Analysis Of The Relationship Between The Eastward Movement Of The Plateau Vortex And The Southwest Vortex And The East Asian Summer Monsoon

The eastward-moving type of plateau vortex(plateau vortex,southwest vortex)is an important synoptic system that affects the strong transition synoptic processes in the eastern Tibetan Plateau.Meanwhile,the onset of the East Asian summer monsoon is characterized by abrupt changes in the wind field and a sudden increase in precipitation.This thesis focuses on the vertical coupling characteristics and physical processes of the plateau vortex and southwest vortex at different levels in various stages of their developments.The daily ERA-interim reanalysis data at 00:00,06:00,12:00,and 18:00,on June 1-5 from 1979 to 2017 are employed to conduct comparative analyses between the years with and without the eastward-moving type of plateau vortex in terms of the corresponding high-and low-level circulation systems,the spatial distribution of precipitation and the wind fields.Specific investigations on the selected cases were also conducted.The main research results and conclusions are as follows:(1)In the same short period,there are significant differences in terms of the high-and low-level circulation systems in the East Asia monsoon region between the years with and without the eastward-moving type of plateau vortex of long life-scale.Besides,circulation systems of plateau vortex with different eastward-moving paths are different.Geopotential height field: In the years when there exists eastward-moving plateau vortex,the large value area westerly jet is significantly eastward compared with the year without an eastward-moving plateau vortex;the latitudes of the centers of South Asian high are basically the same,and its centers with the northeast and southeast paths of the vortexes are shifted to the north and south respectively;the East Asian troughs in the years with the eastward-moving type of plateau vortex are more eastward than the years without.For the northeast,east,and southeast paths of plateau vortex,the west ridge points of the western Pacific subtropical high turn southward and eastward respectively.Water vapor conditions:For the year with the eastward-moving type of plateau vortex,there is a wide water vapor belt from the southeast of Lake Baikal to the northeast of China.Sea-level pressure field: There is weak land-sea pressure contrast in the years with the eastward-moving type of plateau vortex,and the sea level pressure gradually decreases as the plateau vortex moves eastward.(2)The possible impact of the eastward-moving plateau vortex on the East Asian summer monsoon region is also discussed.Precipitation field: In the year with the eastward-moving type of plateau vortex,precipitation is enhanced in the middle and lower reaches of the Yangtze River.Moreover,the plateau vortexes with different eastward paths are in clear agreement with the precipitation areas in the east of the plateau.Compared with the plateau vortex of the northeast path,the precipitation area is northerly than the plateau vortex with the eastward path.Compared with the plateau vortex of the southeast path,the precipitation area is southerly than the plateau vortex with the eastward path.Wind field: The eastward movement of the plateau vortex will affect the low-level wind field of the East Asian summer monsoon region in its active area.In the year with the eastward-moving type of plateau vortex,the averaged wind speed is higher in the plateau vortex activity area with stronger disturbance of lower-layer airflow than the year without the eastward-moving the plateau vortex.The850 h Pa wind fields with different eastward paths of plateau vortexes are also significantly different.The eastward path of plateau vortex is accompanied by a smaller average wind speed on the east side of the Tibetan Plateau,and the southerly and northerly airflows are weak.The northeastward path of plateau vortex is accompanied by strong southerly airflows in most parts of eastern China.The southeastward path of plateau vortex is accompanied by strong northerly airflows in most parts of southeastern China.(3)The interactions between the eastward-moving plateau vortex and the East Asian summer monsoon were studied based on case analysis and historical statistics.It shows that the eastward-moving type of plateau low vortex can trigger the enhancement of precipitation and low-level jet in the East Asian summer monsoon region.In a case analysis of the eastward-moving type of plateau vortex on June 1-5in 1989,a plateau vortex was generated and moved eastward out of the plateau at500 h Pa.At 700 h Pa,it induced a southwest vortex and the two vortices moved eastward together.Dynamic and thermodynamic conditions: There is a large value area of positive vorticity on the east side of the plateau vortex with upward vertical velocity.The divergence field is manifested as the configuration of low-level convergence and high-level divergence.The eastward movement of the plateau vortex provides favorable conditions for precipitation.The water vapor brought by the strong southerly airflow produces precipitation on the south side of the plateau vortex activity path.After precipitation,the latent heat of condensation heats the surface,and a low-pressure center appears on the ground as the plateau vortex moves eastward.The pressure gradient force is enhanced,and the average wind speed in the lower layer is significantly increased compared to the background field.Finally,the six eastward-moving plateau vortexes from June 1-5 in the same period of 1979-2017 were counted.It was found that 83.33% of the eastward-moving plateau vortexes were accompanied by positive anomalies in the low-level wind speed of the East Asian summer monsoon region.