西南雨季降水异常大气环流特征

Characteristics of Abnormal Atmospheric Circulation of Rainy Season Precipitation in Southwest China

摘要: 为了研究造成西南雨季降水异常的大气环流特征，利用西南地区81个气象站1960~2022年的逐日气象降水量观测数据和同期NCEP/NCAR逐月再分析数据集，水平分辨率为2.5˚ × 2.5˚，运用合成分析和显著性检验等现代气候统计诊断分析方法对西南地区63年来雨季降水异常的高低层大气环流场、其他物理量场以及水汽输送场进行了研究，结果表明：1) 西南雨季降水空间分布不均，雨季降水三个大值区，分别位于四川省中部雅安、峨眉山和乐山一带，云南南部地区以及云南西部地区，且多雨区和少雨区呈交替分布。西南雨季降水长期变化趋势呈微弱递减的趋势，且存在明显的年际变化特征，总的来说降水偏多年主要集中在70年代以前，偏少年集中在2005年以后，西南地区降水有一定的年代际变化特征。2) 多雨年南亚高压较常年偏弱，南支槽活跃，贝加尔湖高压增强，整体为经向环流，西南地区产生显著的西南风异常，有利于对西南地区的水汽输送，有利于降水。少雨年南亚高压较往年偏强，高压中心偏东，整体为纬向环流，中南半岛西北风异常风场形势不利于孟加拉湾水汽向西南地区的输送，会导致降水偏少。西南雨季的垂直运动具有区域性，多雨年在云南西南部和四川东部垂直运动较强。少雨年在四川东部垂直运动较强。3) 多雨年孟加拉湾中部存在一气旋性距平水汽输送，其南侧有显著的西南向水汽通量输送带，说明孟加拉湾南部对西南地区的水汽输送加强。在两广和湖南一带有反气旋性距平水汽输送，南侧存在东南向水汽通量距平，表明来自南海的水汽向贵州、重庆及四川东部输送加强。西南地区多雨年的水汽通量散度距平从南向北表现为“辐合–辐散–辐合”的空间分布，水汽汇集的大值区位于四川东部和云南的南部。少雨年云南存在气旋性水汽通量距平，印度半岛东部有反气旋性水汽通量距平，前者后部和后者前部的北向水汽通量距平在中南半岛汇合，减弱了孟加拉湾北部对西南地区的输送。从孟加拉湾南部来的西南向水汽通量距平在中南半岛转为自西向东的水汽通量距平，不利于水汽向西南地区输送。少雨年西南地区水汽汇集的主要地带分别位于四川东部、重庆一带和云南的东部和南部。在云南西部和贵州的水汽通量散度距平很小，虽有水汽的汇集和辐散，但造成当地水汽变化不是很明显。多、少雨年的水汽辐合区均在云南中部和南部、四川东部，但多雨年在这两地水汽辐合强度远远大于少雨年。

Abstract: In order to study the characteristics of atmospheric circulation that cause precipitation anomalies in the rainy season in Southwest China, daily meteorological precipitation observation data from 81 meteorological stations in Southwest China during 1960⁓2022 and monthly NCEP/NCAR reanalysis data set of the same period were used, with a horizontal resolution of 2.5˚ × 2.5˚. By using the modern climate statistical diagnostic analysis methods such as synthesis analysis and significance test, the high and low layer atmospheric circulation field, other physical quantity field and water vapor transport field of the rainy season precipitation anomaly in Southwest China in the past 63 years are studied. The results show that: 1) The spatial distribution of rainy season precipitation is uneven in southwest China. The three regions with high rainfall value are located in Ya’an, Mount Emei and Leshan in central Sichuan Province, southern Yunnan and western Yunnan respectively, and the regions with heavy rainfall and low rainfall are alternately distributed. The long-term variation trend of rainy season precipitation in southwest China showed a weak decreasing trend, and there were obvious inter-annual variation characteristics. In general, the precipitation was mainly concentrated before 1970s, and the precipitation was mainly concentrated after 2005. The precipitation in Southwest China had certain inter-decadal variation characteristics. 2) In rainy years, the South Asian high is weaker than the usual, the south branch trough is active, the Baikal high is strengthened, and the overall meridonal circulation, the southwest region produces a significant southwest wind anomaly, which is conducive to water vapor transport to the southwest region and conducive to precipitation. In the year of low rainfall, the South Asian high pressure is stronger than in previous years, the high pressure center is eastward, and the overall zonal circulation is present. The abnormal wind field situation of northwest wind over Indochina Peninsula is not conducive to the transport of water vapor from the Bay of Bengal to the southwest, resulting in less precipitation. The vertical movement of rainy season in southwest China is regional, and the vertical movement is stronger in southwest Yunnan and eastern Sichuan in rainy years. The vertical movement is stronger in the eastern part of Sichuan in the year of low rainfall. 3) There is a cyclonic anomaly water vapor transport in the middle of the Bay of Bengal in rainy years, and there is a significant southwest water vapor flux conveyor belt In the south side, indicating that the water vapor transport from the south of the Bay of Bengal to the southwest is strengthened. There is an anticyclonic anomaly of water vapor transport in Guangxi, Guangdong and Hunan, and a southeastward anomaly of water vapor flux in the south, indicating that water vapor from the South China Sea is transported to Guizhou, Chongqing and eastern Sichuan. The divergence anomaly of water vapor flux in rainy years in southwest China shows the spatial distribution of “convergence-divergence-convergence” from south to north, and the large value area of water vapor convergence is located in eastern Sichuan and southern Yunnan. There are cyclonic water vapor flux anomalies in Yunnan and anticyclonic water vapor flux anomalies in the eastern part of the Indian Peninsula. The northward water vapor flux anomalies in the rear part of the former and the front part of the latter converge, which weakens the transport of water vapor from the northern part of the Bay of Bengal to the southwest. The southwesterly water vapor flux anomaly from the south of the Bay of Bengal turns to the west-east water vapor flux anomaly in Indochina Peninsula, which is not conducive to the transport of water vapor to the southwest. The main areas of water vapor accumulation in southwest China are located in eastern Sichuan, Chongqing and eastern and southern Yunnan respectively. In western Yunnan and Guizhou, the divergence anomaly of water vapor flux is very small, although there is water vapor convergence and divergence, but the local water vapor change is not obvious. The convergence areas of water vapor in both rainy and rainy years are in central and southern Yunnan and eastern Sichuan, but the convergence intensity of water vapor in rainy years is much greater than that in rainy years.