This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS–WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper‐level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo‐China Peninsula to the Arabian Sea) and northerly cross‐equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid‐level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper‐level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.

The weakening in the relationship between the South China Sea summer monsoon onset (SCSSMO) and the low‐level cross‐equatorial flow (CEF) in May is investigated using the ERA‐Interim reanalysis data sets during 1979–2016. The SCSSMO–SCSCEF relationship has experienced a significant inter‐decadal change, and the correlation coefficient becomes weaker after the late 1990s. The correlation has shifted from the significant negative value in the earlier decade to insignificant in the later decade. This inter‐decadal change is robust under several sensitive tests and largely independent of the El Niño‐Southern Oscillation signal. One possible explanation is the change in mechanisms driving the SCSSMO over the course of the two periods. Before the late 1990s, the northwards march of the intertropical convergence zone, which has a close relationship with the SCSCEF, is mainly responsible for the SCSSMO. After the late 1990s, warming of the western North Pacific favours more tropical cyclones and disturbances during May. The westwards movement of these tropical disturbances would affect the SCSSMO and help explain why the SCSSMO–SCSCEF relationship became weaker in the later decade.

This study investigates interannual variability of boreal winter regional Hadley circulation over western Pacific (WPHC) and its climatic impacts. A WPHC intensity index (WPHCI) is defined as the vertical shear of the divergent meridional winds. It shows that WPHCI correlates well with the El Niño–Southern Oscillation (ENSO). To investigate roles of the ENSO‐unrelated part of WPHCI (WPHCIres), variables that are linearly related to the Niño‐3 index have been removed. It reveals that meridional sea surface temperature gradient over the western Pacific plays an essential role in modulating the WPHCIres. The climatic impacts of WPHCIres are further investigated. Below‐normal (above‐normal) precipitation appears over south China (North Australia) when WPHCIres is stronger. This is due to the marked convergence (divergence) anomalies at the upper troposphere, divergence (convergence) at the lower troposphere, and the accompanied downward (upward) motion over south China (North Australia), which suppresses (enhances) precipitation there. In addition, a pronounced increase in surface air temperature (SAT) appears over south and central China when WPHCIres is stronger. A temperature diagnostic analysis suggests that the increase in SAT tendency over central China is primarily due to the warm zonal temperature advection and subsidence‐induced adiabatic heating. In addition, the increase in SAT tendency over south China is primarily contributed by the warm meridional temperature advection. Further analysis shows that the correlation of WPHCIres with the East Asian winter monsoon (EAWM) is weak. Thus, this study may provide additional sources besides EAWM and ENSO to improve understanding of the Asia‐Australia climate variability.