| Based on 753-station the daily rainfall data provided by the National Meteorological Information Center and the NCEP/NCAR daily reanalysis data from 1961-2013, and HYSPLIT model synchronous driving data from NOAA, the low-frequency characteristics on the precipitation of the typical drought and flood years in the first rain season of South China are analyzed, and their relationships with atmospheric low frequency circulation elements, which include water vapor flux field, OLR field, height field, vertical speed and wind field, are investigated. Then, their low frequency signal sources and propagations are discussed and compared. The main results are as follows:(1)During the pre-rainy season over South China, precipitation in typical flood years have 10～20 days (significantly) and 30～60 days of low-frequency oscillation that show regional differences. Low frequency precipitation at Guangdong and Guangxi area is 10～20 days mainly, but there are two significant low-frequency cycle at 10～20 days and 30～60 days in Fujian and Jiangxi provinces. In contradistinction to the typical flood years, low-frequency oscillation cycle of the drought years precipitation is 10～20 days mainly, and regional difference is not obvious.(2)In typical flood years, the 10-20d low-frequency water vapor transport circulation systems mainly have Mascarene circulation, Somali cross equatorial moisture flow, Arabian Sea circulation couple, New Guinea circulation, the Northwest Pacific circulation and the Caspian Sea circulation. Correspondingly, in typical drought years, the 10～20d low-frequency water vapor transport circulation systems mainly have Mascarene circulation, Somali cross equatorial moisture flow, central equatorial Indian Ocean circulation, the Philippines circulation, the Western Pacific circulation, the Caspian Sea circulation and the Baikal lake circulation.(3)In typical flood years,10～20d low-frequency moisture transport channels and sources have the southwest channel in which water vapor signal reference source is northern Madagascar and the middle equatorial Indian Ocean, in which simulating source is the middle equatorial Indian Ocean; the southeast channel in which reference source is Japanese islands to the southeast of the ocean and the middle equatorial Pacific Ocean, in which simulating source is near the Philippines; the northwest channel in which reference source is northern to the Caspian Sea or Lake Baikal on the southeast side, in which Simulating source is the northern of the Caspian Sea or the east of the Aral Sea; the northeast channel in which reference source is the Bering Sea, in which Simulating source is the Yellow Sea.In typical drought years,10～20d low-frequency moisture transport channels and sources have the southwest channel in which water vapor signal reference source is South Arabian Sea near Somalia and the middle equatorial Indian Ocean, in which simulating source is the middle equatorial Indian Ocean; the partial Southern channel in which signal reference source is Sea southeast of the Philippines, in which simulating source is the South China Sea; the Northwest channel in which cold air signal reference source is northern to the Caspian Sea or Lake Baikal on the southeast side, in which simulating source is the northern part of the Caspian Sea or Balkhash Lake; the Northeast Channel in which reference source is the Okhotsk Sea, in which simulating source does not exist.(4)In typical flood years, sources of 10-20d low-frequency convection key area and channels have the Southwest channel in which the key area is the low-frequency convection zone of the Indian Ocean equatorial flow direction and the Southeast channel in which the key area is northeastern Australia low frequency convection zone. Besides, there is the partial East channel in which the key area is the low-frequency convection zone in the Western Pacific to Eastern side of the Taiwan Island in the typical drought years.(5)In typical flood years, the height field situation shows two ridges with a groove in the middle and high latitudes, at which subtropical high ridge line locates nearby 15°N and the point West ridge extends east of Bay of Bengal. Correspondingly, in typical drought year, the height field situation shows two grooves with a ridge, at which subtropical high ridge line locates nearby 20°N and the point ridge locates in the eastern Philippines.At the 10-20d low-frequency height field, when South China is under the control of negative anomaly low-frequency height field, low-frequency precipitation increases.(6)When low-frequency precipitation is increasing, South China is under the control of strong 10-20d low-frequency updraft. At the same time, little of very weak low-frequency vertical speed exists over the South China Sea and the north of the Yangtze is under the control of significant low-frequency downdraft in typical flood years. Correspondingly, in typical drought year, the South China Sea is under the control of significant low-frequency downdraft and low-frequency weak downdraft exists in the north of the Yangtze.(7)10-20d low-frequency baroclinic disturbancing wave trains show the downstream dispersion characteristics. In typical flood years, there are two disturbance propagation paths. One axis locates between 30～40°N and the other appears the northwest-southeast direction from northern Europe to South China. Low-frequency baroclinic wave packets distribute between 5～25°N by zonal strip and energy disperse downstream. When low-frequency baroclinic wave packets center lie in South China, the low-frequency precipitation increasesin the first rain season. In typical flood years, there is only one disturbance propagation path that the axis locates between 30～45°N. Low-frequency baroclinic wave packets distribute between 10～40°N.There are two propagation paths.The energy of one path is scattered from the upstream of South China, and the other’s is from the Western Pacific propagating westward.(8)In the typical drought and flood years in the pre-rainy season over South China, when the low-frequency moisture convergences, the low-frequency convection enhances, the low-frequency negative anomaly height field strengthens, the low-frequency updraft enhances, and the center of the low-frequency upstream baroclinic wave packets transmits to South China, the low-frequency precipitation increases. |
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