Toward Understanding the Extreme Floods over Yangtze River Valley in June?July 2020: Role of Tropical Oceans※

The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June?July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Ni?o in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June?July 2020 reached its highest value over the last 40 years, and 43％ (57％) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.     

Seasonal Rainfall Forecasts for the Yangtze River Basin in the Extreme Summer of 2020※

Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ), and June–July–August (JJA) 2020 are presented, based on the Met Office GloSea5 system. The three-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least three months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95％ prediction intervals of the three-month forecasts. The forecasts presented here are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced mei-yu front in early summer is skillfully forecast, but the impact of midlatitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region and the potential limitations in anticipating complex extreme events driven by a combination of coincident factors.     

Subseasonal and Synoptic Variabilities of Precipitation over the Yangtze River Basin in the Summer of 2020※

Summer precipitation over the Yangtze River basin (YRB) in 2020 experienced a strong subseasonal and synoptic fluctuation in addition to contributing to an exceptionally large seasonal mean precipitation. The cause of this higher-frequency fluctuation is examined based on observational analyses. Apart from the continuous northward movement of the climatological mei-yu rainband, the mei-yu rainbelt in the summer of 2020 experienced multiple northward and southward swings. The cause of the swings was attributed to the subseasonal variability of southerly winds to the south and northeasterly winds to the north of the YRB. In addition, synoptic-scale variability, characterized by the eastward propagation of low-level cyclonic vorticity and precipitation anomalies, was also commonplace in the summer of 2020. While the strengthening of both the subseasonal and synoptic variabilities in the summer of 2020 was attributed to the increase of the background mean moisture, the synoptic variability was greatly affected by the subseasonal rainfall variability. As a result, both the synoptic-scale and subseasonal variabilities contributed to the north-south swings of the rainbelt. The large-scale modulations by both the seasonal mean and subseasonal anomalies provide insight regarding the optimization of issuing accurate, extended-range forecasts of extreme weather events.     

The Extreme Mei-yu Season in 2020: Role of the Madden-Julian Oscillation and the Cooperative Influence of the Pacific and Indian Oceans※

The middle and lower reaches of the Yangtze River in eastern China during summer 2020 suffered the strongest mei-yu since 1961. In this work, we comprehensively analyzed the mechanism of the extreme mei-yu season in 2020, with focuses on the combined effects of the Madden-Julian Oscillation (MJO) and the cooperative influence of the Pacific and Indian Oceans in 2020 and from a historical perspective. The prediction and predictability of the extreme mei-yu are further investigated by assessing the performances of the climate model operational predictions and simulations. 　 It is noted that persistent MJO phases 1?2 during June?July 2020 played a crucial role for the extreme mei-yu by strengthening the western Pacific subtropical high. Both the development of La Ni?a conditions and sea surface temperature (SST) warming in the tropical Indian Ocean exerted important influences on the long-lived MJO phases 1?2 by slowing down the eastward propagation of the MJO and activating convection related to the MJO over the tropical Indian Ocean. The spatial distribution of the 2020 mei-yu can be qualitatively captured in model real-time forecasts with a one-month lead. This can be attributed to the contributions of both the tropical Indian Ocean warming and La Ni?a development. Nevertheless, the mei-yu rainfall amounts are seriously underestimated. Model simulations forced with observed SST suggest that internal processes of the atmosphere play a more important role than boundary forcing (e.g., SST) in the variability of mei-yu anomaly, implying a challenge in quantitatively predicting an extreme mei-yu season, like the one in 2020.     

Characteristics of Two General Circulation Patterns During Floods over the Changjiang-Huaihe River Valley

Characteristics of the atmospheric general circulation during the catastrophic floods over the Changjiang- Huaihe River Valley(CHRV)are investigated.There are two precipitation patterns over China in the CHRV flood years:the CHRV flood-whole country-wet(P1)pattern and the CHRV flood-south(north)side-dry (P2)pattern.The circulation analysis results show that there are obvious differences between the NH 500- hPa geopotential height fields of P1 and P2 precipitation patterns.The establishment of East Asia-Atlantic (EAA)correlation chain(the South China Sea(SCS)high-the Meiyu trough-the Okhotsk Sea high over East Asia)is a critical condition for excessive summer precipitation over the CHRV,while the European blocking high plays an important role in determining the precipitation pattern over China in the CHRV flood years.Besides,the relation between the EAA correlation chain and the sea surface temperature anomaly (SSTA)in the North Pacific is also studied.     

Moisture Origins and Transport Processes for the 2020 Yangtze River Valley Record-Breaking Mei-yu Rainfall※

The summer of 2020 recorded a record-breaking flood due to excessive mei-yu rain falling over the Yangtze River Valley (YRV). Using the Lagrangian model FLEXPART, this paper investigates moisture sources and transport processes behind this extreme event. Based on climate data from 1979 to 2019, the air-particle (an infinitesimally small air parcel) trajectories reaching the YRV show sectors that correspond to five main moisture sources: the Indian monsoon region (IND, 27.5％ of the total rainfall), the local evaporation (27.4％), the Western Pacific Ocean (WPO, 21.3％), the Eurasian continent (8.5％) and Northeast Asia (4.4％). In the 2020 mei-yu season, moisture from all source regions was above normal except that from Northeast Asia. A record-breaking moisture source from the IND and WPO dominated this extreme mei-yu flood in 2020, which was 1.5 and 1.6 times greater than the climate mean, respectively. This study reveals a significant relationship between the moisture source with three moisture transport processes, i.e., trajectory density, moisture content, and moisture uptake of air-particles. A broad anomalous anticyclonic circulation over the Indo-Northwestern Pacific (Indo-NWP) provides a favorable environment to enhance the moisture transport from the IND and WPO into the YRV. In the 2020 mei-yu season, a record-breaking Indo-NWP anomalous anticyclonic circulation contributed to a higher trajectory density as well as higher moisture content and moisture uptake of air-particles from the IND and WPO regions. This collectively resulted in unprecedented moisture transport from source origins, thus contributing to the mei-yu flood over the YRV in 2020.     

Analysis of Simulated Heavy Rain over the Yangtze River Valley During 11-30 June 1998 Using RIEMS

RIEMS‘ ability to simulate extreme monsoon rainfall is examined using the 18-month (April 1997 September 1998) integrated results. Model-simulated heavy precipitation over the Yangtze River valley during 11-30 June 1998 is compared with the observation, and the relationships between this heavy rainfall process and the large-scale circulations, such as the westerly jet, low-level jet, and water vapor transport,are analyzed to further understand the mechanisms for simulating heavy monsoon rainfall. The analysis results show that (1) RIEMS can reproduce the pattern of heavy precipitation over the Yangtze River valley during 11-30 June 1998, but it is shifted northwestwards. (2) The simulated West Pacific Subtropical High (WPSH) that controls the East Asia Monsoon evolution is stronger than the observation and is extended westwards, which possibly leads to the north westward shift of the heavy rain belt. (3) The Westerly jet at 200 hPa and the Low-level jet at 850 hPa, both of which are related to the heavy monsoon rainfall,are reasonably reproduced by RIEMS during 11-30 June 1998~ although the intensities of the simulated Westerly/Low-level jets are strong and the location of the Westerly jet leans to the southeast,which may be the causes of RIEMS producing too much heavy rainfall in the north of the Yangtze River valley.     

Simulation and Analysis about the Effects of Geopotential Height Anomaly in Tropical and Subtropical Region on Droughts or Floods in the Yangtze River Valley and North China

Previous study comes to the conclusion:based on the anomalies of the South Asian high (SAH),100-hPa geopotential height,and 100-hPa circulation over tropical and subtropical regions,we can predict precipita- tion anomaly in the Yangtze River Valley and North China.To test its validity,a series of experiments have been designed and operated,which include controlled experiment,sensitivity experiment (which has added anomalies into 100-hPa geopotential height and wind field),and four-composite experiments.Experiments based on the composed initial field such as EPR-CF,EPR-CD,EPR-HF,and EPR-HD,can reproduce the floods or droughts in the Yangtze River Valley and North China.It suggests that anomalies of the SAH,100- hPa geopotential height,and circulation over tropical and subtropical regions may probably imply summer precipitation anomalies in the two regions.Sensitivity experiment results show that anomalies of the SAH, 100-hPa geopotential height,and southwest flow in the previous period is a signal of droughts or floods for the following summer in the Yangtze River Valley and North China.And it is also one of the factors that have impact on summer precipitation anomaly in the two regions.Positive anomaly of 100-hPa geopotential height and the anomalous intensifying of the SAH and southwest flow will induce floods in the Yangtze River Valley and droughts in North China;while negative anomaly of 100-hPa geopotential height and anomalous weakening of the SAH and southwest flow will induce droughts in the Yangtze River Valley and floods in North China.     

Magnitude,Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China※

Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12?16 June and 4?8 July 2020) are selected from this period for dynamical characterization, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.     

Typical Circulation Patterns and Associated Mechanisms for Persistent Heavy Rainfall Events over Yangtze–Huaihe River Valley during 1981–2020※

Persistent heavy rainfall events (PHREs) over the Yangtze–Huaihe River Valley (YHRV) during 1981–2020 are classified into three types (type-A, type-B and type-C) according to pattern correlation. The characteristics of the synoptic systems for the PHREs and their possible development mechanisms are investigated. The anomalous cyclonic disturbance over the southern part of the YHRV during type-A events is primarily maintained and intensified by the propagation of Rossby wave energy originating from the northeast Atlantic in the mid–upper troposphere and the northward propagation of Rossby wave packets from the western Pacific in the mid–lower troposphere. The zonal propagation of Rossby wave packets and the northward propagation of Rossby wave packets during type-B events are more coherent than those for type-A events, which induces eastward propagation of stronger anomaly centers of geopotential height from the northeast Atlantic Ocean to the YHRV and a meridional anomaly in geopotential height over the Asian continent. Type-C events have "two ridges and one trough" in the high latitudes of the Eurasian continent, but the anomalous intensity of the western Pacific subtropical high (WPSH) and the trough of the YHRV region are weaker than those for type-A and type-B events. The composite synoptic circulation of four PHREs in 2020 is basically consistent with that of the corresponding PHRE type. The location of the South Asian high (SAH) in three of the PHREs in 2020 moves eastward as in the composite of the three types, but the position of the WPSH of the four PHREs is clearly westward and northward. Two water vapor conveyor belts and two cold air conveyor belts are tracked during the four PHREs in 2020, but the water vapor path from the western Pacific is not seen, which may be caused by the westward extension of the WPSH.     

Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Ni ?no Events?—Role of Midlatitude Circulation in August

It is widely recognized that rainfall over the Yangtze River valley(YRV) strengthens considerably during the decaying summer of El Ni ?no,as demonstrated by the catastrophic flooding suffered in the summer of 1998.Nevertheless,the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998,despite the intensity of the 2016 El Nio having been as strong as that in 1998.A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years,principally in August.The precipitation anomaly was negative in August 2016—different to the positive anomaly of 1998.Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes.The intensified "Silk Road Pattern" and upper-tropospheric geopotential height over the Urals region,both at their strongest since 1980,resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere.This easterly flow reduced the climatological wind,weakened the water vapor transport,and induced the weaker YRV rainfall in August 2016,as compared to that in 1998.Given the unique sub-seasonal variation of the YRV rainfall in summer 2016,more attention should be paid to midlatitude circulation—besides the signal in the tropics—to further our understanding of the predictability and variation of YRV summer rainfall.     

Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Niño Events?——Role of Midlatitude Circulation in August

It is widely recognized that rainfall over the Yangtze River valley (YRV) strengthens considerably during the decaying summer of El Niño, as demonstrated by the catastrophic flooding suffered in the summer of 1998. Nevertheless, the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998, despite the intensity of the 2016 El Niño having been as strong as that in 1998. A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years, principally in August. The precipitation anomaly was negative in August 2016——different to the positive anomaly of 1998. Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes. The intensified "Silk Road Pattern" and upper-tropospheric geopotential height over the Urals region, both at their strongest since 1980, resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere. This easterly flow reduced the climatological wind, weakened the water vapor transport, and induced the weaker YRV rainfall in August 2016, as compared to that in 1998. Given the unique sub-seasonal variation of the YRV rainfall in summer 2016, more attention should be paid to midlatitude circulation——besides the signal in the tropics——to further our understanding of the predictability and variation of YRV summer rainfall.     

The Extraordinary Rainfall over the Eastern Periphery of the Tibetan Plateau in August 2020※

A large amount of accumulated precipitation was recorded over the Eastern Periphery of the Tibetan Plateau (EPTP) in August 2020. Using hourly rain gauge records and the ERA5 reanalysis dataset, we analyzed the unique characteristics of rainfall in August and the accompanying circulation conditions and conducted a comparison with previous data. This record-breaking amount of accumulated rainfall was centered on the northern slope of the EPTP. This location was in contrast with the historical records of the concentration of rainfall over the middle and southern slopes. The hourly rainfall in August 2020 was both more frequent and more intense than the climatological mean rainfall. An amplification effect of the topography was observed, with the precipitation over the EPTP showing a more significant change with terrain height in August 2020. A circulation analysis showed that cold (warm) anomalies existed over the north (south) of approximately 35°N compared with those in the years when the southern EPTP received more rain. The western Pacific subtropical high was more intense and extended to the west, and the low-level cold air from the north was more active. The enhanced low-level southerly winds on the periphery of the subtropical high injected warm, moist air further north than the climatological mean. These winds became easterly near the northern EPTP and were forced to ascend by the steep terrain.     

Synchronous Characteristics of Precipitation Extremes in the Yangtze and Murray–Darling River Basins and the Role of ENSO

The floods caused by the extreme precipitation in the Yangtze River basin(YRB) and Murray–Darling River basin(MDRB), the largest basins in China and Australia, have significant impacts on the society and regional economies.Based on the spatial–temporal analysis of the daily precipitation extremes(DPEs) during 1982–2016, we found that for both basins, the whole-basin-type DPEs have the highest proportion and a synchronous DPE interannual variation characteristic exists in the two basins, with the 3-yr running correlation coefficient of the annual DPE days(DPEDs) reaching almost 0.7(significant at the 0.01 level). The El Ni?o–Southern Oscillation(ENSO), which is one of the most significant climate disturbance factors in the world, plays an important role in modulating the variability of the DPEs in the two basins. Singular value decomposition(SVD) analysis revealed that both the YRB and the MDRB's whole-basin-type DPEs are closely coupled with the procedure that the preceding winter eastern Pacific(EP)-type El Ni?o faded to a central Pacific(CP)-type La Ni?a. This means that the DPEs in the YRB and MDRB may synchronously occur more frequently when the above process occurs. Owing to the atmosphere–ocean interaction from the east–west dipole sea surface temperature(SST) anomaly pattern, the atmospheric circulation disturbance exhibits a pattern in which the equatorial eastern Pacific region is a mass source anomaly with a higher pressure,drier air, and weaker convection, while the equatorial western Pacific region is a mass sink anomaly with a lower pressure, wetter air, and stronger convection. Moreover, two wave trains that originated from the tropical western Pacific were found to extend to the YRB and MDRB. The interaction between the wave train's interphase dynamics and water vapor transport disturbance results in the ascent conditions and enhanced water vapor transport, which leads to the synchronous occurrence of DPEs in the YRB and MDRB on an interannual scale.     

How Frequently Will the Persistent Heavy Rainfall over the Middle and Lower Yangtze River Basin in Summer 2020 Happen under Global Warming?

The middle and lower Yangtze River basin (MLYRB) suffered persistent heavy rainfall in summer 2020, with nearly continuous rainfall for about six consecutive weeks. How the likelihood of persistent heavy rainfall resembling that which occurred over the MLYRB in summer 2020 (hereafter 2020PHR-like event) would change under global warming is investigated. An index that reflects maximum accumulated precipitation during a consecutive five-week period in summer (Rx35day) is introduced. This accumulated precipitation index in summer 2020 is 60% stronger than the climatology, and a statistical analysis further shows that the 2020 event is a 1-in-70-year event. The model projection results derived from the 50-member ensemble of CanESM2 and the multimodel ensemble (MME) of the CMIP5 and CMIP6 models show that the occurrence probability of the 2020PHR-like event will dramatically increase under global warming. Based on the Kolmogorov–Smirnoff test, one-third of the CMIP5 and CMIP6 models that have reasonable performance in reproducing the 2020PHR-like event in their historical simulations are selected for the future projection study. The CMIP5 and CMIP6 MME results show that the occurrence probability of the 2020PHR-like event under the present-day climate will be double under lower-emission scenarios (CMIP5 RCP4.5, CMIP6 SSP1-2.6, and SSP2-4.5) and 3–5 times greater under higher-emission scenarios (3.0 times for CMIP5 RCP8.5, 2.9 times for CMIP6 SSP3-7.0, and 4.8 times for CMIP6 SSP5-8.5). The inter-model spread of the probability change is small, lending confidence to the projection results. The results provide a scientific reference for mitigation of and adaptation to future climate change.     

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

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Changes in Summer Persistent Precipitation over the Middle–Lower Reaches of the Yangtze River and Associated Atmospheric Circulation Patterns

Persistence is an important property of precipitation and its related impacts. However, changes in persistent precipitation and the possible underlying mechanisms in the context of global warming have not yet been discussed in sufficient depth. In this study, the changes in persistent precipitation in summer and related atmospheric circulation patterns over the middle–lower reaches of the Yangtze River(MLRYZR)—a typical monsoon region frequently hit by consecutive rainfall events—are analyzed based on observed daily precipitation and NCEP/NCAR reanalysis data from 1961 to 2019. The results reveal that persistent precipitation events(PPs) tend to happen in a more persistent way, with increased frequency and intensity in the MLRYZR region. Mechanism analyses show that persistent precipitation has happened along with simultaneous enhancement of some large-scale atmospheric circulation patterns,including the Lake Baikal blocking(BB), the Okhotsk Sea blocking(OB), and the western Pacific subtropical high(WPSH). Such enhanced anomalous circulation patterns could persistently reinforce the convergence and supply of water vapor in the MLRYZR region, contributing to the increase in PPs in this region. Based on the above results, we are able to offer some new insights into the long-term changes in precipitation structure and the possible causes. This study is also expected to support attribution studies on regional precipitation changes in the future.     

The Linkage between Two Types of El Ni?o Events and Summer Streamflow over the Yellow and Yangtze River Basins

It is generally agreed that El Ni?o can be classified into East Pacific(EP) and Central Pacific(CP) types. Nevertheless,little is known about the relationship between these two types of El Ni?o and land surface climate elements. This study investigates the linkage between EP/CP El Ni?o and summer streamflow over the Yellow and Yangtze River basins and their possible mechanisms. Over the Yellow River basin, the anomalous streamflow always manifests as positive(negative)in EP(CP) years, with a cor...