LOW FREQUENCY OSCILLATION IN EAST ASIA DURING THE 1991 EXCESSIVELY HEAVY RAIN OVER THE CHANGJIANG-HUAIHE RIVER BASIN

The activity of low frequency oscillation(LFO) widely exists in East Asia during the period of 1991 excessively heavy rain over the Changjiang-Huaihe River Basin(Jianghuai).Both the rainfall amount of Jianghuai and the atmosphere from subtropical area to mid-high latitudes have the dominant period of 10-20 d,while the atmospheres in tropical area and high latitudes have the dominant period of 30-60 d.Compared with normal Meiyu season,the anomaly of the 1991 Meiyu process may be reflected in the following two low frequency synoptic events:(a) The Meiyu process onsets extremely early(in the second dekad of May,which is nearly one month earlier than in normal Meiyu) and is immediately followed by the first episode of heavy rain.(b) In the first dekad of July,there occurs the heaviest episode of rainfall of the whole Meiyu season,and it is even the heaviest rainfall for the recent 30 years in China.For these two periods,corresponding to the adjustment of large-scales ituation from "double blocking high" to "bipolar blocking high",the propagation direction of LFO in East Asia has a distinct seasonal variation,from eastward/northward propagation(passing through Jianghuai) during the first episode to westward/southward propagation during the thirdepisode.Oscillations of different frequency bands are superposed in phase.The LFO activity of the cold and warm/moist airs over Jianghuai can be strengthened through those LFO propagation processes in East Asia,although they may have different directions in three episodes.Particularly,the eastward(westward) propagation in low latitudes makes the southwest(southeast) airflow transport intensively the low frequency warm/moist air to the south of Jianghuai from the Indian Ocean(tropical West Pacific Ocean) in the first(third) episode.Such warm/moist airs meet and interact with the cold air which vigorously invades Jianghuai persistently,and finally three episodes of heavy rain occur in mode of LFO.