Statistical analysis of the impacts of intra‐seasonal oscillations on the South China Sea summer monsoon withdrawal

Abstract

AbstractBased on multiple data sets and methods, this study investigates the impacts of intra‐seasonal oscillations (ISOs) on the South China Sea summer monsoon (SCSSM) withdrawal. A daily SCSSM withdrawal date is established, which can capture reasonably the consistent transition of low‐level zonal wind from westerly to easterly over the South China Sea (SCS). The bandpass‐filtered outgoing longwave radiation and low‐level winds are then composited with respect to the monsoon withdrawal date. It is found that a 30–60‐day oscillation originating from the equatorial Indian Ocean and a quasi‐biweekly oscillation (QBWO) propagating from the equatorial western Pacific collectively contribute to the SCSSM withdrawal. Under the background of slow annual cycle (weak zonal wind during late September), the local convectively inactive phases of these ISOs induce anomalous easterly winds, which could trigger monsoon withdrawal over the SCS. The boreal summer ISO indices also confirm that both the 30–60‐day oscillation and the QBWO could modulate SCSSM withdrawal, which is more likely to occur when the suppressed convection caused by these ISOs is either encroaching on or occupying the SCS.