A possible cause of the low frequency circulation of summer-time drought-fiood abrupt alternation over the Huaihe River basin

The daily rainfall data in April-August during 1960-2007 are used to select drought-flood abrupt alternation events of the Huaihe river basin, and the characteristics of the low frequency oscillation about the summertime daily rainfall of drought-flood abrupt alternation are analyzed.The results are as follow; the 30-60 d oscillation about the summertime daily rainfall of drought-flood abrupt alternation obviously intensify. The positive correlation between the variance contribution of 30-60 d oscillation and the summertime rainfall over most part of the Huaihc river basin is shown with the large (small) variance contribution corresponding to much(rare) summertime rainfall. Furthermore, the significant correlation region is located over the south of the river basin. The opposite patterns of the tonal 30-60 d oscillation phase of both the height and meridional wind over the Eurasian mid-high latitudes are a possible circulation cause for the flood-drought abrupt alternation. In addition, the evolution of the low frequency distribution is given by analyzing the cases in typical years. During the drought period the global tcleconnection variability pattern over the mid-high latitudes has a tonal wavenumer -4 or wavenumer-5 structure with the variability centers over Eurasian ranking as "+—-—+—-" from northeastern Atlantic to East Asia through Europe and Lake Baikal, which is corresponding to the low frequency oscillation distribution. During the flood period the subpolar waveguide emanates from northern Europe to the high-latitude region of Asia along the westerly jet stream, then the strong positive variability center is developed over the sea of Okhotsk, which is favorable to forming and maintaining Okhotsk high blocking. Meanwhile the negative variability centers are found over the mid-latitude region of central Asia with the Indian monsoon weaker than normal, and then negative variability forms over the sea of Japan as a result of the down-stream effect, resulting in the subtropical high shift to more southern than normal. The negative variability is found over the lower latitudes of Bay of Bengal to the high levels of the troposphere over the South China Sca, therefore the connective activities over the South China Sca are weaker than normal. The tonal low frequency circulation pattern over the Eurasian mid-high latitudes during the drought period and the flood period reflects actually the subpolar and subtropical jet stream Rossby waveguide structure and its anomalous propagation.