MJO对我国南海冬季风异常的影响

Impact of the Madden-Julian oscillation on winter monsoon anomaly over the South China Sea

利用长时间的逐日再分析资料和澳大利亚气象局提供的MJO指数,研究了MJO对我国南海冬季风异常的影响和过程。结果表明,随着MJO的对流中心从西印度洋进入西太平洋,南海海面风场出现偏南风-东北风-偏南风异常的振荡现象,表明南海冬季风阶段性的间断和活跃,最明显的偏南风异常和东北风异常分别位于MJO第8—2位相和第5—6位相。通过合成MJO各位相下500 h Pa东亚大槽异常和200 h Pa东亚急流异常,我们进一步证实南海冬季风活跃期(MJO第5—6位相),东亚大槽加深,高空急流加强,我国华南沿海上空的反气旋式环流异常,东南边缘的引导气流利于冷空气南下直达南海。相反,在南海冬季风间断期(MJO第8—2位相),东亚大槽和高空急流均减弱,不利于冷空气的南下。对200 h Pa垂直速度的分析表明,MJO深对流活动的东移,调整东亚局地Hadley环流异常,在高空表现为反气旋/气旋式环流异常交替发展东移,最终消失在北太平洋地区。一方面通过科氏力的作用,引起东亚高空急流的异常,进而影响东亚大槽的位置和强度,从而影响冷空气的南下;另一方面直接加剧南海海面风场异常的南北向振荡。因此,在做南海冬季风季内变化特别是大风天气过程的延伸期预报时,热带MJO活动可以作为一个重要因子考虑在内。

Based On the long-term daily reanalysis data set as well as the real-time multivariate MJO index from the Bureau of Meteorology of Australia, the influences of the MJO on winter monsoon anomalies in South China Sea （SCS） are investigated. The results show that the wintertime surface wind anomaly over the SCS shows south- northeast- south wind pattem variations as the convection centre of MJO moves from the Indian Ocean to the western Pacific. This indicates the enhancement and suppression of the winter monsoon and the most remarkable south and north wind anomalies appear in phase 8--2 and phase 5--6, respectively. Composite results further demonstrate the East Asian trough at 500 hPa and the 200 hPa westerly jet associated to the East Asian local Hadley circulation anomalies can be contributed to the intra-seasonal oscillations of the South China Sea winter monsoon. Therefore, tropical MJO could be taken as an important factor in extended-range forecast of the monsoon anomalies especially the gale wind over the SCS during the winter.