The Influence Of Different Modes And Energy Dispersion Of The Subtropical Westerly Jet On The Anomaly Of Meiyu

By using the NCEP/NCAR global daily mean reanalysis data from 1960 to 2015 and daily precipitation data from 2474 stations in China provided by the National Meteorological Information Center,this paper first analyses the climatic characteristics of the East Asia subtropical westerly jet.Then the empirical orthogonal function decomposition(EOF)method is used to analyze the characteristics of the jet mode during the Meiyu period,and its influence on the Meiyu in the middle and lower reaches of the Yangtze River is discussed.Finally,the characteristics of baroclinic wave packets in different stages of Meiyu season are studied by using the method of WPD.The main conclusions are as follows:(1)The East Asian subtropical westerly jet shows seasonal variation in intensity and locationt,with the strongest intensity in winter and the weakest in summer,and the location of South in winter and North in summer.The intensity of the jet stream in the meiyu period is stronger than that in summer,and the location of the jet stream is slightly south of that in summer.The jet axis rises northward to 37.5°N on June 13,which is the first northward jump in summer.The northward jump of the jet stream occurred one pentad earlier than the beginning of the rainy season.(2)There are significant differences in the characteristics of the east Asian subtropical westerly jet in heavy and light Meiyu years.The horizontal and vertical shear of zonal wind in heavy Meiyu years are stronger than that in light Meiyu years,and the location of jet axis is south of light Meiyu year.In heavy Meiyu years,the jet axis remained stable at about 37.5°N from June 1 to July 17,which may be the reason for the excessive precipitation.In light Meiyu years,the average position of the jet stream axis is northerly.From early June to early July,the jet axis has been pulsating between 35°N and 40°N,and the jet stream axis is variable and unstable,which is an important reason for the low rainfall in Meiyu.(3)The first mode of the 200 hPa zonal wind of meiyu period presents a zonal symmetrical distribution in the spatial field,and the first eigenvector is approximately symbolically opposite around 38°N,which reflects the characteristics of meridional deviation of the subtropical westerly jet.The second mode shows the intensity variation of the subtropical westerly jet.When the location of the jet stream is south,the precipitation is relatively high,when the jet stream is north,the precipitation is relatively low.While the jet stream is farther northern,the Western Pacific subtropical high is stronger.There is obvious northwind anomaly in the range of 30°N-40°N,which is conducive to the southward movement of cold air and the convergence of cold and warm air in the middle and lower reaches of the Yangtze river,and the convergence of updraft is strengthened,which promotes precipitation in the middle and lower reaches of the Yangtze River.When the jet stream is stronger,the precipitation of Meiyu is relatively high and the intensity of easterly wind in low latitude is stronger,the intensity of westerly wind in mid latitude is stronger,which corresponds to the subtropical high of 500 hPa and the intensity of Meiyu is stronger.(4)In heavy Meiyu years,the periodic signal of 30-35 d is relatively significant while 15 d and 22 d is relatively weak.In light Meiyu years,the periodic signal of30-35 d and 20 d is relatively significant.East Asia subtropical westerly jet is relatively stable during the rainy season in heavy Meiyu years,while the jet presents a changeable and unstable characteristic in light Meiyu years.After filtering out the scale changes above 30 d,it is found that the signal in the frequency domain of 2-10 d is prominent,and the periodic signal of 10-15 d is concentrated.(5)In heavy Meiyu years,the large value zone of 3-7d synoptic-scale perturbation waves extends from the west of Asia to the northeast of China,and from the northeast of China to the Aleutian Islands.The jet entrance is located near the Black Sea,corresponding to the center of sub-maximum wave packet,which indicates that there is strong disturbance.The center of the wave packet is obviously to the east in light Meiyu years,which shows that the disturbance energy position of the torrent in heavy Meiyu year is west of that in light Meiyu years.When the center of the wave packet is west and stable,the intensity of Meiyu is stronger.The intensity of 10-15 d weather scale disturbance wave packet is stronger than that of 3-7d,and the intensity of light Meiyu years is stronger than that of heavy Meiyu years.(6)The subtropical westerly jet shows different characteristics before and after Meiyu,and the wave packet along the jet stream also shows different characteristics.In heavy Meiyu years,the perturbation distance of the wave packet in the early and late period of Meiyu is more northerly to the jet stream zone.During the meiyu period,the disturbance in the large value zone of the wave packet located in the north side of the jet zone is very active.and there is a baroclinal wave from the Mediterranean Sea to the east,and the intensity of the wave packet near the middle and lower reaches of the Yangtze river is strong.The maximum value zone of wave packet in light Meiyu years is consistent with that in heavy Meiyu years.During Meiyu period,the wave-packet high-value zone weakens significantly.The intensity of 10-15 d weatherscale disturbance wave packet is stronger than that of 3-7d disturbance wave packet.