The 30-60-day northward-propagating intraseasonal oscillation over South China Sea during pre-monsoon period in a coupled model

The 50-year pentad mean simulation outputs of an atmosphere-ocean coupled general circulation model, the Scale INTeraction EXperiment (SINTEX)-the Frontier Research Center for Global Change (FRCGC) model (SINTEX-F model), are used to understand processes of the northward-propagation of intraseasonal oscillations (ISOs) over the South China Sea (SCS) during the pre-monsoon season. A quite different mean state is found in the SCS before the monsoon onset relative to that of summertime, which is consistent with the observations. Then, 18-year SCS ISOs, with a relatively larger strength and significant northward component, are selected, and the composite and lead-lag correlation analyses are conducted to explore a possible mechanism for the northward propagation of ISOs in the pre-monsoon period. Two mechanisms contribute to the northward propagation of the SCS ISOs. One is the vorticity advection by the barotropic southerly mean flow. First, a barotropic vorticity is induced by vertical advection inside the convection centre. During the pre-monsoon period, the west sides of the upper tropospheric high and the western Pacific subtropical high cover the SCS, thus, there are southerly flows both in the upper and lower troposphere over the SCS. Then the barotropic vorticity induced inside the convection centre shifts to the north because of the advection by the barotropic southerly mean flow. The other is the planetary boundary layer (PBL) moisture advection by the mean flow that lead to a meridional phase leading of the ISO humidity. The northward shift of the PBL moisture promotes a convective instability ahead of the convection centre and leads to the northward propagation of ISOs over the SCS.