气候变暖背景下祁连山区秋季层状云变化特征

利用祁连山区及周边29个气象观测站近41年秋季云形状和气温观测资料,分析了祁连山区秋季层状云出现频率的空间分布与时间变化特征,探讨了秋季层状云出现频率与气候变暖的关系,并选用同期NCEP/NCAR全球再分析资料,对祁连山区秋季层状云的环流特征和水汽输送进行了分析。结果表明：①祁连山区秋季层状云出现频率为8%～26%,呈西少东多的空间分布。②近41年来,祁连山区秋季增温1.2℃,气温变化的倾向率为0.29℃/10a,80年代中期以后发生了增温的突变。③祁连山区秋季层状云的出现频率呈明显的减少趋势,近41年来减少约11%,倾向率为-2.7%/10a,尤其在20世纪80年代中期以后与同期祁连山区显著增温相对应,层状云出现频率减少更为明显,层状云出现频率与气温呈明显的反相变化趋势。④在气候变暖的背景下,祁连山区的层状云出现频率减少,减少的幅度从西北向东南递增。当祁连山区秋季平均气温在升高1℃时,祁连山区层状云出现频率减少2%～10%,祁连山西段、中段减少2%～4%,祁连山东段减少4%～10%。⑤祁连山区秋季层状云偏多与偏少年在欧亚500 hPa环流场上存在明显的差异,层状云偏多年,极涡向亚洲北部伸展,东亚大槽较偏弱,乌拉尔山高压脊偏强,脊前偏北气流引导极地冷空气沿偏西北路径向中国西北地区输送,中亚地区到高原上不断有低值系统发展东移,同时南支槽加强,来自阿拉伯海、南海、东海的暖湿气流向内陆地区的输送明显加强,与进入高原北部的冷空气交绥,从而使祁连山区层状云出现频次增多;层状云偏少年,中亚-中国西北地区暖性高压异常加强,东亚大槽偏强,冷空气活动路径偏东,亚洲大陆至西太平洋冬季风特征明显,偏北风加强,不利于东南暖湿气流向西北内陆地区的输送,冷暖气流在祁连山区交绥次数减少,从而使祁连山区层状云出现频次减少。⑥印度洋沿孟加拉湾的向北的水汽输送,副热带西太平洋的偏东气流在南海和中南半岛附近转为向北的水汽输送,地中海、里海的西风带纬向水汽输送是3支影响祁连山区秋季层状云多寡的水汽输送通道,进而对祁连山区秋季降水产生影响。

The Changes of Autumn Stratiform Clouds in the Qilian Mts. Region under the Background of Global Warming

Based on meteorological data of autumn cloud shape and surface air temperature at 29 meteorological stations in the Qilian Mountains and the surrounding areas during 1961-2001,the temporal variation and spatial distribution of autumn stratiform cloud arising frequency and the relationship between it and climate warming were analyzed,and the NCEP/NCAR global reanalysis data in the corresponding period were selected to analyze the circulation of autumn stratiform cloud in this region.The results are shown as follows.(1) The autumn stratiform cloud arising frequency in the Qilian Mts.region is 8%～26%,presenting a spatial distribution of less in the(north)west and abundant in the(south)east.(2) The autumn mean air temperature increased by 1.2℃ with a rate of 0.29℃/10a in recent 41 years,and the abrupt change occurred in the mid-1980s.(3) The autumn stratiform cloud arising frequency decreased by 11% at a rate of-2.7%/10a,which was corresponding to the synchronous significant warming since the mid-1980s.The autumn stratiform cloud arising frequency decreased more significantly,and the autumn stratiform cloud arising frequency and air temperature presented an anti-phase trend.(4) In the background of climate warming,the autumn stratiform cloud arising frequency in the Qilian Mts.region decreased,with a decreasing amplitude increasing from northwest to southeast.When the autumn average temperature increased by 1℃,the autumn stratiform cloud arising frequency decreased by 2%～10% over most parts of the Qilian Mts.region,2%～4% in the western and central parts,and 4%～10% in the eastern part.(5) There was a significant difference in 500hPa atmospheric circulation between abundant and less years of autumn stratiform cloud in the Qilian Mts.region.In abundant years,polar vortex extends to northern Asia,the eastern Asia deep trough is weaker,and the Ural high pressure ridge is stronger.Due to the north-northwest airflow before the ridge,the polar cold air flows to Northwest China in a northwest path,uninterrupted low-value systems develop and move towards east from Central Asia to the Qinghai-Tibet Plateau.In the meantime,southern trough strengthened and the transportation of the warm and wet air flow from the Arabian Sea,the South China Sea and the East China Sea to the inland areas were markedly strengthened.It met with the cold air which entered the northern plateau region,leading to an increase in the autumn stratiform cloud arising frequency in the Qilian Mts.region.In less years,the warm anticyclone range from Central Asia to Northwest China is abnormally strengthened,the East Asia deep trough is stronger,and cold air moves by east,presenting a significant winter monsoon characteristic from Asia to Western Pacific.North wind strengthened and made against the transportation of warm and wet flows from Southeast to Northwest China,thus there was less possibility for their meeting in the Qilian Mts.region,thus the autumn stratiform cloud arising frequency decreased.(6) The northward water vapor transportation from the Indian Ocean to the Bay of Bengal,the northward shift of east airflow over the subtropic Western Pacific near the South China Sea and Indo-China Peninsula,and the westerlies latitudinal water vapor transportation over the Mediterranean Sea and the Caspian Sea are the three channels for water vapor transportation which affect the autumn stratiform cloud arising frequency and then affect the autumn precipitation in the Qilian Mts.region.