Progressions of the onset and withdrawal of cold surge season over the South China Sea

This study identifies the onset and withdrawal dates of cold surge season over the South China Sea (SCS) and investigates the relevant physical processes around the transition.Climatologically, the cold surge season is initially formed on 30th September and finally ended on 29th March, accompanied by a reversal of low-level meridional winds from southerly to northerly, or vice versa. On the one hand, the seasonal transition is partly attributed to the evolution of Siberian High, as its formation (retreat) contributes to the amplification (weakening) of northerlies from the mid-latitude to the tropics. On the other hand, the change of tropical convection also fulfils its role by intensifying the local Hadley cell. However, the tropical convection is only conducive to the onset process, but has little effect on the withdrawal.Further analysis indicates that the tropical convection is associated with the northward propagation of intraseasonal oscillations (ISOs). The alternation of 10-25-day oscillation causes convection fluctuation over the SCS, while the dry phase of 30-60-day oscillation provides a favorable condition for the maintenance of cold surge onset.