The characteristics of convection onset over the Indochina Peninsula and its possible mechanism are investigated using the reanalysis of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and the black-body temperature at the top of the cloud (T-BB). Results show that the convection onset occurs before the systemic northward progression of the tropical convection and is earlier than the onset of the summer monsoon over this region. The prominent difference between the onset of convection and that of the summer monsoon appears in the westmost part of the western Pacific subtropical high. When the convection bursts, the Indochina Peninsula is under the control of the western Pacific subtropical high. However, the summer monsoon onset follows the splitting of the subtropical high belt and the eastward retreat of the western Pacific subtropical high out of the Indochina Peninsula. The climatological features of T-BB over the Indochina Peninsula indicate that the convective activities have a close relationship with the quasi-biweekly oscillation (QBWO). Based on the onset dates determined by the T-BB data, case-studies and composite analyses are implemented on exploring the possible mechanism of the convection onset. It is found that the onset of convection is mainly influenced by factors as follows. The wet phase of QBWO of the tropical convection propagates northward from Sumatra to the Indochina Peninsula. Over the southern Indochina Peninsula, anomalous southerlies (the Rossby wave response to the tropical convective QBWO) are beneficial to both the northward progression of the convective disturbances and the convergence of the low-level moisture flux. Furthermore, the Indochina Peninsula is easily affected by any cold surge invading southwards from higher latitudes, which intensifies the convective activities when merging with the wet/warm current. Copyright (c) 2007 Royal Meteorological Society.
