Impact of Intraseasonal Oscillations on the Activity of Tropical Cyclones in Summer over the South China Sea. Part I: Local Tropical Cyclones

The South China Sea (SCS) is affected by two intraseasonal components in summer: the Madden-Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO). In the present study, the impacts of the MJO and QBWO on tropical cyclones (TCs) locally formed in the SCS (local TCs) in summer are investigated. The results show that both the MJO and QBWO can affect the genesis frequency, location, and motion of the local TCs. More TCs form in the convectively active phases of the MJO and QBWO in the northern SCS. With the northward propagation of the MJO and QBWO convective signals, the major TC genesis location also shifts northward. Since the western Pacific subtropical high shifts eastward (westward) when convection associated with the MJO and QBWO in the northern SCS is enhanced (suppressed), the steering flow in the major TC genesis region is favorable for the eastward (westward) movement of TCs. Results from the composite analysis of the steering flow indicate that both the MJO and QBWO play an important role in controlling the motion of the eastward-moving TCs.