The Relationship between the Atmospheric Low-Frequency Oscillation over Asia and the Flood in the Huaihe River Valley in the Summer of 2007

Floods and droughts often take place in the Changjiang－Huaihe basin. In the summer of 2007, the severe flood occurred again in the Huaihe River basin. In order to investigate the cause, using the NCEP/NCAR reanalysis data and observed daily rainfall from gauge stations, association of the atmospheric low-frequency oscillation (LFO) over Asia with the severe flood in the Huaihe River basin in the summer of 2007 has been studied in this paper. The result shows that: the low-frequency oscillation of rainfall over the Huaihe River basin in the summer of 2007 was mainly controlled by the 30－70-day oscillation, the variance of this time series can account for about 47% of the total variance, thereby, the rainfall 30－70-day low-frequency oscillation plays an important role in the summer flood of 2007; during the period from phases 5 to 8 (1 to 4) of the rainfall low-frequency oscillation (LFO), the Asia monsoon regions from the northern Arabian Sea to the South China Sea across the Bay of Bengal and from the Huaihe River basin in China to Japan were mainly controlled by the heating source (sink) of the〈Q1〉LFO; at extreme phase 7, the 〈Q1〉 LFO presents the mode distribution of heating sink, heating source, heating sink and heating source from the north to south over eastern China (10°N－45°N, 110°E－120°E), and it is quite the reverse at extreme phase 3; from phases 5 to 8 (1 to 4), the distribution of the 〈Q1〉 LFO over the Asia monsoon region is favorable (unfavorable) for the air flow convergence over the Huaihe River basin and the resulting ascent in this region; because of the change of the circulation low-frequency oscillation (LFO), during the period from phases 5 to 8, more moisture was transported to the Huaihe River basin, convergence and ascent lead to the strong rainfall in this region; reversely, from phases 1 to 4, the divergence of moisture from the western Pacific took place in the Huaihe River basin, which makes against the rainfall occurring in this region.