| The data used in this study consists of the precipitation of 740 stations of China from Climate Center of CMA, the reanalysis data from NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research), the monthly mean SST (Sea Surface Temperature) data from Hadley prediction center of Britain, and so on. The methods of EOF (Empirical Orthogonal Function decomposition), MSSA (Multiple Singular Spectrum Analysis), SVD (Singular Value Decomposition), Wavelet analysis, EEMD (Ensemble Empirical Model Decomposition) and et al, are applied. Some basic studies about the precipitation over two lake region are investigated, including the precipitation climatic characteristics, the impacts of SST of tropical Pacific and Indian Ocean and cold air on the rainy season’s rainfall, and the temporal and spatial variations of drought/flood in summer season. Furthermore, in view of some complete drought/flood events, the drought/flood mechanism has also been preliminarily studied. The main results are as follows:(1) The rainy season in two-lake region lasts 91 days which begins at the second pentad in April and ends at the first pentad in July. The first two EOF models of the precipitation are its most significant rain patterns. The first model shows there are two intensive rainy periods, one of which is in the fist and second ten-day-section of May and the other is in the middle and last ten-day-section in June. The second model shows there are three intensive rainy periods, including one in second ten-day of April, one in second and last ten-day period of May, and one in last ten-day-section of June to the beginning of July. The rainfall in rainy season in two-lake region has multiple period oscillations, such as quasi-7 days, quasi-two weeks,30-60 days and seasonal period, et al. among which, the quasi-two weeks and the 30-60 days oscillations are the most significant ones.(2) The rainfall in two-lake region is closely linked with the SST anomaly in tropic Oceans (refer to the tropic Pacific and Indian Ocean, the same hereafter). While SST anomaly in previous winter is positive (negative) in middle and east Pacific Ocean, negative (positive) in west Pacific and in subtropical part of the middle and east Pacific, as the same time, the SST anomaly in Indian Ocean appears a symmetric model with positive (negative) in north and negative (positive) in south, there will great potentially be more (less) amount of rainfall in rainy season in two-lake region. Correspondently, there will be more (less) rainfall in east part of China in Meiyu season and less (more) in Huaihe River basin. Moreover, in a positive (negative) SST anomaly year, during the rainy period of two-lake region, there easily occurs eastward (westward) wind anomalies in tropical Indian Ocean, but they would change directions to northward (southward) after crossing equator at the longitude of 90 (80) degree. As the same time, in the positive (negative) SST anomaly years, easterly (westerly) wind covers the near equator part of the west Pacific Ocean, producing an (a) anticyclone (cyclone) anomaly system in lower layer which means the location of west Pacific subtropical high is in westward (eastward) and northward (southward) phases, and its intensity is in stronger (weaker) stage.(3) The rainfall in rainy season in two-lake region is also closely linked with the cold air’s activities.it is credible the Moist Potential Vorticity (MPV) is a good index for indicating the trace and intensity of cold air. And also, the MPV has obvious positive relationship with the regional precipitation. For example, in the two major intensive rainy periods in two-lake region, one in the first half of May and one in last 20-day of June, they all linked a big minus MPV center at the mid-lower layers (below 500 hPa), moreover, the big minus MPV areas could extend to higher layers at the second rainy period than the previous one.(4) The flood/drought event happened so frequently that there had average 1.6 months regional floods and 1.3 regional droughts in each year. Though, in two-lake region, drought/flood events easily occurs in winter and autumn seasons based on the variation of precipitation normally percentage, the impacts of drought/flood happened in summer and spring are much more severe, so more researches had focused on. In summer, the Dongting-lake area, and the southern part of Hunan province are the key drought/flood occurring region. The drought/flood in two-lake region mainly presents 4 models, namely:a coherent flood (drought) in almost whole region; flood (drought) in north-west against drought (flood) in south-east; flood (drought) in south-west against drought (flood) in north-east; and flood (drought) in center but drought (flood) in north-west and south-east.(5) There exits obvious inter-decadal variations to the drought/flood in two-lake region with a jump point in eighty years of 20 cenury, which shows more droughts before 1980s and more flood after that. There also exists obvious inter-annual changes with periodic oscillations of quasi-8 years, quasi-5 years, quasi-2 and 4 years. Among which, the quasi-two-years oscillation appeared every clear before 1980s but not any more after that.(6) As persistent drought or flood event happens in two-lake region, the height field at 500 hPa easily appears a different kind of meridional propagation pattern. That is, in the process of persistent droughts (floods), the distribution of 500 hPa height from high to low latitude in East Asia and West Pacific region is the typical pattern of "-+-" ("+-+"), which means when persistent droughts (floods) occurs in two-lake region, the location of west Pacific subtropical high is northward and westward (southward and eastward),and the intensity of block high in mid-high latitude in east-Asia is weaker (stronger).(7) The water vapor fluxes (WVF) of two-lake region’s persistent droughts and floods are obviously different. During droughts, the WVF mainly comes from tropical Indian Ocean, but goes northward steadily with the strong southwest summer monsoon going northward. On the other hand, during floods, there exists a WVF transmit belt in the lower latitude of 10-20, where two WVFes converge, one from the tropical Indian Ocean and one form the tropical Pacific. In addition, there exists an eastern-WVF from the subtropical Pacific in 35-40 latitude of East Asia. It meets together with the lower WVF in two-lake region.In conclusion, based on the above studies, we find that the rainy season of two-lake region has its special characteristics.In climatic view, the prime rainy season in two-lake region begins at the second pentad in April and comes to end at the first pentad in July. The first pattern of the EOF of the precipitation shows there are two more-rain periods, of which one occurs in the first to the second ten-day part of May and one in the second to the last ten-day part of June. The inter-annual variation of the rainfall in two-lake region is significant, with the biggest climatic deviation in autumn and winter. So the drought and flood events happened frequently in the two seasons. The inter-decadal variation is also significant. A notable jump point of the rainfall has happened in almost the beginning of 1980, with more drought events before it and more flood after, as the same time, the quasi-2 day period oscillation also weakened down. A broad sea surface temperature anomaly of the middle-east Pacific and Indian Ocean in winter has greater impact on the rainfall of two-lake region in its next rainy season. Therefore, we think that these research findings will possibly benefit for the local seasonal climate prediction. |
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