Intraseasonal Variability of the South China Sea Summer Monsoon

The objective of this study is to explore, based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed. The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced. Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode. Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies.