Interdecadal Variabilities of SCS Summer Monsoon Intensity

In order to diagnose the interdecadal variabilities of South China Sea (SCS) summer monsoon activities and reveal the inner relationships of the sea surface temperature and the general circulation anomalies to the interdecadal variabilities of SCS summer monsoon activity, by using NCEP/NCAR reanalysis grid data, the interdecadal phases of SCS summer monsoon are divided, the differences of the average fields in the different interdecadal phases and their causes are studied with composite and comparison analysis. The results show that the intensity of SCS summer monsoon is characterized by intedecadal variability that has an abrupt jump occurring around 1976. It can be divided into two phases. The first phase is from 1960 to 1976 and the second phase is from 1980 to 1998. Comparing the southwesterly over SCS in the second phase with that in the first phase, it is found that the average intensity is weaker, the amplitude of annual variation is greater, the periods are shorter and the ascending flow over the south and center of SCS is stronger in the second phase than that in the first phase. The temperatures in whole troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in the increase of geopotential height in lower and middle levels in troposphere over China continent, and it is more than that over the ocean by the thermodynamic forcing process. The weakening of the pressure gradient between the continent and ocean can result in the enhancement of an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. The temperatures in the troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in geopotential height in lower and middle levels of troposphere over China continent increasing more than that over ocean through the thermodynamic forcing process. The weakening of pressure gradient between the continent and ocean can trigger an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. As shown in the distribution of divergence wind field and vertical motion, it implies that the remarkable increase of sea surface temperature over the eastern equatorial Pacific Ocean may play an important role in enhancing the ascending motion over southern and center of SCS.