Understanding The Occurrence Of Long-Cycle Abrupt Drought-Flood Alternations In Southwest China

Drought and flood disasters are the two main meteorological disasters in our country.Due to its unique geographical location and regional climate characteristics,the Southwest is one of the frequent areas of drought and flood hazards in China since ancient times.This thesis performs the phenomenon of long cycle abrupt drought-flood alternations in summer in Southwest China.Abrupt drought-flood alternations in this paper refers to the phenomenon that the drought persisted for a time,and the other period flood lasted.The alternation from droughts to floods occurred in a certain season.It reflects the coexistence of the two extreme events in short terms.The long cycle refers to the drought and the flood lasted 2 months,in which the year of drought to flood means a drought in May and June,a flood in July and August;the year of flood to drought is the opposite.By defining the long cycle abrupt drought-flood alternation index,we analyze the evolution of drought-flood alternation events and the anomaly of atmospheric circulation in summer in Southwest China.The relationship between abrupt drought-flood events and Asian summer monsoon was studied by means of correlation analysis,regression analysis and other climatic statistical methods in this paper.In addition,the BCC_CSM1.1(m)model was used to evaluate the predictive ability of the drought-flood alternations in Southwest China.The main conclusions obtained were as follows:(1)An index of abrupt drought-flood alternation in Southwest China in summer was defined in this paper.We analyze the characteristics of the long cycle abrupt drought-flood alternations in the southwest over the past 50 years.The results show that the alternation events from drought to flood are more than those from flood to drought in the summers during 1961-1970,the alternations from flood to drought are more during 1971-1980,and the alternations from drought to flood are equal to those from flood to drought during 1981-2000.Since the early 21st century,abrupt alternations from flood to drought tend to increase.The characteristics of the anomalous atmospheric circulation and water vapor transportation were analyzed.It was found that the western Pacific subtropical high extended westward and became stronger during the drought period before the conversion from drought to flood.The westerlies at the middle and high latitudes also became stronger,preventing cold air from moving southward.In the vertical,downdraft prevailed and the moisture transport from the Bay of Bengal and South China Sea was weaker than before.Southwest China was located at the moisture divergence area.In addition,there was less rainfall during drought periods.During the flood period after the conversion from drought to flood,meridional circulation at the middle and high latitudes tended to be stronger.The trough to the east of the Ural Mountains deepened and the eastern Asia ridge strengthened.The westerlies at the middle and high latitudes became weaker and updrafts prevailed in the vertical.Also,large amounts of moisture were transported from the Bay of Bengal and South China Sea to Southwest China,which explains why rainfall became abundant there.(2)This paper analyzes the relationship between Asian summer monsoon and the drought-flood alternations in Southwest China.The result shows that the central longitude of the plateau monsoon was anomalously west 2.66°in June during the year from flood to drought and the central latitude was north 0.21° in the early March.The index of Plateau monsoon intensity was unusually weak in December before the year of drought to flood.In March and April during the year from drought to flood,the intensity of Bay of Bengal monsoon,East Asia monsoon.Indian monsoon,Northwest Pacific monsoon and the South China Sea monsoon are all significantly weak.During the year from flood to drought,the Indian monsoon,Northwest Pacific monsoon and the South China Sea monsoon are strong in August.There is a significant negative correlation between the index of alternations and Asian polar vortex.The area and intensity of Asian polar vortex are both weak during the drought-flood year and are strong during the flood-drought year.The area and intensity of Indian subtropical high are small and weak from drought to flood,and the area of South China Sea is also small.The position of the Western Pacific ridge is anomaly south during drought-flood year.Meanwhile,the position of Western Pacific subtropical high is westward.(3)The results of BCC_CSMl.1(m)model show that it has great differences in forecasting drought-flood alternations according to different lead months.The prediction capability of the model on precipitation in May and June is higher than that in July and August.The model can accurately reflect the main modes of temporal and spatial variability of precipitation in southwest China in summer.However,there are some errors in predicting the modal variance contribution and the inter-annual variation of the time coefficient.The BCC_CSM1.1(m)can reasonably predict the characteristics of large scale water vapor transmission,vertical velocity and global sea surface temperature(SST),but the overall forecasting ability for summer precipitation in Southwest China is limited.The prediction effect on atmospheric circulation and the key area of sea surface temperature in May and June is significantly higher than that in July and August,which directly affects the forecast effect of the model on precipitation in early summer and mid-summer in Southwest China.