高原上空纬向环流的季节突变现象

ABRUPT SEASONAL TRANSITIONS OF ZONAL CIRCULATION OVER THE TIBETAN PLATEAU

利用1951-2012年NCEP/NCAR全球2.5°×2.5°日平均再分析风场及温度场资料,分析高原季风区纬向环流的季节突变现象,并探讨季节突变的主要成因。结果表明:(1)由冬季环流型向夏季环流型转变时,纬向环流表现为零速度线突然北跳,东风带持续加强北进,西风急流突然消失;反之亦然。(2)600 hPa零速度线指数可作为纬向环流冬夏转换的判据。20°N为临界值,零线于20°N以北,高原上空为夏季环流型,20°N以南则为冬季环流型。冬夏转换的突变时间分别为31候和59候。(3)纬向环流季节突变的主要原因是高原的热力作用,随着6月高原加热增强,高原及其南部上升气流增强,形成了高原至低纬地区的季风环流圈,使得高原南部东风气流加强,迫使西风急流北退,完成了冬季环流型向夏季环流型的转变。10月高原加热作用减弱,高原至低纬地区的Hadley环流重新出现,西风急流逐渐增强,纬向环流转变为冬季环流型。

Based on the NCEP/NCAR reanalysis data, the abrupt change of zonal circulation and its main cause in the Tibetan Plateau monsoon region are studied. The results suggest that the zero velocity curve and the east- erlies jump northward, while the westerly jet stream disappears and the westerlies move southward when the cir- culation pattern changes from winter to summer and vice versa. The position of a zero velocity curve at 600 hPa is a usable criterion for determining if the change between winter and summer has happened. In winter, it is south of 20 °N, and in summer it is north of 20 °N. An abrupt change of flow field happens in the 59th pentad （in October）, while it happens in the 31st pentad （in June）. The main factor of abrupt seasonal transitions of zonal circulation is the thermodynamic effect of the Tibetan Plateau. Because of the thermal effect in June, the updrafts coming from the Tibetan Plateau and its southern area strengthen. Then the monsoon circulation is formed which makes the easterly flow stronger and the westerly jets move back to the north. As a result, the atmospheric circulation changes into the summer pattern. In October, as the heat effect of the Tibetan Plateau becomes weak, the Hadley circulation reoccurs, the westerly jets strengthen, and the zonal circulation changes into the winter pattern.