Interannual Variation Characteristics Of Biweekly Oscillation Of Precipitation Level During The Meiyu Period In The Yangtze And Huaihe River Basin And Its Relationship With Sea Surface Temperatur

Based on the daily precipitation data of China from 1979 to 2020,atmospheric reanalysis data sets from NCEP/NCAR,monthly sea surface temperature(SST)data from Hadley Center and daily SST data sets from NOAA,we compare and analyze the similarities and differences in the characteristics and causes of quasi-biweekly oscillation(QBWO)of precipitation over the YangtzeHuaihe River Basin(YHRB)during the Meiyu period in 2016 and 2020,as well as the sources of low frequency signals.The interannual variation characteristics of the quasi-biweekly oscillation of Meiyu rainfall over YHRB,its link with SST are further investigated,and the mechanism of the influence of the North Atlantic SST anomaly on the quasi-biweekly oscillation of precipitation in the YHRB is revealed,and the physical mechanism was verified by using CAM5.3 model.The main conclusions are as follows:(1)The standard deviation of 8～16 days(QBWO)precipitation of Meiyu rainfall in the YHRB is the largest,which indicates that there is a significant QBWO in the YHRB.The first mode of the QBWO of Meiyu rainfall in YHRB extracted by the EOF decomposition method is the uniform mode across the whole region,and the second mode is a meridional dipole.The key circulation systems affecting the first two modes of the QBWO of the YHRB are significantly different,the main lowfrequency systems affecting the first mode of the QBWO of the Meiyu rainfall in YHRB are the Lake Baikal low-frequency cyclone with equivalent barotropic structure and the northwest Pacific lowfrequency anticyclone;and the low-frequency cyclone with equivalent barotropic structure located northward over the YHRB is the significant system leading to the meridional dipole mode of lowfrequency precipitation in the critical region.(2)The intensity of the quasi-biweekly oscillations of the YHRB Meiyu precipitation were more pronounced in 2016 than in 2020.The QBWO of Meiyu precipitation in the YHRB in 2016 was influenced by the anomalous anticyclonic circulation over the Northwest Pacific Ocean,the enhanced and eastward westerly jet,and the low frequency cyclonic circulation over Lake Baikal.In 2020,the low frequency cyclonic circulation moving from the south of Japan to the southwest direction,the Northwest Pacific anomalous anticyclonic circulation and the middle and high latitude anomalous cyclonic circulation acted together.2016 has a west-to-east propagation of the boreal anomalous cyclone,while 2020 shows a westward propagation.In addition,through the sea-air interaction on the quasi-biweekly scale,the atmospheric quasi-biweekly oscillation signal in the lower troposphere can continue to propagate to the YHRB,thus affecting the quasi-biweekly oscillation of the plum rainfall in YHRB.(3)There is a clear interannual variability in the intensity of the quasi-biweekly oscillations.The spring tropical North Atlantic SST anomaly enhances the high pressure in the Urals and Okhotsk Sea and deepens the low pressure in Lake Baikal by stimulating a wave train propagating to the middle and high latitudes of Europe and Asia,and causes an anomalous anticyclonic circulation in the northwest Pacific through the modulation of zonal circulation between the Atlantic Ocean and the Pacific Ocean,thus enhancing the QBWO of the first-mode Meiyu precipitation in the YHRB.The SST anomalies of the North Atlantic dipole type,through the excitation of a mid-to high-latitude Eurasian wave train,cause the deepening of the low pressure in the southeastern part of Lake Baikal toward Japan,which enhances the QBWO of the YHRB Meiyu rainfall in the second mode.The CAM5.3 model results verify that the SST anomalies in different modes in the North Atlantic affect the East Asian circulation anomaly,and then affect the QBWO of Meiyu precipitation of YHRB.