Analysis Of The Characteristics And Causes Of Arid Climate In Southwest China In The Past 55 Years

In this chapter,standardized Precipitation Evapotranspiration index(SPEI)is used to analyze and discuss the spatial and temporal evolution characteristics of the drought in southwest China in past 55 years.And analyze the circulation anomaly characteristics and the causes of drought in arid climate.The conclusions are as follows:(1)The spatial distribution of cumulative drought intensity,frequency of drought events and average drought duration in southwest China in past 55 years are relatively consistent.Drought events are more frequent in summer and winter,and the average duration of drought in spring and autumn is longer,which lasts 2-3months.In the past 55 years,the average regional drought intensity in southwest China has been increasing year by year,and the suffering area(drought station ratio)has been expanding year by year.After 2002,the inter-decadal scale of rapid drought trend has appeared in southwest China,and the periodic variation of 3-4years have appeared from 2002 to 2012.(2)The variation of drought in southwest China in the past 55 years is consistent with the increase of temperature and the decrease of precipitation in summer and autumn in this region under the background of global warming.The analysis showed that the seasonal SPEI values in the southwest China are significantly negatively correlated with the temperature changes in this region,the correlation coefficients are-0.396(Spring),-0.533(Summer),-0.247(Autumn)and-0.299(Winter),respectively.In the past 55 years,the correlation coefficients of SPEI values and precipitation in southwest China are 0.368(Summer)and 0.257(Autumn),respectively.Generally speaking,drought in winter and spring is significantly related to the increase of temperature,while drought in summer and autumn is closely related to the increase of temperature and the decrease of precipitation.(3)In arid climate,the ridge of high pressure has been strengthened in the upper500 h Pa and the low-level winter wind is weakened in the middle and low latitudes of east Asia in winter and spring,and the temperature in the troposphere in southwest China is anormaly rising,resulting in the weakened cold air activity in southwest China in winter and spring,which is prone to the high temperature and drought.In summer and autumn,the trough ridge line in the mid-latitude area of Eurasia is slightly to the north in the arid climate.And there is no obvious southwest warm moisture airflow in the southwest China.The west Pacific subtropical high controls the east of the southwest China,and the subsidence airflow in this area is active,so it is easy to develop high-temperature and rainless weather.(4)In the past 13 years,the summer drought trend and the significant reduction of summer precipitation in southwest China are closely related to the weakening of Indian summer monsoon.During the weak Indian summer monsoon years in the early 2000 s,the southwest China is located in the south side of upper westerlies,the westerlies over southwest China weakens,and there is no obvious southwest water vapor transport in the lower layer.The western Pacific subtropical high anormaly enhances and controls the southeast of southwest China,which causes the area affected by the downdraft,and the summer precipitation in southwest China has decreased sharply in recent years.(5)The rapid decrease of Indian summer monsoon is closely associated with a significant reduction of the temperature in the Tibetan plateau in the past 13 years.Tibetan plateau as the hot reservoir,where the temperature anormaly decreases in recent years,combining of the anormaly rising of the Indian Ocean SST,which leads to the sea-land thermal contrast between the Tibetan plateau and the Indian Ocean significantly decreased,contributing to the weakening of the Indian summer monsoon in recent years.(6)In the past 13 years,the Pacific Ocean has shown obvious cold phase of PDO.The change between cold and warm phase of PDO has a significant negative correlation with east-west oscillation of western Pacific subtropical high of-0.36.In the situation of cold phase of PDO,the western Pacific subtropical high is farther eastern than normal,leading to the rainfall in southwest China getting less in recent years.In addition,the anormal increase of troposphere temperature is also one of the reasons for inter-decadal drought in southwest China due to the less rainfall.