春季青藏高原绕流作用变化特征及其影响

受青藏高原地形影响，低层西风气流在高原西侧分支，从南北两侧绕流，于高原东侧汇合，在其边缘附近形成一正一负2个对称的涡旋（下称“涡旋对”）。文中利用NCEP／NCAR提供的1951—2004年再分析资料，研究发现涡旋对在700hPa上最明显。将700hPa正、负涡旋对平均涡度之差定义为春季绕流指数，可以定量地表征高原绕流作用强弱。研究还发现，1951—2004年春季绕流指数存在准18a、准10a、准4a的周期振荡，春季绕流指数能较好地反映高原绕流作用的强弱。利用全国160站1951—2004年气温和降水资料，研究春季绕流作用对中国天气气候的影响发现，强高原绕流作用不仅有利于中高纬冷空气向中国北方输送，使西北、东北部分地区春季气温偏低，而且也有利于冷空气绕高原南下，使江淮部分地区春季气温也偏低；同时也有利于高原东南侧的暖湿气流向华南及江南地区输送，偏南暖湿气流和偏北冷干气流在32°N附近辐合偏强，有利于江淮地区春季降水偏多。此外，春季绕流指数对长江中下游及淮河流域夏季气温具有指示意义，当春季绕流指数偏强（弱）时，同年长江中下游及淮河流域夏季气温偏低（高）。

Variation Characteristics of the Function of Detouring Flow around Tibetan Plateau in Spring

Affected by Tibetan Plateau terrain, the lower-layer westerly is split at the east side of plateau and forced to flow around it. There are two branches of the westerly, one flowing to the south of the plateau and another to the north of the plateau. They converge at the east side of the plateau, forming a vortex couple there. By utilizing the 1951--2004 NCEP/NCAR reanalysis data, we discovered that vortex couple was most obvious at 700 hPa. Therefore, Detouring Flow Index （DFI） in spring was defined as the difference between the average vorticity of the positive and the negative vortexes at 700 hPa to quantitatively characterize the Function of Detouring Flow around Tibetan Plateau （FDFTP）. The results also showed that the DFI in spring presented the quasi- 18 a, 10 a and 4 a period oscillations, which could better reflect FDFTP. In spring, enhanced FDFTP was not only favorable to transport cold air to northern China and induced low temperature in northeast China and northwest China, but also favorable to transport cold air to the Yangtze-Huaihe region and then induced low temperature there. Furthermore, enhanced FDFTP is favorable to transport warm and wet current to southern China. The stronger convergence of cold and warm air near 32°N might cause heavy rain in the Yangtze-Huaihe region. Additional, enhanced FDFTP in spring was related to high temperature at mid and lower reaches of Yangtze River and Huaihe region. The temperature in summer in the Yangtze-Huaihe region was low （high） with the strong （weak） DFI in spring.