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The abrupt changes of zonal circulation in the Tibetan Plateau (TP) region and their likely causes are derived from National Centers for Environmental Prediction and the National Center for Atmospheric Research reanalysis data. The zonal circulation over the TP abruptly changed in summer (31st pentad) and winter (59th pentad). The switch from summer to winter circulation is characterized by a sudden northward shift of the westerlies and the zero-velocity curve and disappearance of the westerly jet. The winter–summer switch is characterized by the reverse pattern. Therefore, the circulation conversion between summer and winter can be judged from the position of the zero-velocity curve. Curves located north of 20 °N indicate summer circulation over the TP and vice versa. The abrupt change of zonal circulation is mainly caused by the thermodynamic effect of the TP. In June, this effect causes a huge monsoon circulation cell extending from the TP to low latitudes. Consequently, the westerlies jump to the north as easterlies develop. This process, which is enhanced by the strong northerly in Coriolis, establishes the summer circulation. In October, the Hadley cell recurs as the thermal effects of the TP diminish, the westerlies rush southward, and the winter circulation is established. 相似文献
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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。 相似文献
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