Comparison of the Double Summer Monsoon Troughs over East Asia

Based on NCEP/NCAR reanalysis data, an investigation has been carried on the comparison of the double summer monsoon troughs over East Asia, which refer to the subtropical summer monsoon trough (subtropical-trough) and the South China Sea summer monsoon trough (SCS trough), respectively. The results show that the SCS trough is stronger than the subtropical-trough either in convergence or convection.The subtropical-trough extends up to higher levels and inclines northward with altitude, while, the SCS trough extends up to a lower level, and its position is seldom changed. The SCS trough establishes early and abruptly with the low level positive relative vorticity appearing suddenly, and retreats slowly,but the subtropical-trough establishes step by step with the positive relative vorticity initially over the Yunnan-Guizhou Plateau and Guangxi areas spreading gradually northeastward, and withdraws rapidly. The onset of SCS trough is obviously indicated by the reverse of the easterly, but the establishment of the subtropical-trough is characteristic of the westerly enhancement. The subtropical-trough has clearly frontal property,yet the SCS trough has not.