Climatology And Inter-annual Variation Of The Sichuan Basin Vortex

The Sichuan Basin Vortex is a kind of Southwest Vortex that appears frequently,its generation and development can bring disastrous heavy rainfalls to Sichuan Basin and the middle and lower reaches of Yangtze river.Using the 6-hourly,0.25°×0.25° ERA-Interim reanalysis data during 1979-2016,all Sichuan Basin Vortexes are detected and analyzed.The results are as follows:(1)The geneses of Sichuan Basin Vortex are mainly concentrated in the southern part of Sichuan Basin and the southern foot of Dabie mountain.The averaged size is 392.74 km,longevity is shorter than a day,migration rate is 18.88%.It is most frequent in June and strongest in July,and it is more frequent at night than in daytime slightly.It can be divided into5 categories according to its movement,including stationary type,eastward migrating type,northeastward migrating type,southeastward migrating type and stationary type.The generation of eastward-migrating,southeastward-migrating and stationary type of Sichuan Basin Vortexes peak in June,while the other types peak in July.(2)Structure and precipitation of 5 different types of long-lived Sichuan Basin Vortexes are discussed by composite analysis,the result are as follows:About horizontal structure,the shape of the vortex on 700 hPa is a circular or an ellipse with a northeast-southwest extended long axis.About vertical structure,in maturation stage,apart from westward-migrating type,vortexes of other categories are cold on lower levels and warm on higher levels with a northwest or west tilt.Eastward-migrating,northeastward-migrating and westward-migrating types can reach higher levels than the others.The strongest ascent motion of southeastward-migrating and westward-migrating types is coincide with the vorticity center and it is located at the east side of the vortex for the others.When it comes to precipitation,except for the westward-migrating type,6 h-precipitation center is located at the east or northeast side of the vortex.And 6 h-precipitation of northeastward-migrating type is the strongest among all types.There is a good correspondence between the strong ascent motion area,the area with large relative humidity,the convergence and divergence centers of the upper and lower layers and the location of precipitation for the Sichuan Basin Vortex.Theinteraction of various physical quantity fields jointly promotes the development of the vortex.(3)Inter-annual variability: Annually,there are 53 Sichuan Basin Vortexes.From1979-2016,the number of vortexes in winter and the number of vortexes first appeared at night has gradually increased.From 1985-2016,the number of vortexes that can move out of the target area has gradually increased as well.In summer,the number of Sichuan Basin Vortex is positively correlated with the precipitation from southeast Tibetan Plateau to lower reaches of Yangtze river,and it is negatively correlated with the temperature in Sichuan Basin and lower reaches of Yangtze river.When the geneses of vortexes is high,there will be more precipitation,lower temperature,lower sensible and latent heat flux.In summers with more geneses of Sichuan Basin Vortex,east Asia-western Pacific region have a geopotential height anomaly with a ‘+-+' pattern,a stronger subtropical high,a stronger westerlies between 23°N and 30°N,which can reinforce the air flow around the west side of the Tibetan plateau and therefore foster the formation of Sichuan Basin Vortexes.