A Comparative Study Of The Combined Effects Of Tropical And Mid-high Latitudinal Variablities On Quasi-Biweekly Rainfall With Those On Intraseasonal Rainfall In Summer In The Middle And Lower Reaches Of The Yangtze River

In this study,a comparative analysis of the combined effects of tropical and mid-high latitudinal varaiablities on quasi-biweekly rainfall with those on intraseasonal rainfall in summer in the Middle and Lower Reaches of the Yangtze River(MLYR)is carried out based on observational and reanalysis data with using various statistical and analytical methods including wavelet analysis,spectral analysis,synthetic,regression,correlation analysis and Linear Baroclinic Model.First,three typical flood years(1991,1998,and 2016)in MLYR are selected,and we diagnosed the characteristics of their significant cycles of summer precipitation and compared the evolution characteristics of the corresponding atmospheric circulation systems,which focused on the influence processes of atmospheric intraseasonal variability(ISO)from the tropics and from the mid-high latitudes,and found that the intensity of the two types of low-frequency precipitation differed significantly in different years,with some years dominated by the quasi-biweekly precipitation and other years dominated by precipitation with 30-60 day cycles.However,in both types of precipitation,there are combined effects of tropical and mid-high latitudinal varaiablities.In order to clarify the difference of these combined effects in the two types of low-frequency precipitation events,the daily summer precipitation from 1981 to 2020 was then decomposed to obtain the quasi-biweekly precipitation components and intraseasonal components for each year.Then,the daily quasi-biweekly precipitation components and intraseasonal components were accumulated to obtain the quasi-biweekly precipitation components and intraseasonal components intensity indices.The strong and weak quasi-biweekly and intraseasonal precipitation years are selected accordingly.Firstly,on the low-frequency time scale,the strong and weak quasi biweekly and intraseasonal precipitation events that occurred in these years were selected.Further,the differences in the low-frequency activities of atmospheric circulation in the tropics and mid-high-latitudes are compared with these events.Secondly,at the interannual time scale,the differences in the connection between summer quasi biweekly and intraseasonal precipitation intensities and summer seasonal background fields were compared.Finally,an in-depth analysis of the individual case of 2020,the strongest rainy year since 1961,was conducted from the perspective of thermodynamic forcing and interannual background modulation of low-frequency variability.The results are summarized as follows:(1)1991,1998,and 2016 are strong quasi-biweekly or intraseasonal rainfall years,and although all the three years are characterized by significant low-frequency oscillations in precipitation in MLYR,there were differences in both the precipitation periods and the sources of ISO that affected the rainfall among these three years.In 1991,two ISO have a common period of 10-30 d but originated from the tropical Indian Ocean and the mid-latitudinal Eurasia,respectively.These ISOs propagated toward northeast and southeast,and merged in MLYR,causing the 20-30 d fluctuation in rainfall.In 1998,the ISO originated alone from the central North Pacific.It propagated westward with two characteristic periods of 30-60 d and 10-30 d,led to the20-40-day oscillation in rainfall over MLYR.In 2016,the origination of ISOs are relatively more complicated,including those from the tropical Indian Ocean with the period of 30-60 d,from the tropical Pacific with the period of 10-30 d,and from the mid latitudinal Eurasia with the period of 10-30 d.They propagated simultaneously toward MLYR,resulting in a 10-20 d oscillation in rainfall.(2)There are significant differences in the intensity of precipitation,propagation direction of rainbelt,water vapor source,the corresponding atmospheric ISO activity and background SST field between strong low-frequency precipitation events in strong and weak quasi-biweekly precipitation years.First,in strong years,the quasi-biweekly rainbelt propagating from southeast to northwest,while in the weak years,the quasi-biweekly rainband propagating from north to south.Second,whether in strong or weak years,the divergence term of moisture budget equation plays a leading role in precipitation,followed by the advection term,in comparison,the moisture tendency has a much smaller contribution to precipitation.Meanwhile,the divergence term,advection term,and tendency term are stronger in strong years than in weak years,resulting in stronger precipitation in strong years than that in weak years.Third,in terms of circulation,the southeast-northwest movement of the WPSH is the key for the southeast-to-northwest propagation of the rainbands in strong years.The westward-propagating mid-latitude and high-latitude wave trains,as well as the northeastward-propagating EAP wave train with the eastward-propagating North Pacific wave train,are jointly influencing the occurrence of quasi-biweekly precipitation.While the cyclonic system over MLYR is consistent with the propagation of rainband from northwest to southeast,which indicates that this system is the key to regulate the movement of rainfall during the weak years.The westward propagating Indian Ocean East Asia wave train and high latitude wave train,along with the eastward propagating North Pacific wave train and mid-latitude wave train,jointly affect the occurrence of quasi-biweekly precipitation.At last,in the interannual background field,when the intensity of quasi-biweekly precipitation is strong,it corresponds to a double-blocking high-level circulation,and the SSTs in the North Pacific Bering Sea and the Philippine Sea show a significant positive correlation with the intensity of quasi-biweekly precipitation.The structure of the summer background circulation field obtained from the two regressions is similar,which means that the SSTs in these two regions can influence the intensity of quasi-biweekly precipitation in MLYR by affecting the summer background flow.(3)There are also significant differences in the intensity of precipitation,propagation direction of rainbelt,water vapor source,the corresponding atmospheric ISO activity and background SST field between strong and weak intraseasonal rainfall years.First,in the years with strong intraseasonal precipitation,the rain bands propagating from south to north,while in weak years,the rainfall is the result of the northward transmission of tropical rainfall and the southward transmission of mid-latitude rainfall in MLYR.Second,whether in strong or weak years,the divergence term of moisture budget equation plays a leading role in precipitation,and it is stronger in strong years than in weak years,resulting in stronger precipitation in strong years than in weak years.Third,in terms of circulation characteristics,in both strong and weak years,it is the intraseasonal oscillation of WPSH and northern cyclonic system south of the MLYR that regulates the alternation of dry and wet phases of precipitation.In strong years,the above two systems are associated with the EAP wave train and the eastward propagating Eurasian wave train.While in weak years,they are combined influenced by the westward-propagating Eurasian wave train and the eastward-propagating North Pacific wave train.At last,in the interannual background field,when the intensity of intraseasonal precipitation is strong,it corresponds to a three-blocking high-level circulation.And it is found that the SST in the regious from the Indian Ocean to the western Pacific shows a significant positive correlation with the intraseasonal precipitation intensity.The structure of the summer background circulation field obtained from the two regressions is similar,which means the SSTs from the Indian Ocean to the western Pacific can affect the intensity of seasonal precipitation in MLYR by influencing the summer mean flow.(4)There are significant differences in the intensity and evolution of low-frequency precipitation in strong quasi-biweekly precipitation years and strong intraseasonal precipitation years,as well as the characteristics of atmospheric ISO activity in the tropics and mid-high latitudes,and the corresponding background fields.First,both types of low-frequency precipitation are dominated by the divergence term of moisture budget.In terms of intensity,the wet phases of the strong quasi-biweekly years are stronger than that of the strong intraseasonal years.As for the propagation direction of precipitation,the rainfall band in strong quasi-biweekly years has the characteristic of propagating from southeast to northwest,while in the strong intraseasonal rainfall years,it has the characteristic of propagating from south to north.This difference is mainly caused by the low-frequency northward WPSH,which has a southwest-northeast distribution in the strong quasi-biweekly years and an east-west distribution in the strong intraseasonal years.Second,the low-frequency oscillations in the potential heights of both the WPSH and the area north of the MLYR are the key systems controlling the occurrence of both types of low-frequency precipitation.In strong quasi-biweekly years,the westward-propagating mid-latitude and mid-high latitude Eurasian wave trains,together with the eastward-propagating EAP wave trains and the eastward-propagating North Pacific wave trains,influence the quasi-biweekly oscillations in the potential heights of these two systems,leading to the occurrence of quasi-biweekly precipitation.In the strong intraseasonal precipitation years,the EAP wave train and the eastward propagating Eurasian wave train modulate the above two systems,which are responsible for the alternating wet and dry phases of precipitation.Thirdly,there are also differences in the mean summer background fields,and the quasi-biweekly precipitation intensity is mainly related to the double blocking background field,which is further positively correlated with the SST in the North Pacific Bering Sea and the sea off the Philippines,while the intraseasonal precipitation intensity is mainly related to the three-blocking background field,and this background field is further positively correlated with the SSTs from the Indian Ocean to the western Pacific.(5)The 10-20 d precipitation in MLYR in the early summer of 2020 is significant and the strongest since 1981,and the low-frequency precipitation is influenced by combined effects from tropical and mid-high latitudinal varaiablities.It is the combined effects of the 10-20 d East Asia-Pacific wave train,the Eurasian wave train,the mid latitude wave train and the North Pacific wave train that causes the strong10-20 d oscillation characteristic of the rainfall.The strong oscillations of the 10-20 d wave trains is found to be related to the background SST.On the one hand,the tropical western Indian Ocean SST warms significantly in the early summer of 2020 and anchors the convection overhead,which enhances the meridional thermodynamic contrast across the IO to the high latitudes,in turn enhances the atmospheric baroclinity and the Central Asian westerly jet stream,thus facilitating the kinetic energy conversion of the background flow to the 10-20 d wave train,making the 10-20 d activity of the Eurasian wave train unusually active.On the other hand,the warm SST in the western Indian Ocean enhances the 10-20 d oscillation of the convection overhead,and this10-20 d oscillation of the convection triggers anomalous descent over the tropical western Pacific,causing the strong 10-20 d oscillation of the EAP wave train.In addition,anomalously warm SSTs in the Kuroshio extension cause anomalous meridional thermal gradients that weaken and shift the westerly winds northward at the exit of the North Pacific westerly jet stream.The weakened background westerly winds and reduced wave number of the North Pacific wave train favor the westward retreat of the wave train,which in turn affects the 10-20 d variation of the potential height field in the Baikal region.In the early summer of 2020,the 10-20 d oscillation intensity of the EAP wave train(the geopotential height over Lake Baikal)ranked first(second)in the past 40 years,and together led to the 10-20 d component of early summer precipitation in MLYR broke records.Comparing the circulation characteristics,tropical and mid-high latitude atmospheric ISO activities and background SST fields between 2020 and the strong quasi-biweekly precipitation years,they are all relatively consistent,indicating that the general rule of strong quasi-biweekly precipitation years is basically applicable in 2020.