大气季节内振荡对东亚夏季风活动的影响研究

利用ERA40逐日再分析资料、NCEP/NCAR2逐日再分析资料、中国740个测站日降水资料、上海台风研究所提供的西太平洋热带气旋资料、Kaplan等重建的月平均SSTA资料、NOAA逐日长波辐射（OLR）等资料，应用离散功率谱分析、带通滤波、EOF分析等统计方法，研究了东亚夏季风(EASM）的移动特征、东亚地区季节内振荡(ISO)的基本特征、季节内振荡对东亚夏季风活动的影响、季节内振荡对东亚夏季风异常活动的影响机理。主要结论如下： （1）综合动力和热力因素定义了可动态描述东亚夏季风移动和强度的指数，并利用该指数研究了东亚夏季风的爆发和移动的季节内变化及其年际和年代际变化特征。研究发现，气候平均东亚夏季风前沿分别在28候、33候、36候、38候、40候、44候出现了明显的跳跃。东亚夏季风活动具有显著的年际变率，主要由于季风前沿在某些区域异常停滞和突然跨越北跳或南撤引起，造成中国东部旱涝灾害频繁发生。东亚夏季风的活动具有明显的年代际变化，在1965年、1980年、1994年发生了突变，造成中国东部降水由“南旱北涝”向“南涝北旱”的转变。 （2）东亚季风区季节内变化具有10~25d和30~60d两个波段的季节内振荡周期，以30-60d为主。存在三个主要低频模态，第一模态主要表征了EASM在长江中下游和华北地区活动期间的低频形势；第二模态印度洋-菲律宾由低频气旋式环流控制，主要表现了ISO在EASM爆发期间的低频形势；第三模态主要出现在EASM在华南和淮河活动期间的低频形势。第一模态和第三模态是代表东亚夏季风活动异常的主要低频形势。 （3）热带和副热带地区ISO总是沿垂直切变风的垂直方向传播。因此，在南海-菲律宾东北风垂直切变和副热带西太平洋北风垂直切变下，大气热源激发菲律宾附近交替出现的低频气旋和低频反气旋不断向西北传播，副热带西太平洋ISO以向西传播为主。中高纬度地区，乌拉尔山附近ISO以向东、向南移动或局地振荡为主；北太平洋中部ISO在某些情况下向南、向西传播。 （4）季风爆发期，伴随着热带东印度洋到菲律宾一系列低频气旋和低频反气旋， 冷空气向南输送，10~25天和30~60天季节内振荡低频气旋同时传入南海加快了南海夏季风的爆发。在气候态下，ISO活动表现的欧亚- 太平洋（EAP）以及太平洋-北美（PNA）低频波列分布特征(本文提出的EAP和PNA低频波列与传统意义上的二维定点相关得到的波列不同)。这种低频分布形式使得欧亚和太平洋中高纬度的槽、脊及太平洋副热带高压稳定、加强，东亚地区的低频波列则成为热带和中高纬度ISO相互作用影响东亚夏季风活动的纽带。不同的阶段表现不同的低频模态，30~60d低频模态的转变加快了EASM推进过程中跳跃性；30-60d低频模态的维持使得EASM前沿相对停滞。 （5）30-60d滤波场，菲律宾海域交替出现的低频气旋和低频反气旋不断向西北传播到南海-西太平洋一带。当南海-西太平洋地区低频气旋活跃时，季风槽加强、东伸，季风槽内热带气旋（TC）频数增加；当南海-西太平洋低频反气旋活跃时，季风槽减弱、西退，TC处于间歇期，生成位置不集中。 （6）在El Nino态下，大气季节内振荡偏弱，北传特征不明显，但ISO由中高纬度北太平洋中部向南和副热带西太平洋向西的传播特征显著，东亚地区ISO活动以第三模态为主，EASM集中停滞在华南和淮河流域，常伴随着持续性区域暴雨的出现，易造成华南和江淮流域洪涝灾害，长江和华北持续干旱。在La Nina态下，大气季节内振荡活跃，且具有明显的向北传播特征，PNA低频波列显著，东亚地区ISO活动以第一模态单峰为主；EASM主要停滞在长江中下游和华北地区，这些地区出现异常持续强降水，华南和淮河流域多干旱；在El Nino态向La Nina态转换期，ISO活动以第一模态双峰为主，长江中下游常常出现二度梅。

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.