Leading Pattern of Spring Drought Variability over East Asia and Associated Drivers

Drought events have become more frequent and intense over East Asia in recent decades,leading to huge socioeconomic impacts.Although the droughts have been studied extensively by cases or for individual regions,their leading variability and associated causes remain unclear.Based on the Standardized Precipitation Evapotranspiration Index(SPEI)and ERA5 reanalysis product from 1979 to 2020,this study evealuates the severity of spring droughts in East Asia and investigates their variations and associated drivers.The results indicate that North China and Mongolia have experienced remarkable trends toward dryness during spring in recent decades,while southwestern China has witnessed an opposite trend toward wetness.The first Empirical Orthogonal Function mode of SPEI variability reveals a similar seesawing pattern,with more severe dryness in northwestern China,Mongolia,North China,South Korea,and Japan but increased wetness in Southwestern China and southeast Asia.Further investigation reveals that the anomalously dry(wet)surface in North(Southwestern)China is significantly associated with anomalously high(low)temperature,less(more)precipitation,and reduced(increased)soil moisture during the previous winter and early spring,regulated by an anomalous anticyclone(cyclone)and thus reduced(increased)water vapor convergence.The spring dry-wet pattern in East Asia is also linked to cold sea surface temperature anomalies in the central-eastern Pacific.The findings of this study have important implications for improving the prediction of spring drought events in East Asia.

A Typical Mode of Seasonal Circulation Transition: A Climatic View of the Abrupt Transition from Drought to Flood over the Middle and Lower Reaches of the Yangtze River Valley in the Late Spring and Early Summer of 2011

In this study, the seasonal transition of precipitation over the middle and lower reaches of the Yang-tze River Valley (YRV) from late spring to early summer is investigated. The results show that the seasonal transition of precipitation exhibits multi-modes. One of these modes is characterized by an abrupt transition from drought to flood (ATDF) over the middle and lower reaches of the YRV in the seasonal transition of precipitation. It is shown that the ATDF event from May to June 2011 is simply one prominent case of the ATDF mode. The ATDF mode exhibits an obvious decadal variability. The mode has occurred more frequently since 1979, and its amplitude has apparently strengthened since 1994. From the climatic view, the ATDF mode configures a typical seasonal circulation transition from winter to summer, for which the winter circulations are prolonged, and the summer circulations with the rainy season are built up early over the YRV.

Drought and Flood Analysis and Impact on Food Production in Fushun

Based on monthly precipitation data during 1961-2008 in 50 stations in Fushun,drought and flood indicators of three counties were calculated with Z index method. The geographical and seasonal distribution characteristics of Fushun were analyzed,and so was the impact of droughts and floods on food production. It shows that,since 1961,there are 7 poor harvest years in Fushun,with quadrennial caused by continuous seasonal floods or droughts,two years by year drought,one year by summer flood.

The Analysis on the Features of the Atmospheric Circulation in Preceding Winters for the Summer Drought and Flooding in the Yangtze And Huaihe River Valley

Based on the annual variation of the rainfall departure percentage in summer in the lower-middle reaches of the Yangtze River and the Huaihe River valley, 7 cases for the abnormal drought and flooding summers (the drought years: 1981. 1984, 1985; the flooding yearst 1980, 1982, 1983, 1987) are selected. First we analyse the general circulation characteristics of the summer drought and flooding, and then the evolution processes of the general circulation patterns from preceding winters to summers are studied. It is found that during the two kinds of preceding winters for the drought and flooding summer, not only the general circulation patterns in the high-mid latitudes, the local Hadley cells in East Asia but also the activities of the cold surge in the lower latitude are different obviously. Spring, especially April, is the turning period of the general circulation in preceding winter for the drought or nooding summer evolution towards opposite direction. Hereafter, the drought or flooding circulation pattern is established and developed.

The Relation between Atmospheric Intraseasonal Oscillation and Summer Severe Flood and Drought in the Changjiang-Huaihe River Basin

The intraseasonal oscillation (ISO) is studied during the severe flood and drought years of the Changjiang-Huaihe River Basin with the NCEP/NCAR reanalysis data and the precipitation data in China. The results show that the upper-level (200 hPa) ISO pattern for severe flood (drought) is characterized by an anticyclonic (cyclonic) circulation over the southern Tibetan Plateau and a cyclonic (anti-cyclonic) circulation over the northern Tibetan Plateau. The lower-level (850 hPa) ISO pattern is characterized by an anticyclonic (cyclonic) circulation over the area south of the Changjiang River, the South China Sea, and the Western Pacific, and a cyclonic (anticyclonic) circulation from the area north of the Changjiang River to Japan. These low-level ISO circulation patterns are the first modes of the ISO wind field according to the vector EOF expansion with stronger amplitude of the EOF1 time coefficient in severe flood years than in severe drought years. The analyses also reveal that at 500 hPa and 200 hPa, the atmospheric ISO activity over the Changjiang-Huaihe River basin, North China, and the middle-high latitudes north of China is stronger for severe flood than for severe drought. The ISO meridional wind over the middle-high latitude regions can propagate southwards and meet with the northward propagating ISO meridional wind from lower latitude regions over the Changjiang-Huaihe River Basin during severe flood years, but not during severe drought years.

Impacts of Upper Tropospheric Cooling upon the Late Spring Drought in East Asia Simulated by a Regional Climate Model

Responses of late spring （21 April 20 May） rainfall to the upper tropospheric cooling over East Asia are investigated with a regional climate model based on Laboratoire de M6t6orologie Dynamique Zoom （LMDZ4-RCM）. A control experiment is performed with two runs driven by the mean ERA-40 data during 1958-1977 and 1981 2000, respectively. The model reproduces the major decadal-scale circulation changes in late spring over East Asia, including a cooling in the upper troposphere and an anomalous meridional cell. Accordingly, the precipitation decrease is also captured in the southeast of the upper-level cooling region. To quantify the role of the upper-level cooling in the drought mechanism, a sensitivity experiment is further conducted with the cooling imposed in the upper troposphere. It is demonstrated that the upper-level cooling can generate the anomalous meridional cell and consequently the drought to the southeast of the cooling center. Therefore, upper tropospheric cooling should have played a dominant role in the observed late spring drought over Southeast China in recent decades.

Future Changes of Drought and Flood Events in China under a Global Warming Scenario

This study investigates the impact of global warming on drought/flood patterns in China at the end of the 21st century based on the simulations of 22 global climate models and a regional climate model(RegCM3) under the SRES(Special Report on Emissions Scenarios) A1B scenario.The standardized precipitation index(SPI),which has well performance in monitoring the drought/flood characteristics(in terms of their intensity,duration,and spatial extent) in China,is used in this study.The projected results of 22 coupled models and the RegCM3 simulation are consistent.These models project a decrease in the frequency of droughts in most parts of northern China and a slight increase in the frequency in some parts of southern China.Considering China as a whole,the spatial extents of droughts are projected to be significantly reduced.In contrast,future flood events over most parts of China are projected to occur more frequently with stronger intensity and longer duration than those prevalent currently.Additionally,the spatial extents of flood events are projected to significantly increase.

DIAGNOSTIC ANALYSIS OF PERSISTENT DROUGHT/FLOOD EVENTS IN SUMMER OVER THE TWO-LAKE REGION OF CHINA

Based on the daily regional mean rainfall,the Z-index method is used to identify persistent flood and drought events lasting for at least 10 days over a region where Dongting Lake and Poyang Lake sit(referred to as the"two-lake region"hereafter).The National Centers for Environmental Prediction(NCEP)reanalysis data are then utilized to perform a preliminary diagnostic analysis on these events.The results indicate that the composite standardized geopotential height at 500 hPa presents two different meridional wave trains from north to south over the East Asian-Pacific region,i.e.,a"-+-"pattern for the droughts and a"+-+"pattern for the floods,respectively.The developing,maintaining and decaying phases in the drought and flood events are closely related to the intensity and location of a subtropical high and an extra-tropical blocking high.It is shown that the East Asian summer monsoon is strong(weak)with the occurrence of persistent drought(flood)events.Droughts(floods)are accompanied by a weak(strong)tropical convergent system and a strong(weak)subtropical convergent system.Furthermore,the persistent drought(flood)events are associated with a divergence(convergence)of vertically integrated water vapor flux.In the vertical profile of water vapor flux,divergence(convergence)in the mid-and lower-levels and convergence(divergence)in the higher levels are evident in the droughts(floods).Both the divergence in the droughts and the convergence in floods are strongest at 850 hPa.

CHARACTERISTICS OF RAINFALL VARIATION OVER EAST CHINA FOR THE LAST 50 YEARS AND THEIR RELATIONSHIP WITH DROUGHTS AND FLOODS

With the precipitation data of 113 stations in East China during the last 50 years,the characteristics of the precipitation,including Precipitation Concentration Degree (PCD) and Precipitation Concentration Period (PCP) and their tendencies,are analyzed.The results show that the PCD in the northern part of the region is markedly higher than that in the southern part,but the PCP in the south is much earlier than that in the north by about one and a half months,which displays significant regional differences in precipitation.With the global warming,precipitation over East China shows an increasing tendency,but PCD displays a trend that is neither increasing nor decreasing.At the same time,the PCP is later than before,which can be mainly found in Jiangxi and southern Henan provinces.As a result,there are strong associations between the precipitation,PCD and PCP,which can be shown in the years with more precipitation,stronger PCD and later-than-usual PCP.In a word,the abnormal distribution of precipitation,PCP,and PCD over East China results in more extreme events of precipitation and more droughts and floods.

Numerical Study of Ural Blocking High＇s Effect Upon Asian Summer Monsoon Circulation and East China Flood and Drought

In terms of Kuo-Qian pesigma incorporated coordinate five-level primitive equation spheric band (70°N-30°S)model with the Ural high's effect introduced into it as initial and boundary conditions, study is made of the high's influence on Asian summer monsoon circulation and dryness / wetness of eastern China based on case contrast andcontrol experiments. Rusults show that as an excitation source, the blocking high produces a SE-NW stationarywavetrain with its upper-air atnicyclonic divergent circulation oust over a lower-level trough zone) precisely over themiddle to lower reaches of the Changjiang River, enhancing East Asian westerly jet, a situation that contributes toPerturbation growth, causing an additional secondary meridional circulation at the jet entrance, which intensifies theupdraft in the monsoon area. As such, the high's presence and its excited steady wavetrain represent the large-scalekey factors and acting mechanisms for the rainstorm over the Changjiang-Huaihe River catchment in the easternpart of the land.

Mechanism and Forecasting Methods for Severe Droughts and Floods in Songhua River Basin in China

The influence of various factors, mechanisms, and principles affecting runoff are summarized as periodic law, random law, and basin-wide law. Periodic law is restricted by astronomical factors, random law is restricted by atmospheric circulation, and basin-wide law is restricted by underlying surface. The commensurability method was used to identify the almost period law, the wave method was applied to deducing the random law, and the precursor method was applied in order to forecast runoff magnitude for the current year. These three methods can be used to assess each other and to forecast runoff. The system can also be applied to forecasting wet years, normal years and dry years for a particular year as well as forecasting years when floods with similar characteristics of previous floods, can be expected. Based on hydrological climate data of Baishan (1933-2009) and Nierji (1886-2009) in the Songhua River Basin, the forecasting results for 2010 show that it was a wet year in the Baishan Reservoir, similar to the year of 1995; it was a secondary dry year in the Nierji Reservoir, similar to the year of 1980. The actual water inflow into the Baishan Reservoir was 1.178 × 10 10 m 3 in 2010, which was markedly higher than average inflows, ranking as the second highest in history since records began. The actual water inflow at the Nierji station in 2010 was 9.96 × 10 9 m 3 , which was lower than the average over a period of many years. These results indicate a preliminary conclusion that the methods proposed in this paper have been proved to be reasonable and reliable, which will encourage the application of the chief reporter release system for each basin. This system was also used to forecast inflows for 2011, indicating a secondary wet year for the Baishan Reservoir in 2011, similar to that experienced in 1991. A secondary wet year was also forecast for the Nierji station in 2011, similar to that experienced during 1983. According to the nature of influencing factors, mechanisms and forecasting methods and the service objects, mid-to long-term hydrological forecasting can be divided into two classes:mid-to long-term runoff forecasting, and severe floods and droughts forecasting. The former can be applied to quantitative forecasting of runoff, which has important applications for water release schedules. The latter, i.e., qualitative disaster forecasting, is important for flood control and drought relief. Practical methods for forecasting severe droughts and floods are discussed in this paper.

Causes and Predictability of the 2021 Spring Southwestern China Severe Drought

In the spring of 2021,southwestern China(SWC)experienced extreme drought,accompanied by the highest seasonal-mean temperature record since 1961.This drought event occurred in the decaying phase of a La Niña event with negative geopotential height anomalies over the Philippine Sea,which is distinct from the historical perspective.Historically,spring drought over SWC is often linked to El Niño and strong western North Pacific subtropical high.Here,we show that the extreme drought in the spring of 2021 may be mainly driven by the atmospheric internal variability and amplified by the warming trend.Specifically,the evaporation increase due to the high temperature accounts for about 30%of drought severity,with the contributions of its linear trend portion being nearly 20%and the interannual variability portion being about 10%.Since the sea surface temperature forcing from the tropical central and eastern Pacific played a minor role in the occurrence of drought,it is a challenge for a climate model to capture the 2021 SWC drought beyond one-month lead times.

Spatial-Temporal Variations of Dominant Drought/Flood Modes and the Associated Atmospheric Circulation and Ocean Events in Rainy Season over the East of China

By using Season-reliant Empirical Orthogonal Function (S-EOF) analysis, three dominant modes of the spatial-temporal evolution of the drought/flood patterns in the rainy season over the east of China are revealed for the period of 1960-2004. The first two leading modes occur during the turnabout phase of El Nino-Southern Oscillation (ENSO) decaying year, but the drought/flood patterns in the rainy season over the east of China are different due to the role of the Indian Ocean (IO). The first leading mode appears closely correlated with the ENSO events. In the decaying year of El Nino, the associated western North Pacific (WNP) anticyclone located over the Philippine Sea persists from the previous winter to the next early summer, transports warm and moist air toward the southern Yangtze River in China, and leads to wet conditions over this entire region. Therefore, the precipitation anomaly in summer exhibits a 'Southern Flood and Northern Drought' pattern over East China. On the other hand, the basin-wide Indian Ocean sea surface temperature anomaly (SSTA) plays a crucial role in prolonging the impact of ENSO on the second mode during the ENSO decaying summer. The Indian Ocean basin mode (IOBM) warming persists through summer and unleashes its influence, which forces a Matsuno-Gill pattern in the upper troposphere. Over the subtropical western North Pacific, an anomalous anticyclone forms in the lower troposphere. The southerlies on the northwest flank of this anticyclone increase the moisture transport onto central China, leading to abundant rainfall over the middle and lower reaches of the Yangtze River and Huaihe River valleys. The anomalous anticyclone causes dry conditions over South China and the South China Sea (SCS). The precipitation anomaly in summer exhibits a 'Northern Flood and Southern Drought' pattern over East China. Therefore, besides the ENSO event the IOBM is an important factor to influence the drought/flood patterns in the rainy season over the east of China. The third mode is positively correlated with the tropical SSTA in the Indian Ocean from the spring of preceding year(-1) to the winter of following year(+1), but not related to the ENSO events. The positive SSTA in the South China Sea and the Philippine Sea persists from spring to autumn, leading to weak north-south and land-sea thermal contrasts, which may weaken the intensity of the East Asia summer monsoon. The weakened rainfall over the northern Indian monsoon region may link to the third spatial mode through the 'Silk Road' teleconnection or a part of circumglobal teleconnection (CGT). The physical mechanisms that reveal these linkages remain elusive and invite further investigation.

Causes of a Typical Southern Flood and Northern Drought Event in 2015 over Eastern China

The spatial distribution of summer precipitation anomalies over eastern China often shows a dipole pattern,with anti-phased precipitation anomalies between southern China and northern China,known as the“southern flooding and northern drought”(SF-ND)pattern.In 2015,China experienced heavy rainfall in the south and the worst drought since 1979 in the north,which caused huge social and economic losses.Using reanalysis data,the atmospheric circulation anomalies and possible mechanisms related to the summer precipitation anomalies in 2015 were examined.The results showed that both El Niño and certain atmospheric teleconnections,including the Pacific Japan/East Asia Pacific(PJ/EAP),Eurasia pattern(EU),British–Baikal Corridor pattern(BBC),and Silk Road mode(SR),contributed to the dipole pattern of precipitation anomalies.The combination of these factors caused a southwards shift of the western Pacific subtropical high(WPSH)and a weakening of the East Asian summer monsoon.Consequently,it was difficult for the monsoon front and associated rain band to migrate northwards,which meant that less precipitation occurred in northern China while more precipitation occurred in southern China.This resulted in the SF-ND event.Moreover,further analysis revealed that global sea surface temperature anomalies(SSTAs)or sea-ice anomalies were key to stimulating these atmospheric teleconnections.

Studies on the Northern Early Summer Teleconnection Patterns, Their Interannual Variations and Relation to Drought / Flood in China

By using the one-point correlation method, calculations have been made of the northern early summer 500 hPa teleconnection patterns. Seven teleconnection patterns are revealed, namely, the Western Atlantic (WA), the Eastern Atlantic (EA), the Eurasian (EU), the Bengal / Northern Pacific (BNP), the Western Pacific (WP), the East Asian / Pacific (EAP), and the Huanghe / East Asian (HEA) patterns. Their centers are determined and their yearly intensity indices (1951-1990) are calculated. On this basis the relationship between their interannual variations and the drought / flood in China is examined. It is noted that the EU, HEA and EAP wave trains are closely related to the drought / flood in China. The HEA and EAP patterns strongly influence the precipitation in eastern China. For example, the fierce floods experienced in 1991 early summer over China are related to the weak EAP and strong HEA patterns.

THE PRELIMINARY STUDY ON POSSIBLE SCENARIOS OF FLOOD AND DROUGHT IN CHINA IN THE CASE OF GLOBAL WARMING

According to Prof. Zhu Kezhen’s(Chu K.C.)historical climatic division,the last 500 years in China can be divided into several alternately cold and warm periods.The periods of 1470-1520,1620-1720,1840-1890 had cold winters,while those of 1550-1600,1770-1830 had warm winters.Based on such division,in four kinds of periods,i.e.cold, warm,cold-warm,and warmcold (transition period),the differences between flood/drought degree in 120 stations in China and average of flood/drought degree in the last 500 years have been calculated. Positive anomaly indicates drought-prone area,while negative anomaly indicates flood-prone area. This historical experience provides a background to analyze the possible scenarios in the case of global warming in the future.The final results suggest that in the case of global warming the hazards of flood probably increase in many parts of China,such as southeast coast area,southwest,northwest, some parts of northeast and inner Mongolia while the hazards of drought probably decrease in the North China Plain,the middle reaches of the Huanghe River and its southern adjacent area. This distribution is basically consistent with that of precipitation in warming periods in this century and that resulted from climatic model in the case of CO2 doubling.

RELATIONSHIP BETWEEN THE 30 TO 60 DAY OSCILLATION OF ATMOSPHERIC HEAT SOURCE AND THE DROUGHT AND FLOOD EVENTS IN JUNE IN THE SOUTH OF CHINA

Based on the NCEP/NCAR reanalysis data and the observed precipitation data in the south of China from 1958 to 2000,the impact of 30 to 60 day oscillation of atmospheric heat sources on the drought and flood events in June in the south of China is discussed.During the flood(drought) events,there exists an anomalous low-frequency anticyclone(cyclone) at the low level of the troposphere over the South China Sea and the northwestern Pacific,accompanied with anomalous low-frequency heat sinks(heat sources),while there exists an anomalous low-frequency cyclone(anticyclone) with anomalous heat sources(sinks) over the area from the south of China to the south of Japan.On average,the phase evolution of the low-frequency in drought events is 7 to 11 days ahead of that in flood events in May to June in the south of China.In flood events,low-frequency heat sources and cyclones are propagated northward from the southern South China Sea,northwestward from the warm pool of the western Pacific and westward from the northwestern Pacific around 140°E,which have very important impact on the abundant rainfall in June in the south of China.However,in drought events,the northward propagations of the low-frequency heat sources and cyclones from the South China Sea and its vicinity are rather late compared with those in flood events,and there is no obvious westward propagation of the heat sources from the northwestern Pacific.The timing of the low-frequency heat source propagation has remarkable impact on the June rainfall in the south of China.

PRELIMINARY STUDY ON THE FLOODING AND DROUGHT CALAMITY DURING PAST 1500 YEARSIN THE HAI'AN REGION, JIANGSU PROVINCE

Collection and arrangement of the historical records of climatic changes andenvironment evolution, especially in the aspect of calamities, are made on the history documents ofpast 1500 years about Hai'an region, Jiangsu Province. There existed two obvious flooding-droughtfrequently-occurring periods: one was from 1550 AD to 1850 AD and another was 1000 AD to 1200 AD.The period of 1550 AD to 1850 AD is interrupted by two relatively arid and cold climatic periods:one was from 1630 AD to 1700 AD and another was 1750 AD to 1820 AD. The main characteristic of thecalamity periods is that they occurred by turns, and sometimes, both drought and flooding occurredin the same year. The instability of the climatic changes in the Little Ice Age may be the mainreason of the frequently-occurring flooding and drought in Hai'an region. Research results also showthat the frequently-occurring periods of flooding and drought is in close relationship with thesolar activity, and therefore, occurrence of the flooding and drought may be in relation with theintensity of the solar activity. This hypothesis may need further study in the future.

Dryness-wetness change and regional differentiation of flood-drought disasters in Guangdong during 1480-1940AD

Based on the historical records of the drought and flood disasters during 1480-1940AD, this paper reconstructs the sequences of wetness index （WI） and drought and flood disasters. We find a good identical relationship between the fluctuation of WI sequence and the δ^18O record of the GISP2 ice core in Greenland, which shows an apparent monsoonal disposal pattern of moisture and temperature. By applying the Morlet Wavelet Transform Method to deal with the data, several apparent periodicities, such as 7-8a, 11-15a, 20-23a and ca 50a, are revealed, among which some can be attributed to the solar forcing. Based on results of Cluster Analysis of dry-wet changes, we resume the regional differentiation pattern of flood-drought disasters all over Guangdong during different climatic intervals in the LIA and, find the western and northern parts of Guangdong have undergone drastic changes in drought-flood regional differentiation, but the eastern part is relatively stable, and the area of the Pearl River Delta shows stable condition of more flood disasters.

THE RELATIONSHIP BETWEEN THE ATMOSPHERIC HEATING SOURCE/SINK ANOMALIES OF ASIAN MONSOON AND FLOOD/DROUGHT IN THE YANGTZE RIVER BASIN IN THE MEIYU PERIOD

NCEP/NCAR reanalysis data and a 30-year precipitation dataset of observed daily rainfall from 109 gauge stations are utilized in this paper.Using the REOF we analyzed the spatial distribution of precipitation in the 109 stations in the Yangtze River Basin in Meiyu periods from 1978 to 2007.The result showed that the spatial distribution of precipitation in the Yangtze River Basin can be divided into the south and north part.As a result,relationships between an atmospheric heating source(hereafter called <Q_1>) over the Asian region and the precipitation on the south and north side of Yangtze River in Meiyu periods were separately studied in this paper.The results are shown as follows.The flood/drought to the north of Yangtze River(NYR) was mainly related to the <Q_1> over the East Asia summer monsoon region:when the <Q_1> over the Philippines through Western Pacific and the south China was weakened(strengthened),it would probably result in the flood(drought) in NYR;and the precipitation on the south side of Yangtze River(SYR)was related to the <Q_1> over the east Asia and Indian summer monsoon region:when the <Q_1> over the areas from south China to the northern East China Sea and Yellow Sea and south-eastern Japan was strengthened(weakened),and the <Q_1> over the areas from the Bay of Bengal to south-eastern Tibetan Plateau was weakened(strengthened),it will lead to flood(drought) in SYR.