Analysis of the characteristics of 30-60 day low-frequency oscillation over Asia during 1998 SCSMEX

The wavelet analysis is performed of the mid- and low-latitude circulation index at 850 hPa over East Asia, the East Asian monsoon index and the preci pitation over the middle and lower reaches of the Yangtze River during 1998 South China Sea Monsoon Experiment (SCSMEX) from May to August. Analysis s hows that distinct 30-60 day low-frequency oscillation (LFO) exists in all of the above elements during the experiment period. Analysis of low-frequen cy wind field at 850 hPa from May to August with 5 days interval is perform ed in this paper. Analysis results reveal that: (1) A low-frequency monsoon circulation system over East Asia, characterized by distinct 30-60 day low -frequency oscillation, exists over 100 degrees -150 degreesE of East Asian area from the middle and eastern parts of China continent and the South Ch ina Sea to the western Pacific in both the Northern and Southern Hemisphere . The activity of East Asian monsoon is mainly affected by the low-frequenc y systems in it: (2) All of the tropical monsoon onset over the South China Sea in the fifth pentad of May, the beginning of the Meiyu period and heav y rainfall over the middle and lower reaches of the Yangtze River in mid-Ju ne and the heavy rainfall after mid-July are related to the activity of low -frequency cyclone belt over the region, whereas the torrential rainfall ov er the upper reaches of the Yangtze River in August is associated with the westward propagation of low-frequency anticyclone into the mainland; (3) Th ere are two sources of low-frequency oscillation system over East Asia duri ng SCSMEX. i.e. the equatorial South China Sea (SCS) and mid-high latitudes of the middle Pacific in the Northern Hemisphere. The low-frequency system over SCS propagates northward while that in mid-high latitudes mainly prop agates from northeast to southwest. Both of the heavy rainfall over the mid dle and lower reaches of the Yangtze River in June and July are associated with the northward propagation of the above-mentioned SCS low-frequency sys tems From the tropical region and the southwestward propagation From mid-hi gh latitudes respectively and their convergence in the middle and lower rea ches of the Yangtze River; (4) There are two activities of low-frequency cy clone and anticyclone belt each in the East Asian monsoon system during May to August. However the activity of these low-frequency circulation systems is not clearly relevant to the low-frequency circulation system in the Ind ian monsoon system. This means that the low-frequency circulation systems i n Indian monsoon and East Asian monsoon are independent of each other. The concept previously put forward by Chinese scholars that the East Asian mons oon circulation system (EAMCS) is relatively independent monsoon circulatio n system is testified once more in the summer 1998.