冬季热带西太平洋MJO活动强弱年的环境场特征

THE ENVIRONMENTAL CHARACTERISTICS OF STRONG/WEAK MJO ACTIVITY OVERTHE TROPICAL WESTERN PACIFIC IN WINTER

利用1948—2011年NCEP等再分析资料,采用合成分析等方法对比分析了冬季(冬半年)热带西太平洋MJO(Madden-Julian Oscillation)活动强、弱年的环境场特征。结果表明,冬季热带西太平洋MJO的活动具有显著的年际和年代际变化。MJO活动强年,对流层低层在菲律宾以东洋面上空有异常气旋式环流,赤道东太平洋上空为较强的东风距平,赤道印度洋到赤道西太平洋上空是异常西风,西太平洋地区有较强辐合,从而导致热带西太平洋地区积云对流活动显著加强;而MJO活动弱年的环流特征相反。热带MJO以东传为主,有少量西传波动。在MJO活动强年,无论东传还是西传其时空谱值都显著大于MJO活动弱年,其中心频率较MJO活动弱年偏高。MJO活动的异常和海温及东亚冬季风紧密相连,在MJO活动强年,海平面气压和500 h Pa位势高度异常场表现为中高纬度的正异常和低纬地区的负异常,东亚冬季风活动偏强,中国大陆中部气温普遍偏低,同时,黄河以南长江以北地区降水偏多,而长江以南地区降水偏少;台湾附近海域受强东亚冬季风影响,海表温度偏低,东太平洋上海温距平呈现La Nia型的异常分布,而在MJO活动弱年上述特征基本相反。

Comparative analysis is done of the environmental characteristics of the winter of strong/weak Madden-Julian Oscillation （MJO） activity over the tropical western Pacific based on the NCEP reanalysis data. It is shown that the activity of MJO has a significant interannual and interdecadal variability in tropical western Pacific during winter. In the strong MJO winter, there exits anomalous cyclonic circulation east of the Philippines and a strong anomalous easterly controls the equatorial eastern Pacific region and the westerly anomalies extend from the Indian Ocean to the western Pacific at lower troposphere, strengthening the convergence in the western Pacific region and significantly increasing cumulus convection over the equatorial western Pacific. All these features are the opposite in the weak MJO winter. Eastward propagation is the main feature of MJO, though there may be little westward propagation. Compared to a weak MJO winter, the value of spatial and temporal spectra and the center frequency are higher in a strong MJO winter. The anomalous activity of MJO is closely linked to the sca surface temperature （SST） and the East Asian winter monsoon; in the strong MJO winter, there are positive anomalies in the high latitudes and negative anomalies in low latitudes in both the sea level pressure and 500hPa geopotential height and the temperature is lower over the central part of Chinese mainland, which means a strong East Asian winter monsoon; and positive rainfall is located in the area between the Yellow River and Yangtze River while less rainfall is in the south of the Yangtze river. The SSTA is negative near the island of Taiwan due to the effect of the strong East Asian winter monsoon, and shows a La Nifia type of anomaly in the eastern Pacific. In the weak MJO activity winter, the characteristics mentioned above are just the opposite.