Relationship Between Northward Propagation Of Tropical Intraseasonal Oscillation In East Asian Monsoon Region And Summer Rainfall Over Southern China

Southern China is located in east Asian monsoon region. In summer, the precipitation of southern China area is more than normal, many flood multi-disasters occur and it has done serious harm to national economy and people’s life. So there is an important significance in studying and precisely predicting the change law and the influencing factors on summer rainfall over southern China. Many studies show that the summer rainfall in China has an obvious low-frequency character, and the influencing factors on summer rainfall in China are complicated. In recent years, how the tropical intraseasonal oscillation(ISO) influence rainfall in China has become a hot research topic.Based on previous research, using the daily rainfall data of stations in China and the NCEP/NCAR reanalysis data from 1981—2011 to investigate the basic statistical facts between BSISO and rainfall in southern China, and to study the relationship between summer low-frequency rainfall regime over southern China and ISO in atmosphere by examining systematically the propagation features. The key research is the effect of northward propagation of the tropical ISO in east Asian monsoon region on the rainfall in southern China and the distribution of circulations. Further more, to make an primary investigation of the concrete features and influencing factors in northward propagation of ISO.The main conclusions are summarized as follows:(1) In China, the fractional variance of summer low-frequency rainfall by the tropical ISO signals has the highest value 55%~60% in Yangtze River region and south China region.It is demonstrated that ISO from low-latitude area plays important roles in influencing the low-frequency change of summer rainfall in southern China.(2) In summer, there is a close linkage between ISO signals avtive area and the low-frequency rainfall in southern China. For the ISO signals from equatorial Indian Ocean region, when ISO signals propagate from Indian Ocean to Maritime Continent, it is beneficial to more precipitation over Southern China. When ISO signals propagate from Bay of Bengal to West Pacific region, it will lead to less precipitation in Southern China. When ISO signals propagate northwestward from West Pacific region and haven’t arrived in China, the rainfallin Southern China is less than normal. The negative rainfall anomaly center is located in South China. When the ISO signals arrive in China, the rainfall in Southern China is more than normal. ISO signals activity will influence the frequency of low-frequency rainfall in most area of Southern China, with about 10% more than the climatology frequency.(3) In summer, there are five kinds of large scale low-frequency rainfall regime. And the moist ISO signals influence the low-frequency rainfall events in different regions of southern China, mainly propagate northwestward from the tropical marine to the southeast of China.The southeast China rainfall regime is intimately associated with the moist ISO signals propagating northwestward from the equatorial mid-western Pacific Ocean. For both the Yangtze River regime and South of Yangtze River regime, the moist ISO signals over the northern South China Sea. For the South China regime, the moist ISO signals show a significant northward propagation from the Philippines. The moist ISO signals corresponding to all kinds of rainfall regimes show a character of propagating northwestward. ISO signals’ strength enhanced during propagation and finally reach different regimes in southern China. It is demonstrated that there is a close linkage between the five rainfall regimes over southern China and the northward propagation of the tropical ISO. It is further found that the interaction between the northward propagation of low-latitude ISO signals and the southward propagation of high-latitude ISO signals and the latitude ISO signal arriving can also make a clear influence on the low-frequency rainfall in southern China.(4) In summer, ISO signals in east Asian monsoon region show a significant character of northward propagation from low latitude, and there is a significant impact on low-frequency rainfall anomaly distribution in southern China when ISO signals propagate northward to different latitudes.Identify ISO northward propagation events along 115°E profile and pick out two typical kinds of events: far north boundary events and far south boundary events. The propagation characters and the precipitation distribution in southern China show a significant difference between the two kinds of events. For far north boundary events, ISO signals propagate northwestward from Philippines and west-Pacific region to 25°N, the signals show a significant north propagation, and during propagation, the precipitation anomaly in south China has changed from negative to positive, the rain belts show an northward extension. For far south boundary events, ISO signals from equatorial northern Indian Ocean show slowly propagation rate, the northward propagation is weak and ISO signals located in south.In this case, Philippines and south of Yangtze River show active convection. During propagation the rain belts show a southern extension from Yangtze River region.(5) A preliminary study on influencing factors of ISO singals’ northward propagation shows obvious differences in circulation and vertical shear of basic flow between two kinds of northward propagation events. When ISO signals show a significant northward propagationand reach more northward latitudes, the South Asia High is located more westward and the Subtropical High is more eastward. In South China Sea, the height fields show negative anomaly beneath 500 hPa. The ISO signals’ northward propagation will be prevented when the westward extension of Subtropical High reach to China. ISO signal will be located in South China Sea and barely propagation northward. Also, the vertical shear of basic flow in Asia monsoon area is a factor in influencing ISO signals’ northward propagation. By analyzing the difference of wind shear between two kinds of events, it is demonstrated that strong easterly wind shear in South Asia Monsoon area and strong northern wind shear in South China Sea and Philippines is beneficial to the northward propagation of ISO.Otherwise, it is adverse to the northward propagation.This work show some facts of the relationship between the propagation of ISO signals and the summer rainfall in southern China, and do some analysis on the factors and the characteristics on influencing northward propagation of ISO. All of the research was a foundation for further studies on about the mechanism of the meridional propagation influence summer rainfall in southern China and the mechanism of meridional propagation of ISO.