Analysis Of The Relationship Between The Asian Blocking High And The Subtropical Westerly Jet In The Meiyu Season In The Jianghuai Region

In this paper,the NCEP/NCAR daily analysis data from June and July of1960-2018,the daily precipitation data of China's national stations in the same period,Quantitative characterization of blocking highs,Eliassen-Palm flux(E-P flux),and vorticity source equation are used to analyze Characteristics,Correlation,interaction force,thermal mechanisms and the impact on the Yangtze-Huaihe Meiyu of blocking highs and subtropical westerly jet in Asia.The results show that:(1)In the Yangtze-Huaihe Meiyu season(June-July)of last 59 years,there are a total of 363 blocking events in Asia,and the centers are mainly distributed in the three key areas of Ural Mts(40-80°E),Baikal(80-120°E)and Okhotsk Sea(120-160°E).The frequency and days of blocking events in the three key areas are from high to low: the Okhotsk Sea,Ural Mts and Baikal.The double-blocking situation is dominated by the Ural Mountains-Okhotsk Sea,which accounts for60% of the double-blocking days in Asia.The average life cycle of the blocking event is about 7 days,and the longest holding time is 13 days.(2)Analysis of the spatial distribution characteristics and climate change characteristics of Asian subtropical westerly jets revealed that the 200 h Pa zonal wind field of the Yangtze-Huaihe Meiyu season had three large wind speed centers along the axis of the Asian subtropical westerly jets at 40°N,located at50°E of Near the Caspian Sea,80-105°E of northern Xinjiang in China,135-160°E of Japan and the east of it,they are located in the three key blocking highs of Asia,respectively,and appear as westerly jets.The three-wave quasi-stationary Rossby wave pattern in the up and down directions.The quasi-stationary Rossby wave train meridional velocity decelerates as it passes eastward along the westerly jet.The location of the large center of the westerly jet has a sudden change in the east-west direction from the end of June to the middle of July.(3)The mid-latitude(30-60°N)near the subtropical westerly jet region in Asia has the strongest negative correlation between the blocking highs and the jet intensity in the critical jet region.The strong impact of the westerly jet may not be conducive to the formation of the blocking highs and may be conducive to the collapse of the blocking highs;Strong meridional wind may be conducive to the formation and maintenance of the blocking highs,and the anti-cyclone movement above the blocking highs will also hinder the westerly propagation of the westerly jet,weaken the westerly airflow,and branch the westerly airflow to bypass the blocking highs.;The high latitudes(north of 60°N)on the northwest side of the subtropical westerly jet region are positively correlated with the jet intensity in the jet region,but the positive correlation between the two is relatively small.The relationship between the obstructing high pressure at the mid-latitudes(south of60°N)in the Okhotsk region and the intensity index of the subtropical westerly jet in Asia has the most significant relationship.(4)Based on the analysis of the typical years of the Okhotsk region where the blocking high pressure is more and less,the results are as follows: During the Yangtze-Huaihe Meiyu season in 1998 when the Okhotsk Sea region was blocked by high pressure,the 200 h Pa high-altitude transient eddy momentum flux at45°N played an important role,while the transient eddy heat flux in the south and north of the region the volume is dominant,and the EP flux divergence is in a‘+-+” situation at an altitude of 200 h Pa.The high-altitude westerly jet weakens,but increases on the north and south sides of it.The propagation of the Asian subtropical westerly jets is blocked,the westerly wind flows around and produces a north-south branch.In the Okhotsk Sea region,during the Yangtze-Huaihe Meiyu season in 1963,where the high pressure is biased,the transient eddy heat flux dominates around 200 h Pa in the mid-latitude region,and the EP flux divergence is in the mid-latitude region(30°-50°N)200 h Pa high altitude is EP flux divergence,the high altitude westerly jet is enhanced,and the divergence is basically zero in both the north and south areas.The year when the Okhotsk Sea region has low blocking high pressure does not affect the normal propagation of subtropical westerly jets in Asia.(5)There are obvious interdecadal changes in the blocking frequency and days in the three key blocking areas,and there is a increase trend,which is more obvious in the Okhotsk Sea.It is consistent with increase trend of the cumulative rainfall in the Yangtze-Huaihe Meiyu season of the past 59 years.The amount of rainfall in the Yangtze-Huaihe Meiyu season is closely related to blocking activity,there are significantly more(less)blocking events in the Okhotsk Sea and Ural-Okhotsk in Meiyu positive(negative)abnormal years,while the blocking events in the Ural and Baikal regions are not significantly related to Meiyu.In the Yangtze-Huaihe Meiyu season,the number of blocking days in the Okhotsk may be related to the early sea surface temperature anomaly signal ENSO.(6)From the dynamic mechanism of mid-scale atmospheric propagation,we can further understand the mechanism of the influence of the anomalous activity of the subtropical westerly jet on the Meiyu in China.In Yangtze-Huaihe Meiyu's positive(negative)anomalous year,The wave train distribution and propagation on the East Asian subtropical jet system is systematically westward(east),the wave flux is concentrated(divergent)and the intensity is strong(weak),the westerly jet is strong(weak),low altitude The southwestern jet strengthens(weakens),the entire layer of water vapor fluxes in the Jianghuai Basin converges(divergence),the lower layer has strong(weak)convergence and the upper layer has strong divergence(zero divergence),and the vertical ascent movement is enhanced(weakened),which is(not)beneficial to more precipitation of the Yangtze-Huaihe Meiyu Season.