer. That is, if fewer (more) TCs enter into the SCS from the NWP, more (fewer) TCs are generated over the SCS. Further analyses show that variability of the western Pacific subtropical high (WPSH) is responsible for the out-of-phase relationship. As the WPSH shifts eastward (westward), fewer (more) non-local TCs formed in the NWP can enter into the SCS because TCs are recurved northward (moved westward). Due to the eastward (westward) movement of the WPSH, positive (negative) low-level vorticity anomalies, weak (strong) vertical wind shear between the upper and lower troposphere, and anomalous upward (downward) motion in the middle troposphere are induced in the northern SCS. The changes in the relative vorticity, vertical wind shear and vertical velocity are favorable (unfavorable) for the local TC genesis in the SCS. These variations of non-local and local TCs result in an out-of-phase relationship between TCs formed locally in the SCS and non-locally in the NWP.