Interannual variability and triggers of the South China Sea summer monsoon withdrawal

The interannual variability of the South China Sea (SCS) summer monsoon (SM) withdrawal and associated factors are examined using NCEP/NCAR reanalysis, CMAP rainfall, and OLR data. During late-withdrawal years of the SCSSM, a significant cyclonic anomaly is observed over southern China and northern SCS, accompanied by pronounced anomalous low-level westerly winds over the Bay of Bengal that extend eastward to the Philippine Sea. At the upper level, significant easterly wind anomalies are seen over the SCS. Corresponding to these circulation anomalies, a dipole rainfall anomaly mode is observed, with significant positive (negative) rainfall anomalies over the SCS and Philippine Sea (equatorial Indian Ocean). The seasonal progressions of several monsoonal parameters (e.g., zonal wind, rainfall, convection activity) over the SCS also show significant differences between late- and early-withdrawal years. Observational evidence shows that tropical cyclone (TC) activities over the SCS and Philippine Sea and the active/suppressed phases of 30‒60 day oscillations are two important factors that may contribute to the interannual variability of SCSSM withdrawal. Both statistical evidence and case studies verify that the 30‒60 day oscillation impacts SCSSM withdrawal by modulating convection activity and anomalous zonal winds over the SCS. In addition to the TC activities and 30‒60 day oscillations, the interannual variability of the SCSSM withdrawal can also be affected by synoptic systems, including easterly wave and cold front.