摘要
使用NCEP/NCAR再分析资料对Webster与Yang的季风指数(WYI)进行计算和修改。WYI定义为850与200 hPa的纬向风差,但通过分析150—100 hPa和200 hPa的环流场、散度场与垂直运动场,发现200 hPa层并不能真正反映亚洲季风系统上层环流的变化,尤其是其最主要的环流特征即热带东风急流的变化,其核心位于150—100 hPa。纬向风切变U850-U(150+100)的值比U850-U200的值远大得多,更能真实反映季风的强度,并且与低层辐合耦合在一起的高层辐散最大位于150 hPa,在对流层高层取150 hPa比200 hPa更能反映季风系统的耦合关系。因此,在对流层上层选择150—100 hPa重新定义季风指数(DHI)为IDH=U8*50-U(*150+100),不但可以更好地表征亚洲纬向风切变中心的强度变化,也可以代表对流层上下层季风系统的变率。分别用季风指数DHI和WYI对亚洲夏季风的长期变化进行研究,发现DHI比WYI更合适。DHI的变化表明亚洲夏季风存在明显的年代际变化及突变,20世纪70年代末之后显著减弱,这主要是由于150—100 hPa层东风的减弱,但这种东风的减弱现象在200 hPa不明显。突变后总的来说:亚洲地区高层东风减弱,表明夏季风减弱;海陆气压差和海陆温差的减小导致季风减弱;相应高空辐散和水汽输送在印度半岛、中南半岛中部、中国华北与东北地区都是减弱的,也表明夏季风减弱。最后比较NCEP/NCAR和ERA-40两种再分析资料研究亚洲夏季风的强度及其长期变化的差异,以作参考。
Webster and Yang monsoonal index WYI-the zonal wind shear between 850 and 200 hPa-was calculated and modified on the basis of NCEP/NCAR reanalysis data. After analyzing the circulation fields and divergence fields of both 150 - 100 hPa and 200 hPa, however, we found out that the level of 200 hPa could not reflect the real change of the upper-tropospheric circulation of Asian summer monsoon, especially the characteristics and variations of the tropical easterly jet which is the most important feature of the upper-tropospheric circulation and whose core is located in the layer of 150 - 100 hPa. The zonal wind shear U850 - U（150＋100） is much larger than the U850 - U200, so it can reflect the strength of the monsoon more appropriately. In addition, divergence is the largest at the level of 150 hPa rather than 200 hPa, so 150 hPa in the upper-troposphere can better reflect the coupling of the monsoon system. Therefore, WYI is redefined as （DHI） IDH = U^＊850 - U^＊（150＋ 100）, which is able to characterize the variability of not only the strength of the center of zonal wind shear in Asia, but also the monsoon System in the upper- and lower-troposphere. DHI is in agreement with WYI in describing the interannual variation of Asian summer monsoon, but superior to WYI in characterizing the long-term variation. DHI shows that there were obvious interdecadal variations in the Asian summer monsoon, and the climatic abrupt change of the Asian summer monsoon occurred in the year of 1980. The Asian summer monsoon was stronger before 1980 and it weakened after then due to the weakening of the easterly in the layer of 150 - 100 hPa, while the easterly at 200 hPa did not weaken significantly. Generally speaking, after the climate jump year, the easterly in the upper troposphere weakened in Asia, indicating the weakening of the Asian summer monsoon; the land-sea pressure and thermal differences reduced, resulting in the weakening of the monsoon; the corresponding upper divergence as well as the water vapor transport in Indian peninsula, central Indo-China peninsula, North China and Northeast China decreased, indicating the weakening of the summer monsoon as well. The difference between NCEP/NCAR and ERA-40 reanalysis data in studying the strength and long-term variation of the Asian summer monsoon is also compared in the end： the vertical velocity of the NCEP/NCAR data is the biggest at 850 hPa rather than in the layer of non-divergence, while the ERA-40 has its peak value in the layer of 300 - 400 hPa, showing that the latter is more reasonable in depicting the vertical velocity than the former; the DHI of the ERA-40 experienced two weakening processes in the mid- and late 1960s and in the mid- and late 1980s respectively, while the DHI of the NCEP/NCAR data had only one weakening process in the late 1970s; the surface pressure of the ERA-40 had a negative anomalous center in the district of Mongolia, while that of the NCEP/NCAR had a positive anomalous center in North China, Northeast China and Mongolia. According to the previous study that there is something wrong with the sea surface pressure of the NCEP/NCAR data in east Eurasia in the period of 1948-1967, therefore, the ERA-40 is more reasonable in describing the surface pressure than the NCEP/NCAR data.
出处
《气象学报》
CAS
CSCD
北大核心
2006年第6期770-779,共10页
Acta Meteorologica Sinica
基金
"九五"国家攀登B项目:南海季风实验研究
国家基础研究发展规划项目(No2004cb418303)
关键词
季风指数
年际变化
年代际变化
突变
Monsoon indices, Interannual variation, Interdecadal variation, Climate abrupt change.
