The Characteristics Of The Low Frequency Oscillation Before The Persistent Heavy Rainfall Events Over Yangtze And Huaihe River Vallev

Based on the Persistent Heavy Rainfall Events (PHREs) with the "double-blocking-high" as its background over the Yangtze and Huaihe River Valley (YHRV) during 1975-2010, using the NCEP/NCAR Daily Re-analyze Index, OLR, MJO Index and the low-frequent data after the first level Butterworth method, the Low Frequent Oscillation (LFO) Features relates to the events are analyzed by the composite and compare methods. The results showed:the 30-60d LFO and 10-30d LFO over different troposphere and areas work together to influent the anomalous systems, which play significant roles to the happen and persistence of the PHREs.The PHREs are happened under the background of Stable circulation:the South Asia High (SAH) eastern move and the West Pacific Subtropical High (WPSH) western move, the southerly at the middle-low troposphere over the west of the WPSH and the northerly at the upper Troposphere over the southeast of the SAH meet at the South of China, which constitutes a stable continuous monsoon vertical circulation and then favor to the start of the PHREs; The stable maintenance of high latitude blocking high is one of the important conditions for the PHREs process to be sustained. The anomaly high over the north of East Asia and the anomaly low over the southeast of the Baikal constitutes the "dipole blocking", indicating the northerly along the trough of Baikal and the Southerly along the west of the WPSH persistently join at the YHRV. And the "+-+" anomaly high pressure wave distribution is the important conditions for the occurrence and duration of the PHREs.The Low Frequency Oscillation plays an important role for the PHREs, the 30-60d LFO related to the events are mainly over the low and middle troposphere at low or middle latitudes, whose main feature is eastern or northern propagation. While, the 10-30d LFO mainly at the middle and high latitudes over different troposphere and with southern propagation as its main features. The 30-60d LFO and 10-30d LFO work together and influenced the background of Stable circulation via influence the evolution of SAH, WPSH and the blocking highs:(1) At the middle and lower troposphere, the westward and the northward of the WPSH is affected by the 30～60d LFO from the middle and low latitudes and the 10～30d LFO from the high latitudes:With the 30～60d low frequency tropical convection disturbance spread from west to the West Tropical India ocean (MJO phase 8,1), an anomalous 30～60d anticyclone existed at the middle of the Tropical India Sea and then northwestern move to the Bay of Bengal, the low frequency easterly enhanced at the Marine-Continent Areas and the Philippines, a low frequent anticyclone over the eastern of Philippines westward move to the Northwestern Pacific and the Southeast Coast of China; and a 10～30d low frequency high over the Okhotsk Sea one week before the events southward propagated to the Southeast Coast of China also play a significant role to the happen of the events.(2) At the upper troposphere, the strength increase and the eastern move of SAH is mainly affected by adjustment of trough distribution and circulation on the westerly wind belt over the Eurasian in the Northern Hemisphere. Two weeks before the events, there are two trough at the eastern part of the Iran plateau and the eastern coast of China and stayed stable, the SAH northward strength and the eastern edge stayed around 110°E; One week before, the western trough turned week and disappeared, the wave over the middle latitudes disappeared too, while the SAH eastern propagated. The features are influenced by the 10～30d LFO over the middle-high latitudes at northern hemisphere. When the two trough stay stable over the west and the east, there are two LF cyclone over the Iran plateau and the eastern coast of China. When the troughs be weak and disappearing, the LF cyclone over the Iran plateau turn to the LF anticyclone, and the LF cyclone over the eastern coast of China western move and be weak, the SAH westward expansion and be zonal distribution; when the events are beginning, the LF anticyclone over the Iran plateau became weak and turn to be influenced by the LF cyclone, the west trough eastward propagated and stay near the Lake Balkhash, the LF anticyclone keep weaken and northward move, when the LF anticyclone southward moved from the Japan Sea and be the east of the eastern coast of China, the Trough at the east of China northward propagation and the SAH eastern move. The Low Frequency anticyclone or cyclone over the Iran plateau and the eastern coast of China can be the 2 important LF systems for the PHREs.(3) The Ural High and the Northeastern Asia High were influenced by the anomaly high to their west, and the Ural high is related to the downstream development effect of Rossby wave train in the middle and high latitudes of the northern hemisphere. About -9 days, an anomaly high on the West European 20°E eastern propagated to the Ural. The enhancement of the Ural high and the Northeastern Asia High have the 30-60d LF features:about 2 weeks ago, the 30-60d LF high over West European 20°E and the West of the Lake Baikal northeastward move to the Ural mountain and the north of East Asia, which are favor to the enhancement of the High Centers, and the Lake Baikal trough extending to its southeastern is related to the 10-30d LF low from the high latitudes. The north East Asia high with the low over the southeast of Baikal formed stable phase-locked structure, along the southwest low trough south of the north wind and the west of subtropical high southerly wind converge in the YH and favor to the persistence of the PHREs.In this study, we have a further understanding of the low frequency oscillation and its propagation characteristics before the PHREs, which is look forward to be the reference when do the extended period forecast of the PHREs over the YHRV.