亚洲夏季风季节内振荡对云南主汛期降水的影响Ⅰ:云南主汛期季节内振荡特征及其传播过程

Impact of Intra-Seasonal Oscillation in Asian Summer Monsoon on Precipitation in Main Rainy Season of Yunnan Ⅰ:Features and Propagation Processes of Intra-Seasonal Oscillation in Main Rainy Season of Yunnan

利用云南省124个气象观测站降水资料和NCEP OLR再分析资料,分析了云南主汛期(6-8月)季节内振荡(ISO)的活动特征及其传播的年际差异,并着重分析了云南主汛期ISO活跃年热带印度洋ISO向云南传播的两条路径和两个亚洲季风系统ISO分别对云南主汛期ISO的影响。云南主汛期平均降水量与区域平均OLR呈显著负相关,用低频(30～60天)OLR表征云南夏季风ISO,其强度具有明显的年际差异。在云南主汛期ISO活跃年,ISO主要来自于两条传播路径:一条是从副热带西太平洋或中国东南部的三次西传,强度较大,分别造成云南主汛期内3次低频对流显著活跃期;另一条是从热带印度洋沿孟加拉湾西岸的西南-东北向传播,到达云南时加剧了云南主汛期的低频对流。在云南主汛期ISO不活跃年,主汛期仅有两次弱的低频对流,主要来自于副热带两次弱的纬向低频OLR传播,第一次是从副热带西太平洋的西传,第二次是从阿拉伯海北部的东传。在云南主汛期ISO活跃年,热带印度洋低频对流一方面沿孟加拉湾西岸向西南-东北方向传播,激发了孟加拉湾西南季风ISO继续向云南传播;另一方面东传到南海以南的热带洋面并向南海北传,激发了南海夏季风ISO北传到副热带中国东部地区,再向云南西传,越过云南后与从东北方向传来的低频对流在孟加拉湾以北地区交汇,完成了一个经纬向接力传播的周期。因此,正是热带印度洋ISO通过两条路径对南海夏季风ISO和孟加拉湾西南季风ISO的激发,使得东亚夏季风和南亚夏季风这两个亚洲季风系统共同作用于云南主汛期ISO。

Using the precipitation data at 124 stations of Yunnan and OLR data from NCAR/NCEP, the ac- tivity features and annual differences of Intra-Seasonal Oscillation （ISO） during main rainy season （June- August） in Yunan, and two paths along which tropic Indian ISO propagates to Yunnan and the impact of two Asian Summer Monsoon systems to ISO during main rainy season have been analyzed. The precipitati- on in main rainy season of Yunnan has prominent negative correlation with OLR averaged in Yunnan re- gion. Summer monsoon ISO in Yunnan which is described as Low Frequency （LF） （30~60 d） OLR shows obvious intra-annual differences. In the stronger IS（） years of main rainy season in Yunnan, LF convection propagates along two paths： One is strong westward propagation from tropic western Pacific ocean or southeastern China, which causing three times active LF convections during main rainy season of Yunnan; Another is propagation along southwest-northeast direction from tropic Indian ocean along the western bank of Bay of Bengal （BOB）, which strengthens LF convection activities during main rainy season in Yunnan when it arrives in Yunnan. Otherwise, in the weak ISO years of main rainy season in Yunnan, just two times weak LF convections happen, which caused by two times weak zonal propagation of I.F OLR from southern subtropical region： The first is westward propagation from subtropical western Pacific ocean, and the second is eastward propagation from Northern Arabian Sea. In the stronger ISO years, in developing process of tropic Indian LF convection, it propagates along southwest-northeast directions a- long the western bank of BOB, and excites the southwest Monsoon in BOB to propagates to Yunnan con- tinually. On the another hand, it propagates eastward from BOB to South China Sea, and excites SCS Summer Monsoon （SCSSM） to propagate to subtropical eastern China and then turn to westward to Yun- nan. When passing Yunnan, in northern BOB it meets LF convection that propagates northeastward from western bank of BOB, accomplishing a whole circle of relay propagation along zonal and meridional direc- tions. Therefore, that is the excitation of ISO in tropic Indian to ISO in SCSSM and ISO in BOB southwest monsoon respectively along two paths, causes eastern Asian summer monsoon and southern Asian summer monsoon, these two Asian Monsoon systems impact together to ISO in main rainy season of Yunnan.