Propagation Diversity Of Biweekly Oscillations In Summer Level Drop In The Middle And Lower Reaches Of The Yangtze River

The summer precipitation in the middle and lower reaches of the Yangtze River has obvious characteristics of intraseasonal oscillation,but its propagation path has not been unified.Based on observational precipitation data and ERA5 reanalysis data,this paper analyzed the causes and interannual variation characteristics of summer quasi-biweekly precipitation propagation diversity over the middle and lower Reaches of the Yangtze River from 1979 to2019.The main conclusions are as follows:(1)The 10-20-day quasi-biweekly oscillation of summer precipitation over the middle and lower reaches of the Yangtze River is obvious,with 3-4 quasi-biweekly precipitation events occurring annually on average.On the quasi-biweekly scale,the precipitation fluctuated with a period of 13 days.In terms of its propagation characteristics,quasi-biweekly precipitation events are divided into three types: southward,northward and local vacillation.The frequency of oscillation events propagating southward is the largest,while that propagating northward is the least.(2)In the southward propagating event,the wave train obviously propagating eastward can be seen at middle and high latitudes,and propagating southward over the eastern coast of East Asia.In the events of northward propagation and local vacillation,mainly influenced by low latitude systems,the wave train propagating from east to west can be seen in the northwest Pacific Ocean,and the role of the boreal summer intraseasonal oscillation(BSISO)may need more consideration.In these two types of events,the precipitation peak usually occurs in phase6 and 7 of the BSISO2 index.The characteristics of the vapor flux field and its divergence field are similar to those of the circulation field.(3)In the interannual variation characteristics of the spread difference of quasi-biweekly precipitation,the SST anomalies in the characteristic years of the southward spread are higher in the east and lower in the west.In the negative phase years of the Indian Ocean dipole(IOD),the quasi-biweekly signals in the middle latitudes are active,while the quasi-biweekly activity in the low latitudes is weak.The SST in the central equatorial Pacific is negative anomaly in the year of northward propagation.In the year of negative SST anomaly,the quasi-biweekly signal in middle latitude is weak,and the quasi-biweekly signal in low latitude is active.In the years with local vacillation characteristics,the SST of the South China Sea is negative anomaly,and in the years with negative SST anomalies,there are active quasi-biweekly signals in middle and low latitude at the same time.(4)RegCM 4.6 can better simulate the intensity range and propagation characteristics of precipitation in the middle and lower reaches of the Yangtze River,and the number of quasibiweekly precipitation oscillation events and the number of each propagation characteristic event have a good correspondence with the actual precipitation.In the boundary sensitivity test,the southward propagation events of the mid-latitude test decreased significantly,while the northward propagation events increased significantly.The northward propagation events of low latitude experiments decreased most significantly.