Asymmetric effects of atmospheric circulation on the South China Sea summer monsoon onset

Abstract

Observational analyses suggest a significant positive correlation between the year-to-year convection over the South China Sea (SCS)/western Pacific (WP) and the SCS summer monsoon (SCSSM) onset date. An investigation shows that there is an asymmetric relationship between the area-mean outgoing longwave radiation and the SCSSM onset date index. The analysis found that the influence of the intraseasonal scale circulation is the main cause of this asymmetric relationship. On the interannual scale, circulation and sea surface temperature (SST) anomalies are distributed symmetrically. During the convection active (inactive) years, the SST anomaly field indicates a La Ni n˜ a (El Ni n˜ o)-like pattern. The SCS-WP low-level westerly (easterly) anomaly is enhanced, leading to an increase (decrease) in moisture and ascending (descending) motion. The enhanced convection results in weakening (enhancement) of the subtropical high and provides a favorable condition for early (late) SCSSM establishment. On the intraseasonal time scale, the convection intensity is stronger during the active years than during the inactive years. The northwestward propagation is significant during the active years. This feature is not observed during inactive years. This difference is attributed to the asymmetric meridional distribution of the atmospheric convective instability and moisture disturbance during active years, not during inactive years. In active years, the atmosphere is potentially more (less) unstable and the moisture is to the north (south) of the intraseasonal oscillation (ISO) convective center, which is conducive to generating a favorable unstable environment for the development of new convection north of the ISO convection center, leading to northward convection propagation. The moistening in the SCS-WP is primarily attributed to the phase leading horizontal advection term. An ISO moisture budget analysis reveals that the largest positive contribution is the vertical moisture advection term.