Analysis of climatic features and major meteorological disasters over the Three Gorges Region of the Yangtze River Basin in 2021

Tong Cui,Xianyan Chen ,Xukai Zou,Qiang Zhang,Hongling Zeng,Linhai Sun

National Climate Center, China Meteorological Administration, Beijing, China

Keywords:Three Gorges Region Climatic anomaly Weather and climate events Extremely warm early autumn

ABsTRACT Based on daily observation data in the Three Gorges Region (TGR) of the Yangtze River Basin and global reanalysis data,the authors analyzed the climate characteristics and associated temporal variations in the main meteorological factors in 2021,as well as the year’s climatic events and meteorological disasters.The 2021 average temperature was 0.2°C above the 1991—2020 average and the 13th-warmest year since 1961.Seasonally,winter and autumn were both warmer than usual.The annual mean precipitation was 12.8% above normal,and most regions experienced abundant rainfall throughout the year.The seasonal variation in precipitation was significant and the TGR had a wetter-than-normal spring and summer.The number of rainstorm days was higher than normal;the wind speed was above normal;and the relative humidity was higher than normal.In terms of rain acidity,2021 was tied with 2020 as the lowest since 1999.From mid-September to early October 2021,the TGR experienced exceptional high-temperature weather,which was driven by abnormal activity of mid-and high-latitude atmospheric circulation over the Eurasian continent and the western Pacific subtropical high (WPSH).In addition,a strong blocking high over the Ural Mountains accompanied by intense mid-latitude westerly winds prevented cyclonic disturbances from extending to the subtropical region.As a result,under the combined effect of the weaker-than-normal cold-air activities and the anomalous WPSH,the TGR experienced extreme high-temperature weather during early autumn 2021.

1.Introduction

As a key project for the governance and protection of the Yangtze River,the Three Gorges Hydropower Complex Project,located at the junction of the Sichuan Basin and the middle and lower reaches of the Yangtze River,is the world’s largest water conservancy and hydropower project,providing numerous benefits.It was mainly built to ensure the safety of people living in the middle and lower reaches of the Yangtze River Basin in the flood seasons,and has functions including power generation,shipping,water supply,and ecological conservation (Zhou et al.,2017).Studies have shown that climate anomalies are the most important natural factors affecting the operation of the Three Gorges Project.Especially in recent years,due to global warming,extreme climate events such as extreme precipitation,high temperatures and regional drought have occurred frequently;and these events themselves as well as associated geological disasters represent a challenge to the safe operation and administration of the Three Gorges Project(Zhang et al.,2000 ;Deng et al.,2011 ;Yang et al.,2017 ;Chai et al.,2019 ;Zhu et al.,2021).

For a long time,scientists have used observation,remote sensing,and reanalysis data to study the climatic features and regulations of climate change in the TGR.These results help people understand the characteristics of climate evolution in the Yangtze River Basin (Zeng et al.,2009 ;Guo et al.,2011 ;Chen et al.,2015,2021 ;Song et al.,2017 ;Sun and Liu,2018 ;Zhang et al.,2020 ;Cui et al.,2022).In addition,the climate effect created by the Three Gorges Reservoir also receives considerable attention.Lin (1985) was the first to estimate the trends of temperature and precipitation variation after the reservoir’s completion,by comparing the differences between land and lake climate types.Since then,with the development of numerical models,numerous scientists have used regional climate or mesoscale models to simulate the climate effects of the Three Gorges Reservoir (Zhang et al.,2004 ;Miller et al.,2005 ;Wu et al.,2011 ;Li et al.,2019),which has led to a deeper understanding of the climate characteristics and changes in the TGR.

Fig.1.Annual (a) temperature anomaly (units:°C;1991—2020 base period) and (b) precipitation anomaly (units: %) in the TGR (black curve),UR (red curve),and SWC (blue curve) from 1961 to 2021.(c) Mean precipitation pH and (d) precipitation pH for six meteorological stations from 1999 to 2021.

The climate in the TGR is closely related to the variations in the East Asian monsoon and corresponding atmospheric circulations,as well as being affected by ENSO,sea ice,and other factors,resulting in substantial interannual variability and a complex spatial distribution across the region (Huang and Huang,2012 ;Li et al.,2012 ;Kuang et al.,2014 ;Xu et al.,2016 ;Li and Lu,2017 ;He et al.,2018 ;Shen et al.,2019).This paper provides climate information on the TGR for 2021,with an analysis of climate monitoring,disastrous events,and possible causes of climate anomalies.Section 2 introduces the data and methods;Sections 3 and 4 report the features of the main meteorological elements,the weather,and the climate disasters that took place,along with their impacts;Section 5 gives a brief analysis of the causes of the abnormal high-temperature events in autumn;and Section 6 sets out our conclusions.

2.Data and methods

Utilized in this paper are the daily observational data from 33 national meteorological stations in the TGR (see Chen et al.,2021 for details about their locations) from 1961 to 2021,including temperature,precipitation,wind speed,fog,relative humidity,and other variables.In order to compare the regional climate state of the TGR with the surrounding areas like Southwest China (SWC) and the upper regions of the Yangtze River Basin (UR),we also used the data from a total of 716 stations in the aforementioned two regions.These data were provided by the National Meteorological Information Center of the China Meteorological Administration,Chongqing City Meteorological Bureau,and Hubei Province Meteorological Bureau.The atmospheric circulation data we used were from the ERA5 global reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts,including data on the daily mean zonal wind at 200 hPa,the geopotential height field at 500 hPa,and tropical convection (as measured by outgoing longwave radiation (OLR)).The calculation of the western Pacific subtropical high (WPSH) index follows that of Liu et al.(2012).Unless otherwise noted,the average values in the TGR indicate the arithmetic average for all 33 stations,and anomalies are relative to the “normal period ”of 1991—2020.

3.Main climatic features

3.1.Temperature

In 2021,the annual mean temperature was 17.6 °C in the TGR,which was 0.2 °C above the climatology of 1991—2020 and the 13thwarmest year since 1961 (Fig.1 (a)).Before (1961—2003) and during(2004—2010) the period of experimental impounding,average temperatures were 17.1°C and 17.6 °C respectively,with a consistent long-term upward trend.The mean annual temperature was 17.5 °C (0.3 °C above the 1961—2010 average) since normal impoundment in 2011,with 6 of the 20 warmest years having occurred during this period.Except for 2011,2012,2014,and 2020,in which the air temperature was below normal,the average annual temperature in each of the remaining seven years was above or close to normal.The correlation coefficient of the annual temperature anomaly between the TGR and UR (TGR and SWC)was 0.97 (0.84),both exceeding the 99% confidence level.Generally,the decadal and interannual fluctuations of the annual mean temperature anomaly in the TGR were relatively consistent with those in UR and SWC.However,the positive temperature anomalies in the aforementioned two regions have been slightly higher than those in the TGR since 2014.

In most of the eastern and the mid-south area of the TGR,the 2021 average temperature was generally higher by 0.2—0.4 °C,and by 0.4—1.0 °C in some localized areas;for instance,the annual mean temperatures for Wufeng County and Enshi City were the highest on record since 1961.The northern area of the TGR observed negative temperature anomalies,by 0.2—1.4 °C (Fig.2 (a)).

During the winter of 2020/21(from December 2020 to February 2021),the air temperature in the TGR was above normal;temperatures fluctuated significantly in the spring of 2021;and they were below normal in the summer,above normal at the beginning of autumn,and below normal at the end of the season.The mean temperatures in February(2.8 °C above normal) and September (2.0 °C above normal) were the highest and second highest since 1961,respectively.The number of an-

Fig.2.Annual mean (a) temperature anomaly (units:°C) and (b) precipitation anomaly (units: %) in 2021 over the TGR relative to the period 1991—2020.

nual high-temperature days (defined by the daily maximum temperature reaching or exceeding 35 °C) reached 26 for the TGR,which is 2.2 days less than the 1991—2020 average.

3.2.Precipitation

Fig.1 (b) shows the 2021 precipitation anomaly (as a percentage of normal) in the TGR.The 2021 mean precipitation total for the TGR was 1341.8 mm,which was 12.8% more than normal and 12.3% less than the precipitation received in 2020.Before water started to be impounded in the Three Gorges Reservoir in 2003,the climatological annual precipitation during 1961—2003 was 1204.1 mm;and during the experimental impoundment period (2004—2010),the TGR had deficient rainfall for several years.During the first few years after impoundment in 2011,the average annual precipitation was below normal,but the average annual precipitation in 2014—2017 and 2020—2021 was in both cases above normal.The correlation coefficient of the annual precipitation anomaly between the TGR and UR (TGR and SWC) was 0.57 and 0.69,respectively,both exceeding the 99% confidence level.This indicates that the variation in precipitation was generally consistent with that of the larger surrounding area.However,the range of the annual precipitation interannual variability in the TGR was relatively larger than that in the aforementioned two regions,especially from the 1980s to 1990s.

Precipitation over most areas in 2021 was near to or above the 1991—2020 long-term average (Fig.2 (b)),and more by 40%—60% in the northern TGR;only Yichang City,Changyang County,and a few other places observed negative rainfall anomalies,by approximately 10%.

During the year,the temporal distribution was inhomogeneous,as spring and summer precipitation were both above normal,while winter and autumn received below-normal rainfall.During August,the TGR received a total of 307.5 mm of rainfall (110% above normal),and it was the second-wettest August since 1961.In 2021,the number of average rainstorm days (defined as daily precipitation reaching or exceeding 50 mm) was 4.3 (above normal by 1 day),making it the fifth-most year on record.Compared with normal years,most regions of the TGR had more rainstorm days,and part of the northern area had 2—3 more rainstorm days.

3.3.Acid rain

In 2021,the average pH of precipitation at the observation stations in six areas along the Yangtze River (Chongqing City’s Shapingba District,Fuling District,Wanzhou District,and Fengjie County,and Hubei Province’s Badong County and Yichang City) was 5.96,indicating normal rainfall (Fig.1(c)).The rainfall acidity has continued to weaken(since 2005),and 2021 was tied with 2020 as the lowest on record since 1999.The precipitation acidity was strongest in autumn (pH=5.85) and weakest in spring (pH=6.14).From the average of the aforementioned six stations,the TGR received normal precipitation every month.

The annual average pH of precipitation measured by the representative six stations was 6.03,6.07,5.95,5.52,6.01,and 6.16,respectively.In fact,at Yichang,Badong,Fengjie,and Shapingba stations,the annual average pH of precipitation has increased for many consecutive years (Fig.1 (d)).Since 2015,that in Wanzhou has also increased significantly.On the whole,the rain acidity in the TGR has tended to weaken over the past 17 years.

3.4.Wind, fog, and relative humidity

In 2021,the average wind speed in the TGR was 1.46 m s-1(0.34 m s-1higher than normal),and tied with 2015 as the fifth-highest on record since 1961.In terms of spatial distribution,the wind speed in some of the southwestern and mid-north areas was relatively higher than that in other parts of the TGR —generally,1.5—2.0 m s-1,with more than 2.0 m s-1in some localized areas.Meanwhile,the wind speed was 1—1.5 m s-1in most other areas.The maximum annual wind speed reached 2.83 m s-1in Qijiang District,creating a new record since 1961;and the minimum wind speed occurred in Pengshui County,at 0.64 m s-1.

The TGR is a fog-prone area in China.The number of foggy days in 2021 was more than 50 in most parts of the TGR,and varied quite significantly from place to place.There were 50—100 foggy days in the western,mid-north,and eastern areas;and the number of foggy days at three stations (Kaizhou,Qianjiang,Fengdu) exceeded 200,which was the most on record for each of them.However,the number of foggy days at Wanzhou and Pengshui stations was less than 10.

In 2021,the average relative humidity was 81% in the TGR,which was the highest since 1961,and the relative humidity at each station was between 75% and 88% —larger in the mid-west area and smaller in the eastern area.The average annual relative humidity was 3%—6%higher than normal over most of the eastern TGR area;while that in Shapingba District and other places was below normal.

4.Major climate events and meteorological disasters

4.1.Torrential rain in the flood season

The TGR and its surrounding areas experienced torrential rain events during the flood season.During May—September,the numbers of rainstorm days in Beibei (10 days),Yunyang (7 days),and Yubei (9 days) of Chongqing City were the highest in local history since 1961,followed by those in Wuxi,Liangping,and Qianjiang,and the numbers for Fengdu and Zigui of Hubei Province were the third largest in history.

Fig.3.The (a) mean 500-hPa geopotential height field (blue isolines;units: gpm) and associated anomalies (shaded;1991—2020 base period) during 17 September and 5 October,(b) intensity index,and (c) ridge line position index of the WPSH (110°—130°E).The red dashed isolines indicate the climatological extent of 5860,5880,and 5890 gpm in (a),and blue curves indicate the climate mean in (b) and (c).

Between 14 and 17 May,torrential rain occurred over many parts of the TGR,affecting several areas including Hefeng,Xuanen,Enshi,and Jianshi,accompanied by thunderstorms and catastrophic gales,with wind speeds at or exceeding 17 m s-1.The maximum precipitation(64.8 mm) per hour occurred in Huping Village of Hefeng County,and the daily precipitation at Shuiquan Village reached 302.5 mm.The torrential rain caused flooding and destroyed many crops,with lots of people affected,across Enshi Autonomous Prefecture.

On 8 August,168.6 mm of rainfall led to flash flooding that swept through Fuling in Chongqing City,thereby breaking the local record.From 25 to 30 August,most of the TGR experienced intense rainfall and flooding again.The top three amounts of accumulated rainfall occurred in Kaizhou (259.2 mm),Wuxi (242.4 mm),and Yunyang (209.7 mm) of Chongqing City during this period.According to the local Emergency Response Department,the floods destroyed many transportation and power supply facilities.

4.2.Continuous overcast and rainy weather

In April,the temperature was lower than normal by 1.4 °C.Kaizhou(16.7 °C) and Wushan (15.3 °C) were the coldest on record in local history.Yichang and Zigui were the second coldest in history.The spring precipitation in the central area of the TGR was 30%—50% more than normal,with the number of rainfall days being about 40—60 (4—15 days above normal).The continuous overcast and rainy weather in spring caused serious waterlogging in many cropland areas,and a prevalence of crop diseases in the TGR.Geological disasters such as collapsed highways,landslides,and mountain torrents occurred in Enshi,Yichang,and other places in spring.

Fig.4.The (a) zonal wind anomalies at 200 hPa (shaded;units: m s -1) and (b) OLR anomalies (shaded;units: W m -2).The isolines in (a) represent the extent of 35 m s -1 and 0 m s -1 of zonal wind (red for 2021;blue for climatic average).

4.3.Extremely warm early autumn

Vast areas south of the Yangtze River were under the influence of high temperatures in early autumn (Zhou et al.,2022).The end date for this high-temperature event was the latest on record for southern China,at 36 days later than normal.During 17 September to 5 October,the average temperature in the TGR reached 25.6 °C (4.2 °C above normal)and the average maximum temperature reached 31.9 °C (5.6 °C above normal),both of which were the highest on record for the same period since 1961.The average number of high-temperature days reached 3.6,ranking the second-most on record for the TGR.Compared with normal,the end date of this high-temperature event was the latest on record for Yunyang (5 October),Badong (4 October),and 10 other stations.Wuxi (5 October),Yichang (3 October),and four other stations reported their second-latest high-temperature events since 1961,and five stations located in Chongqing City,including Kaizhou,observed their third-latest high-temperature events.In early October,there was still a wide range of high-temperature weather in the TGR,which is historically rare.

4.4.Extreme cold events

The occurrence of cold-air activity in autumn and winter was more frequent in 2021.Two strong cold surges swept through mid-eastern China from 6—8 January and 4—9 November respectively,the latter of which is the fourth most severe since 1961 based on the comprehensive strength index (Zhou et al.,2022).During the first of these two cold surges,most regions of the TGR experienced rainy and snowy weather,and the snow depth at Enshi station reached 10 cm,with a lower-thannormal temperature.The amplitudes of the daily temperature drop at most stations ranged from 4 to 8 °C,and the minimum temperature over most parts of Yichang City remained below freezing for 30 consecutive hours.On 7 January,64 generator sets in cascading hydropower stations on the main stream of the Yangtze River (including the Three Gorges,Gezhouba,Xiluodu,and Xiangjiaba stations) were in peak operation,which was 21 more than before the cold wave arrived (on 4 January).In total,the peak output reached 33.76 million kW on 7 January,and the daily power supply nearly reached 600 million kW h,both of which were new records for the same period historically.

For four days in November,cold waves affected the TGR,including intense temperature drops and damaging winds.The maximum temperature drop during this event was generally 8—12 °C,resulting in moderate snowfall in high-altitude areas across the TGR.On 7 November,the maximum wind speed exceeded 12 m s-1in Xingshan,Yichang,and other places,with the maximum wind speed exceeding 15.3 m s-1in some areas.

5.Brief analysis of the causes of abnormal high-temperature events in autumn

In autumn,the TGR experienced an episode of unusually warm and dry weather (19 days from 17 September to 5 October),and the accumulated rainfall amount was less than the climatological average by 21.3%,resulting in record-breaking temperatures of 2—6 °C higher than normal.Most areas from Chongqing City to the middle and lower reaches of Yangtze River Basin were affected during the period.

The increase in greenhouse gas concentrations in the atmosphere has directly led to a rise in global temperature through thermodynamic effects,which will likely trigger an increase in the frequency and intensity of extreme warmth (Seneviratne et al.,2021).This global background of frequent high-temperature events has been reflected in the TGR and surrounding areas in recent years.In addition,the anomalous high temperatures are closely related to the abnormal activity of mid-and highlatitude atmospheric circulations and the WPSH.

As the 500-hPa height field shows (Fig.3 (a)),the anomalous atmospheric circulation over mid and high latitudes showed a “positive—negative—positive ”pattern across the Eurasian continent.The northern part of Eurasia was controlled by positive height anomalies that extended to the Arctic.The blocking high over the west of the Ural Mountains was stable and strong,which hindered the eastward movement of the westerlies and synoptic disturbances.The regions covering the Black Sea to central Siberia were controlled by a transversal trough,indicating that the midlatitude circulation was flat with stronger zonal circulation.Compared with the same period in normal years,the WPSH was intensified,with a larger area,and its ridge point reached farther west with a northerly generally displacing the ridge line;there was only a brief southward withdrawal for three days at the end of September,and then it jumped north again to 35°N (Fig.3 (b,c)).Actually,even the high temperatures in South China throughout September 2021 were directly related to the anomalous behavior of the WPSH (Zhou et al.,2022).

Another notable feature was a larger-than-normal scope of the midlatitude westerly jet center (the areas where the zonal wind at 200 hPa exceed 35 m s-1),and its location being situated more to the north than normal,which can be seen in Fig.4 (a).This powerful and northwarddisplaced westerly jet strengthened the zonal winds and impeded the cyclonic disturbances from affecting the subtropics,resulting in the abnormal position of the WPSH.As a result of the above atmospheric circulation patterns,cold waves could not reach the Yangtze River Basin easily.The WPSH remained stable over the middle and lower reaches of the Yangtze River Basin,which were dominated by downward airflow,resulting in high temperatures and little rainfall over the TGR and surrounding areas.

Moreover,the anomalous tropical circulation pattern,coupled with the stronger-than-normal East Asian continental high,impelled the continuous stability of the WPSH.Fig.4 (b) shows a map of the OLR anomalies during the period with high temperatures.As can be seen,the OLR anomalies remained negative across the south of the South China Sea to the Philippines,indicating strong convective activities over this region.As a result,the Hadley circulation was enhanced not only by the convective activity over the tropical region,but also the enhanced descending motion over the subtropical area (26°—33°N),which was conducive to a strengthening of the WPSH and its dominance in the southern region of China.

6.Conclusion

The 2021 average temperature in the TGR experienced anomalous seasonal variability with large temperature swings,and an episode of unusually warm and dry weather during autumn in particular.Most regions received abundant rainfall throughout the year,and the number of rainstorm days was higher than normal.The annual average wind speed was above normal,and the average relative humidity was higher than normal.In terms of rain acidity,2021 was tied with 2020 as the lowest since 1999.In 2021,the main meteorological disasters occurring in the TGR included torrential rain and flooding in the rainy season,overcast and rainy weather with low temperatures in spring,and two severe cold events in stages during the year.

From 17 September to 5 October,the TGR experienced a rare hightemperature event,resulting in record-breaking temperatures of 2°C—6°C higher than normal.The abnormal activity of mid-and high-latitude atmospheric circulation combined with the vigorous WPSH produced anomalously warm conditions across the TGR.The powerful zonal circulation over the midlatitudes,coupled with strong convection over the tropics and other factors,jointly led to weaker cold-air activity,and the WPSH persistently dominated the middle and lower reaches of the Yangtze River,resulting in the continuous high temperatures in autumn in the region.

Funding

This work was jointly supported by the funds of the Strategic Cooperation Agreement Project between the China Meteorological Administration and the Three Gorges Corporation [Grant No.0704182 ] and the Comprehensive Monitoring Program for Operational Safety of the Three Gorges Project [Grant No.SK2021015 ],which was financed by the Ministry of Water Resources of China.

Acknowledgments

The authors thank our colleagues at Hubei Provincial Meteorological Bureau and Chongqing Meteorological Bureau for their contributions.We deeply appreciate the editor’s and the anonymous reviewers’ helpful suggestions for improving our manuscript.