中东急流的季节转变特征及与印度夏季风爆发早晚的关系

CHARACTERISTICS OF MIDDLE EAST JET STREAM DURING SEASONAL TRANSITION AND ITS RELATION WITH INDIAN SUMMER MONSOON ONSET

采用1968—2009年NCEP/NCAR逐候再分析资料,分析了季节转换期间中东急流的变化特征及其热力机制。结果表明:中东急流中心的强度和位置存在明显的季节变化特征,第67候—次年第24候偏强且位置稳定偏南(27.5°N)、第38—44候偏弱且位置稳定偏北(45°N)。200 hPa纬向风场EOF第1空间模态反映了中东急流在冬、夏季的位置,冬季主要位于埃及和沙特阿拉伯上空,夏季主要位于黑海东部至咸海东部上空,且中东急流在冬季比在夏季强。第26—31候和第54—61候分别是中东急流春夏季和秋冬季的季节转换期,其200 hPa西风演变与500～200 hPa平均南北温差演变的对应关系很好,表明南北温差的季节性转变导致了200hPa西风的季节性转变。个例分析表明,印度夏季风爆发日期早晚与中东急流季节转换日期早晚的关系非常密切。印度夏季风爆发较早(晚)时,由于500～200 hPa南北温差大值区域向北推进较早(晚),因此,中东急流向北推进较早(晚),同时40～90°E对流层低层西风急流出现也较早(晚)。

By using the pentad mean NCEP/NCAR reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream （MEJS） and its thermal mechanism during seasonal transition are studied. Results show that the intensity and South-North location of MEJS center exhibit obvious seasonal variation characteristics. MEJS is stronger and locates at 27.5°N from the 67 th pentad to the 24 th pentad next year. MEJS is weaker and locates at 45 °N from the 38th pentad to the 44 th pentad. The first Empirical Orthogonal Function （EOF） mode of 200-hPa zonal wind field shows that MEJS mainly locates over Egypt and Saudi Arabia in winter, and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26 th-31st pentad is the spring-summer transition of MEJS, and the 54 th-61 st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500～200 hPa SNTD （South-North temperature difference） and 200-hPa zonal wind velocity matches well, indicating that the former leads to the latter according to the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger 500～200 hPa SNTD moves northward earlier with the westerly jet in the lower troposphere over 40～90 °E appearing earlier than normal, and vice versa.