Climatic Features of Intraseasonal Oscillations of Summer Rainfalls over Mid-lower Reaches of the Yangtze River in the Flood and Drought Years

In order to understand further climatic characters of the intraseasonal oscillation and its relationship with the interannual variation of summer rainfalls over the mid-lower Reaches of the Yangtze River, the composite comparison analysis is applied to the intraseasonal oscillation in between the flood and drought years.Firstly, eight flood years and 7 drought years are identified by using 740 stations daily rainfall datasets in China from 1951 to 2004.And then differences in the period, the intensity and the phase cycle of the intraseasonal oscillation of summer rainfalls in between the drought years and the flood years over the mid-lower Reaches of the Yangtze River are discussed.The main results are as follows.The period of the intraseasonal oscillation of summer rainfalls over the mid-lower Reaches of the Yangtze River is relatively longer in the flood years, with the dominating 30 —60-day oscillation.While it is relatively shorter against a droughty interannual background, mainly being 10—30-day.Generally, the 10—30-day oscillation has greater amplitude than the 30—60-day oscillation in both flood and drought years, but the situation is opposite in certain areas over the lower Reaches of the Yangtze River or in some years.As for the intraseasonal oscillation, the amplitude is greater in the flood years than in the drought years.In the drought years, significant northward propagation can be observed to reach 50°N or so.But in the flood years, both the northward propagation from the low latitude and the weak southward propagation from the mid-high latitudes are obvious.And two branches of intraseasonal oscillations merge over the mid-lower Reaches of the Yangtze River and form strong oscillation centers.The phase composite analysis of the intraseasonal components of summer rainfalls in China and the corresponding low-level circulation show that the modes of the low-frequent anomalous circulation influencing the low-frequent summer rainfalls in China are similar in flood and drought years.However, the anomalous rainfalls and circulations on the intraseasonal scale are stronger under the flood background than the drought background, being an important cause for more rainfalls in flood years than in drought years.In conclusion, the intraseasonal oscillation has an important impact on the annual variation in aspects such as the period, the propagation, the intensity, the phase cycle and so on.