By using Wavelet analysis and Lanczos filter, a study is performed of the influences of low frequency oscillation of Monsoon on the torrential rain from the landing typhoon Bilis based on the dataset of CMA-STI Tropical Cyclone Optimal Tracks, NCEP/NCAR reanalysis and intensive surface observations. Results show that before the Bilis's landfall, 2006 East Asian low frequency southwesterly experienced remarkable northward propagation. After its landing, the strongest low frequency southwesterly and powerful low frequency vapor convergence were just on the south side of the Bilis, resulting in sharply increased rainfall. In addition, a broad belt of high-valued vapor fluxes extended from the eastern Arabian Sea via the Bay of Bengal, Indo-China Peninsula and the South-China Sea into South China. The belt was linked with the low frequency southwesterly forming a moist tongue stretching from the Bay of Bengal to South China, which supplied continuously abundant vapor for the Bilis along with the surge propagating poleward, allowing its long stay over land instead of erosion and disappearance.

Using the dataset of CMA-STI Tropical Cyclone Optimal Tracks,NCEP/ NCAR reanalysis,and the intensive surface observations,the large scale circulation features of the maintenance and increase in torrential rains from the landing typhoon Durian (0103) were studied. The results showed that Durian,after landing,was just located in the low pressure zone,forming a pattern of north trough and south vortex with a strongly developing mid-latitude westerly trough,which blocked the strengthening and westward extension of western Pacific subtropical high,allowing Durian's cyclonic circulation to be maintained. The northwest flow behind the trough transported the cold air from the mid-latitude to the low-latitude and converged southeast warm moist air in Durian's northern periphery,leading to the occurrence of heavy precipitation. In addition,the Somalia and 100°E cross-equatorial flow increased significantly with the result that the southwest monsoon was enhanced and a whole band of high-valued vapor fluxes stretched from the eastern Arabian Sea via the Bay of Bengal,central and southern Indo-China Peninsula and the northern South China Sea into Guangxi,which transported abundant vapor into northeast of Durian,making for the increasing of precipitation during this period. When in an extremely active stage of the 10-20 day intraseasonal oscillation of low latitude summer monsoon,low-frequency west winds and vapor northward spread to the south of Guangxi,thus responsible for convergence at lower levels and providing plentiful vapor,which played an important role in the sharp increase in rainfall from Durian.

By using the dataset of CMA-STI Tropical Cyclone Optimal Tracks, NCEP/NCAR reanalysis and intensive surface observations, a study is performed of the influences of a low-latitude monsoon surge on the longer persistence and increase in torrential rains from the landing tropical storm Bilis. Results suggest that the southwest monsoon was anomalously active after Bilis came ashore. The westerly winds in Bilis’s south side might give rise to the poleward movement of the SW monsoon, thus enlarging the pressure gradient between Bilis and the anticyclonic circulation to the south with the result of greatly intensified SW monsoon, which fueled plentiful water vapor, heat and momentum into the declining Bilis and allowed its long stay over land instead of erosion and disappearance. Before Bilis’s landfall, the 2006 East Asian monsoon surge, characterized by the atmospheric ISO, experienced remarkable northward propagation. After landfall, the strong surge and powerful low frequency vapor convergence were just on the south side of Bilis, resulting in sharply increased rainfall. In addition, a broad belt of high-valued vapor fluxes extended from the eastern Arabian Sea via the Bay of Bengal, Indochina Peninsula and the South China Sea into the south of China. The belt was linked with the SW monsoon surge forming a moist tongue stretching from the Bay of Bengal to the south of China, which supplied continuously abundant vapor for Bilis along with the surge propagating poleward.