The Intraseasonal Oscillation In East Asian Summer Monsoon And Its Relationship Between The Flood And Drought Of Yangtze Valley

Intraseasonal oscillation (hereafter ISO) has been an important issue since the 1970s.The overall goal of the research is to promote a better understanding of the ISO inEast Asian Summer Monsoon (hereafter EASM) in terms of its structure andmigration,as well as interannual variations. The relationships between the ISO ofEASM and the precipitation of the Yangtze valley are explored too.With the daily NCEP/NCAR reanalysis datasets (2.5°*2.5°grid) and the precipitationdatasets of 730 weather stations in China,the climatology of the ISO in the EASMand its relationship with the summer precipitation of the Yangtze valley are studied.The role of the atmospheric circulation in the ISO is discussed too. The waveletanalysis is used to identify the ISO period; the LANCZOS filter is used to acquire theintraseasonal time series. The results show that,there are two kinds of ISO in EastAsian summer monsoon,10-20-day ISO and 30-60-day ISO. Their contributions tothe overall variations differ among different years. The 10-20-day mode years and the30-60-mode years are selected according the importance of the influence. Theclimatology of two ISOs are analyzed and contrasted to each other with the compositeanalysis of the selected typical ISO cycles.For support the conclusion of the climatology and for illuminate the relation betweenISOs and the precipitation of the Yangtze valley,the chosen flood and drought yearsof the different ISO mode years are analyzed. The paper widened and sharpened theunderstanding of the East Asian summer monsoon ISO. The main conclusions arepresented as follows: (1) The wavelet analysis showed that there are two kinds of ISO in EASM,10-20-day ISO and 30-60-day ISO. The 10-20-day ISO,the 30-60 day ISO,and theseasonal cycle may each become dominate in different years. According to theexplain variations,the years from 1979-2005 are divided into 3 catalogs whichare 30-60 mode years,10-20 mode years and the other years. It is obviouslydifferent ISO modes are not correspond with the precipitation of the Yangtzevalley while check the ISO modes of the Yangtze valley typical flood and droughtyears. There are flood years and drought years in 30-60 mode years,so it is truewith the 10-20 mode years. The most flood and drought years in differentintraseasonal mode years are selected to do the case study. 1988 and 1998 are thedrought and flooding years for 30-60 mode years, while 1981 and 1996 are thedrought and flooding years for 10-20 mode years.(2) The ISO cycles are divided into phases in order to do the composite analysis. Theresult show that the ISO in low level is presented as the anomalyanticyclones/cyclones emerge by turns in the north South China Sea and westPacific. The anomaly anticyclones/cyclones migrate meridionally (10-20 mode) orzonaly (30-60 mode) along southeast China regulating the west Pacific SubtropicalHigh entering or quitting South China Sea. The circulation in low level couplingthe circulation in high level regulate the south Asia High east-extending or weakenwest-moved for active or break of the precipitation of Yangtze valley. 30-60 modeis a joint action of the ISOs of Pacific at north 30, Northern air flow and the ISOsin low latitudes. The 10-20 day ISO is the northwestern-propagating of the tropicaldisturbance.(3) 30-60 mode reflects the establishment and the dying of Mei-yu front. The mostdistinct property of the 30-60 mode is described as follows. The northwestwardmigratingintraseasonal cyclones (anticyclones) which origin from east ofPhilippine converge on the southwestward-migrating intraseasonal anticyclones,and then both west migrated to dominate the Yangtze valley. 30-60 day ISO is therespond of the intraseasonal variation of the west Pacific Subtropical High and theintraseasonal variation of the North air flow.(4) 10-20 mode reflects the activity in low latitude areas especially the propagatingtropical west Pacific summer monsoon ISO. The most distinct property of the 10-20 mode is the northward-migrating intraseasonal anticyclones/cyclones which (5) Examine the two ISOs of summer of 1998. Low-frequency cyclones which originfrom east of Philippine converge on the southwestward-migrating intraseasonalanticyclones,and then both west migrated to dominate the Yangtze valley in activepeak phase. In 1988, low-frequency cyclones in active peak phase are from midlatitude.The 2nd ISO in 1998 are as the same as the 1st one. Two high precipitation periodsmake 1998 a Yangtze valley flooding year. In the 2nd ISO, there are no lowfrequencysystems propagated from west pacific, plus the obvious activity in midlatitudemade the west Pacific Subtropical High move north, a situation against theprecipitation in Yangtze Valley.(6) North-propagating low-frequency cyclones which are origin from tropical areasdominate the active peak phases in the 2nd and 3rd ISO in 1996. In the meanwhile,the pattern in the mid-latitude and high-latitude area is favorable for the north airflow propagates south. Both two reasons make 1996 a Yangtze valley flooding year.The sorth-propagating low-frequency cyclones which are origin from mid-latitudeareas dominate the active peak phases in the 2nd and 3rd ISO in 1981. In themeanwhile, the pattern in the mid-latitude and high-latitude area is not favorablefor the north air flow propagates south. Both two reasons make 1981 a Yangtzevalley drought year.