The Effect Of The Atmospheric Intra-seasonal Oscillation On Movement Of East Asian Summer Monsoon

Using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis daily data ERA40, NCEP/NCAR2 reanalysis daily data, the daily rainfall data from 740 stations in China, the North-West Pacific tropical cyclone data provided by Shanghai Typhoon Institute, the monthly mean sea surface temperature anomaly (SSTA) reconstructed by Kaplan el al. and the daily outgoing long-wave radiation (OLR) data of NOAA, basing on the discrete power spectral method, the band-pass filter and the EOF (empirical orthogonal function) analysis et al. statistical methods, this Paper study on the characters of the East Asian summer monsoon (EASM) movement, the basic features of intra-seasonal oscillations (ISO), the relationship between ISO and the movement of EASM and the mechanism how ISO influence the abnormal activities of EASM. The preliminary results are as following:(1) EASM index was defined by using the thermal and dynamic factors. It can describe EASM's movement and intensity objectively. And then the intra-seasonal variability, inter-annual and inter-decadal variations of the EASM were analyzed by using EASM index. On average, EASM undergo six abrupt northward shifts at the 28th Pentad, 33rd pentad, 36th Pentad, 38th pentad, 40th pentad and 44th pentad. The movement of EASM showed prominent interannual variability, which is caused by the front of EASM stayed at one region without less movement for long time and jump northward or retreat southward abruptly, bringing the serious flood and drought disasters to East China. The movement of EASM also exhibited interdecadal variability and had three interdecadal abrupt changes in 1965, 1980 and 1994, respectively,in result, East China underwent from the pattern of south- drought and northern- waterlog before 1979 to south-waterlog and north- drought.(2) The 10-25-day and 30-60-day oscillations are predominantly in East Asia, especially the 30-60-day oscillation. By the EOF analysis, there are three important low-frequency models in summer. The first model shows the prominent southeast-northwest East Asian (EANW) teleconnection wave-train, it appears in the Yangtze River Meiyu Period and North China rainy season. The second model exhibit low-frequency dipole in Baikal and Okhotsk, the cyclone circulation control from the Indian Ocean to Philippine. It can describe the low-frequency circulation in the EASM onset period. The third model shows the prominent southwest-northeast East Asian (EANE) teleconnection wave-train, it usually occurs in the periods that EASM stay in South China and Huaihe River Valley. After the EASM onset, the low-frequency circulation usually shows the first or the third mode mainly in some year, this is the important reason that causes the abnormal activity of EASM.(3) The propagations of the tropical and sub-tropical ISO rely on the vertical shear of the mean flow, the ISO propagate along the perpendicular direction of the vertical shear wind. Therefore, under the northeastern vertical shear in South China Sea-Philippine and northern vertical shear in subtropical Pacific, ISO from Philippine moves northwest to near of South China Sea (SCS), and then propagate to northward. The ISO form subtropical West Pacific move westward mainly. In mid-high latitudes The ISO from Ural propagate eastward and southward or oscillation in one place. Under some conditions, ISO from the middle of North Pacific move southward and westward.(4) The different low-frequency circulations occur in different EASM's movement period. During the onset period, EASM are mainly affected by low-frequency cyclones (or anti cyclones) belt from low latitudes and the low-frequency dipole appearing in Baikal and Okhotsk. The phase lock of the low-frequency cyclones from 30-60-day and 10-25-day intra-oscillations in SCS are important reasons that cause the jump of EASM. After mid June, the low-frequency circulations take on Pacific-North American (PNA) and Europe-Asia-Pacific (EAP) low-frequency wave-train of the 30-60-day ISO. EAP and PNA low-frequency wave-train make the trough and ridge over mid-high latitudes and the Western Pacific Subtropical high steady and strengthen. East Asian (EA) low-frequency wave-train is the link between the intra-oscillations from low latitudes and mid-high latitudes, which affect the activity of EASM. The shift of the low-frequency mode and the phase lock with 10-25-day oscillation make the EASM abrupt movement (advance northward or retreat quickly), while EASM usually stay at one region for long time by the model of 30-60-day ISO.(5)The low-frequency cyclones and anti cyclones from Philippine propagate northwestward to SCS-West Pacific (WP) alternately. The low-frequency cyclone make the monsoon trough (MT) strengthen and extend eastward, the WP tropical cyclones (WPTC) are active and appear frequently, the location that WPTC appear concentrate. While the low-frequency anti cyclone make MT weaken and shrink westward,WP TC are inactive. The locations where WPTC appear do not concentrate.(6) When the anomaly of sea surface temperature (SSTA) in the tropical Pacific takes on the El Nino mode, the activity of ISO is weak ,and it exhibits the third low-frequency mode mainly. EASM usually stayed at south China and Huai River Valley, the continuous rainstorms usually appear bringing serious flood in these regions, while the rainfall in the lower-middle reaches of the Yangtze River and North China is more less than normal and the drought occur. When SSTA in the tropical Pacific takes on La Nina mode, the activity of ISO is very active and it shows the single pole of the first low-frequency mode mainly. EASM usually stayed at the lower-middle reaches of the Yangtze River and North China for long time, the persistent precipitation appeared in these regions, while the continual drought usually appear in South China and the Huai River Valley. When SSTA in the tropical Pacific takes on La Nina mode and El Nino mode during early stage and in summer, the activities of ISO often show the first low-frequency mode with double poles, the two Meiyu Periods usually occur in the lower-middle reaches of the Yangtze River.