冬、 春季青藏高原东侧涡旋对特征及其对我国天气气候的影响

受高原地形影响， 低层西风气流在高原西侧分支， 从南北两侧绕流， 在高原东侧汇合， 并在南（北）侧形成定常的正（负）涡度带。本文利用NCEP/NCAR提供的1951-2004年再分析资料， 发现这两个涡度带在700 hPa上最明显， 常年存在一正一负两个对称的涡旋（下称“涡旋对”）， 且冬, 春季较强。根据各月涡旋对的位置及强度， 本文定义冬, 春季绕流指数为正, 负涡旋对平均涡度之差， 定量地表征高原绕流作用的强弱， 绕流指数大则高原绕流作用强。结果表明, 1951-2004年中2/3的年份高原绕流作用春季强于冬季， 高原绕流作用不仅是高原大地形的动力作用造成的， 而且受到热力作用的影响。冬季绕流指数以年代际变化为主， 近50年冬季高原绕流作用有显著的增强趋势； 春季绕流指数年代际和年际变化均不明显。冬、 春季， 强高原绕流作用均有利于中高纬冷空气向我国北方输送， 使东北及新疆北部地区气温偏低。春季强高原绕流作用还有利于高原东南侧的暖湿气流向华南及江南地区输送， 使西南、 华南部分地区气温偏高； 偏南暖湿气流和来自中高纬的偏北冷干气流在31°N附近辐合， 有利于江淮地区降水。无论冬, 春季， 亚洲中纬度西风强度是影响高原绕流作用的重要因子。

Characteristic of a Couple of Vortexes on the East Side of Tibetan Plateau from Winter to Spring and Their Impact on the Weather and Climate in China

Affected by Tibetan Plateau terrain, the lower-level west wind is split on the east side of Plateau and forced to flow around it. There are two branches of the westerly, one flow to the south of the Plateau forming a positive vorticity belt, and to the south of the Plateau forming a negative belt. They converge on the east side of the Plateau. Utilizing the NCEP/NCAR reanalysis data from 1951 to 2004, we discovered that the two vorticity belts were the most obvious on 700 hPa, and there were a couple of vortexes on the east of the Plateau all the year round. According to the monthly position and intensity of the vortexes, a Detouring Flow Index (DFI) from winter to spring is defined as the difference between the average vorticity of the positive vortex and the negative one, quantitatively characterized the Function of Detouring Flow around Tibetan Plateau (FDFTP). From 1951 to 2004, the FDFTP is stronger in spring than that in winter in 2/3 years. Interdecadal variation of the DFI is dominant in winter, and the DFI is obviously enhanced in last 50 years; both interdecadal and annual changes of the DFI are not significant in spring. From winter to spring, the enhanced FDFTP is favorable to transport cold air to northern China, induced low temperature in northeast China and northern part of Xinjiang Province. In spring enhanced FDFTP is also favorable to transport warm and wet current to southern China, induced high temperature there. The meeting of cold and warm air near 31°N may cause heavy rain in the Yangtze-Huaihe regions. From winter to spring, intensity of the westerly belt in middle latitude in Asia is one of important effect factors to the FDFTP.