Interdecadal Features Of SCS Summer Monsoon Intensity Interannual Variabilites

Using NCEP/NCAR reanalysis grid data, averaged temperature and precipitation of 730surface stations of China etc., the interdecadal variabilities of South China Ssea (SCS) summermonsoon (SCSSM) intensity have been studied with composite and comparison analysis andwavelet analysis etc..The results show that the intensity of SCS summer monsoon is characterized byintedecadal change around 1976s. Two phases, 1960-1976 (first phase hereafter) and1980-1998 (second phase hereafter), are divided. In contrast to southwesterly over SCS in thefirst phase, the weaker SCSSM intencities, larger amplitude and shorter period of interannualvariation and stronger ascending flow are found in second phase. The change of ocean-landthermal contrast is the main reason for the weaker SCSSM intencities. Otherwise, the secondphase tends to lower summer temperature in the whole troposphere over the Chinese mainlandand higher temperature over the surrounding ocean, leading to the negative geopotential heightanomalies in mid-lower troposphere over Chinese mainland and positive anomalies over theocean due to thermodynamic forcing. The weakening of pressure gradient between thecontinent and ocean can excited an anomalous anticyclonic circulation over the continent,leading to the weakening of the southwesterly over the north and central SCS. As shown in thedistribution of divergence wind field and vertical motion, it implies that the remarkableincrease of sea surface temperature over the eastern equatorial Pacific Ocean may play animportant role on enhancing the ascending motion over south and central SCS.There are larger amplitude of interannual variation and much stronger intensity of SCSSMin the second phase than that in the first. In addition, the differences of many elements (includewind,geopotential height,temperature,relative humidity etc.) between strong and weakSCSSM period behave as a consistent pattern in either first or second phase, but the patterns oftwo phases are greatly different as following: 1) in the second phase, the correlationcorrfficient for the monsoon intensity and the elements distributes as EAP-like pattern and sodoes parameters's interannual variabilities. In the first phase, the EAP pattern isn'tremarkable.2) in the first phase, the correlation corrfficient for the monsoon intensity and theelements distributes as EU-like pattern and so does parameters's interannual variabilities. In thesecond phase, the EU pattern is indistinctive. 3) See from the numerical value, that in thesecond phase is greater than that in the first phase. It is noticeable that the intensities ofSCSSM have a close connection with the general circulation over mid-hiqh latitude during thefirst phase. In comparison with the first phase, the abnormity of SCSSM may have more effecton the north area.From the first phase to the second phase, the effect of the sea surface temperature (SST) inthe tropical ocean on SCSSM becomes more significant and the western North Atlantic Oceanis of less importance by influencing SCSSM. Since the tropical ocean plays a more importantrole during the second phase, at least as one of causes, the more remarkable SCSSMinterannual variability is driven by enhanced tropical SST interannual variability.