Xianyan Chen, Xukai Zou4 , Qiang Zhang, Hongling Zeng, Tong Cui

National Climate Center, China Meteorological Administration, Beijing, China

Keywords:Three Gorges Region Yangtze River Climate summary Weather and climate events

ABSTRACT This report provides a broad overview of the climate and the major weather and climate events over the Three Gorges Region of the Yangtze River (TGR) in 2019. The year 2019, a 0.3 °C warmer year than normal, had a colder winter and warmer spring, summer, and autumn. Annual precipitation in 2019 was 13% less than normal.Below average normal rainfall amounts were received in all four seasons, with 28% and 16% less-than-normal in winter and summer, respectively. The annual mean wind speed in the TGR was higher than normal, and relative humidity was near normal for all four seasons. The intensity of acid rain in 2019 was the weakest since 1999. The major climate events and meteorological disasters in the TGR in 2019 included heat waves, drought,and rainstorms. Heat waves occurred frequently and persisted for long durations. Summer and autumn drought occurred in central and eastern regions of the TGR. The autumn rains of West China occurred earlier this year,which brought much more rainfall than normal in central and western regions of the TGR.

1. Introduction

The Three Gorges Dam, built in Yichang, Hubei Province in 1997,is one of the world’s largest hydropower projects. With the completion of the dam and impoundment of the reservoir, the Three Gorges Reservoir (TGR), extending 660 km along the Yangtze River, connects the Sichuan Basin and the lower and middle reaches of the Yangtze River.

Covering most regions of Chongqing and the southwest of Hubei Province, the TGR lies in the upper reaches of the Yangtze River ( Fig. 1 ).With four distinct seasons, a long and hot summer, short and warm winter, cool but rainy autumn, the climate of the TGR area is affected by both the valley and water. Studies have shown that annual temperature has increased significantly in the TGR during the past 40 to 50 years,while annual precipitation has shown no obvious increasing or decreasing trend but extreme precipitation has changed to become heavier.Also, turning points occurred in the time series of temperature, relative humidity, and fog days in the early 1990s, whereas this was not the case for precipitation ( Zeng et al., 2019b ; Lv et al., 2018 ; Zhang et al., 2010 ). The impact of the TGR on the climate of the reservoir area is complicated and controversial. It is very important to continuously monitor and analyze the meteorological factors of the TGR and its surrounding areas, which can contribute to the scientific and objective assessment of the impact of the TGR. The Annual Climate Report over the TGR area was first released in the year 1996, and the National Climate Center has continued to carry out climate monitoring over the area and make efforts to analyze and understand the year-to-year variations and long-term trends of a changing climate in the TGR area.

This report provides detailed climate information for the TGR in 2019 and analyzes the interannual fluctuations and seasonal variation of air temperature, precipitation, and other main meteorological elements, as well as the major weather and climate events that took place throughout that year.

2. Data

Daily observational data from 33 meteorological stations covering the period 1961—2019 across the TGR area were used in this study. In order to compare the climate change between the TGR area and its surrounding areas, temperature and precipitation observations of 716 stations in Southwest China and the upper regions of the Yangtze River were also used in this study. Those data and some metadata information presented in this paper were originally sourced from the National Meteorological Information Center of China and two regional meteorological divisions —Chongqing Municipal Meteorological Bureau and Hubei Provincial Meteorological Bureau. The stations are reasonably well spatially distributed across the study areas( Fig. 1 ).

Fig. 1. Distribution of meteorological stations over the TGR, Southwest China, and the upper regions of the Yangtze River.

Unless otherwise specified, the terms “above normal ”, “below normal ”, “near normal ”, or “close to normal ”in this paper refer to the longterm average for the period 1981—2010.

3. Temperature and precipitation

3.1. Temperature

The regional mean annual temperature across the TGR was 17.5 °C in 2019, which was 0.3 °C above normal and close to that of last year( Zou et al., 2020 ). A warming trend, especially in the last 20 years,was observed in the TGR during the period 1961—2019. Temperatures of all years were warmer than normal since 1997, except for the years 2000 and 2012. Since the start of the 21st century, two record annual temperatures have been observed. The 2006 temperature was 18.3 °C,claiming the title of the hottest individual year on record; the secondand third-warmest years were 2013 (18.1 °C) and 1998 (17.9 °C), respectively. Furthermore, 2015—19 was the second-warmest five-year period on record regionally. The rate of increase in annual temperature during the past 59 years was consistent with the changes in annual temperature of the upper regions of the Yangtze River and Southwest China ( Fig. 2 ).Since 2014, the temperature anomalies in the southwest and the upper reaches of the Yangtze River were slightly higher than those in the TGR.

Fig. 2. Variations of annual temperature anomalies for the TGR, the upper regions of the Yangtze River, and Southwest China, during the period 1961—2019.

Compared with the long-term average of 1981—2010, warmer climate was observed in most of the TGR in 2019, and the annual temperatures across the central TGR were more than 0.4 °C above normal,while temperatures of several stations in southwestern areas were below normal ( Fig. 3 ).

The temperature variation between months alternated greatly between cold and warm during this year ( Fig. 4 ). February, May, June,and July were colder than normal, with 1.4 °C below normal in May,while March, April, and the months from August to October and December were warmer than normal. The TGR experienced its fourth-warmest April on record in 2019. The seasonal mean temperatures in the TGR for winter (December 2018 to February 2019), spring (March to May),summer (June to August), and autumn (September to November) were 7.0 °C, 17.4 °C, 26.4 °C, and 18.3 °C, respectively. The seasonal temperature was 0.4 °C below normal in winter, and 0.4 °C and 0.5 °C above normal in spring and autumn, respectively. Temperature in summer was close to normal.

Fig. 3. Annual temperature anomalies in 2019 for the TGR (units: °C) with respect to the period 1981—2010.

Fig. 4. Monthly temperature anomalies (bars) in 2019 for the TGR (units: °C)with respect to the period 1981—2010. The actual temperature is shown as the red line.

2019 had a total of 30 hot days (daily maximum temperature exceeding 35 °C) for the TGR, which was 5 days more than normal and had been the fourth consecutive year with high temperature days exceeding the climatology since 2016 ( Fig. 5 ). More-than-normal hot days were observed in central and southern parts of the TGR, while less-thannormal hot days occurred in small parts of northern and southwestern TGR.

Fig. 5. Variations of annual hot days for the TGR during the period 1961—2019.

3.2. Precipitation

The total area-averaged rainfall in the TGR in 2019 was 1035.1 mm,which was 13% less than normal ( Fig. 6) . Before the impoundment period (1961—2003) and during the experimental impoundment period(?2004—10) of the TGR, the annual mean precipitation over the area was 1204.1 mm and 1141.8 mm, respectively. Being less rainfall over the area since 2000, the precipitation began to be more in 2014. Compared with the rainfall variations of the upper regions of the Yangtze River and Southwest China, the decadal and interdecadal changes of annual precipitation in the TGR were similar but stronger than the others, which may be due to the reason that the area of the TGR is much smaller than that of the southwest and the upper reaches of the Yangtze River and the number of meteorological stations in the TGR area being far less than that in the other two regions (F ig. 6) . The variation characteristics of temperature and precipitation in the TGR resemble those in the upper regions of the Yangtze River and Southwest China, which indicate the changes of climate in the TGR are mainly influenced by the atmospheric circulation of the background area.

Fig. 6. Variations of annual precipitation anomalies for the TGR, the upper regions of the Yangtze River, and Southwest China, during the period 1961—2019.

The spatial difference of the precipitation distribution was large across the TGR during 2019. Much of the western TGR had aboveaverage precipitation for the year, which was generally 10%—25% wetter than normal, while the eastern area had 10%—40% drier-than-normal conditions ( Fig. 7 ).

Fig. 7. Annual rainfall anomalies in 2019 for the TGR (units:%) with respect to the 1981—2010 average.

The rainfall amounts were less than normal for all four seasons. Winter precipitation was 51.6 mm, which was 28% less than normal, of which February was the fifth lowest in history. The winter of 2019 was relatively dry across most of the TGR, except for the parts of the west and northeast that were slightly wetter than normal. The total rainfall in spring was 286.6 mm, which was 8% less than normal, and it was the driest spring for the last eight years. The distribution was uneven in summer 2019, with a precipitation total of 450.3 mm for the region as a whole, which was 16% less than normal. Autumn 2019 received 265.8 mm of precipitation, 4% less than normal, in which October was the seventh wettest and September was the fourth lowest in history.

The annual number of rainstorm days (daily precipitation greater than or equal to 50.0 mm) in the TGR in 2019 was 2.2 days, which was 1.1 days less than normal and the ninth lowest since 1961 ( Fig. 8 ). The number of rainstorm days was less than normal or close to normal across most of the TGR.

Fig. 8. Variations of annual rainstorm days for the TGR during the period 1961—2019.

4. Wind, relative humidity, and fog

Being 0.46 mslarger than normal, the average annual wind speed in 2019 in the TGR was 1.48 msand recorded as the third largest since 1961 as well as 2017. The annual mean wind speed in most areas of the TGR was about 0.6—1.5 ms, however it was larger in the western and eastern region with a mean value of about 1.5—2.0 ms. The difference between the highest and lowest monthly wind speed in the TGR was small. The maximum monthly mean wind speed was 1.7 ms, which occurred in April, and the minimum wind speed of 1.3 msoccurred in January.

A 78% regional-average annual mean relative humidity was recorded in the TGR in 2019, which was close to the 1981—2010 average. The annual mean relative humidity of each station was about 70% to 84%, which was larger in the western and central regions and smaller in the northeast. The interannual variation of relative humidity in the TGR was small. The annual mean relative humidity was close to normal in most parts of the TGR, and was 2%—6% less than normal in some regions of the northern TGR. Relative humidity was close to normal for all four seasons for the region as a whole.

The TGR of the Yangtze River is a fog-prone area in China. There were a number of areas of notable foggy days. The annual number of foggy days exceeded 100 days over the western part of the TGR and some areas along the Yangtze River (Qijiang, Fuling, Zhongxian,Nanchuan, Qianjiang, Banan, and Wulong) in 2019, with 206 days in Qijiang and 188 days in Nanchuan, respectively. Fewer foggy days occurred in the central and eastern parts of the TGR in 2019. Kaizhou,Wanzhou, Hefeng, and Enshi had fewer than 10 foggy days.

5. Acid rain

A weakening trend in acid rain has been observed in the past 20 years in the TGR. The regional-average precipitation pH value in the TGR was 5.80, making 2019 the lowest year on record since 1999. The pH values of Chongqing, Fuling, Wanzhou, Fengjie, and Badong were 5.69, 5.83, 5.88, 6.04, and 5.92, respectively, in the year 2019, where the degree of acid rain was normal according to the standard of acid rain monitoring. The annual precipitation pH value in Yichang was 5.48,which was classified as light acid rain.

The acid-rain intensity reached its maximum for 2019 in autumn,with a pH value of 5.71, and the intensity of acid rain was the weakest in summer, with the pH valve of 5.96. The average pH values of precipitation in the other two seasons were 5.73 (winter) and 5.87 (spring).The monthly acid-rain intensity was weak during May—July, while the intensities in other months were generally normal and the strongest acid rain was observed in December, with a precipitation pH value of 5.53.

6. Major climate events and meteorological disasters

In 2019, the main climate events and meteorological disaster types in the TGR included heat waves, drought, heavy rain and flooding. The heat waves hit the TGR frequently and lasted for a long time. Drought phases occurred during the summer and autumn in central TGR. The autumn rains began earlier in West China, and more than normal rain amounts were received in the central and western TGR.

6.1. Frequent and persistent heat waves

During the year 2019, continuous sunny and hot weather occurred in the TGR, with nearly 30 hot days (daily maximum temperature exceeding 35 °C) for the region as a whole, which was 5.4 days more than normal. The annual number of hot days had been more than normal for the fourth consecutive year since 2016. Strong heat waves have hit the TGR frequently in recent years, resulting in many high temperature extremes. A total of six regional heat waves occurred in the TGR during the period from June to September 2019, within which the duration of 19 days for the heat wave between 9 and 27 August was particularly severe.

The heat waves mainly hit central and eastern TGR in 2019. The number of hot days was 34 at Fengjie station, which was the third largest in history. The maximum temperature exceeded 40 °C in some regions of the TGR, such as Shapingba, Beipei, Jiangjin, Fengdu, Wanzhou, Yunyang, Fengjie, Kaizhou, Badong etc., and there were 13 days at Fengjie,which broke the historical record. A recorded maximum temperature of 42.4 °C was observed on 17 August at Fengjie.

6.2. Summer and autumn droughts in central and eastern TGR

The meteorological drought in the TGR was generally lighter than normal during the year 2019, while persistent droughts occurred in the central and eastern TGR due to the continuous high temperature in summer and less rainfall in summer and autumn, with Changshou,Yunyang, Zigui, and Enshi being hit by heavy droughts. The meteorological drought began in early July, aggravated in August, and persisted in September. More than 970 000 people in 15 counties and cities of Enshi and Yichang were affected by the droughts, including 112 000 ha of crop areas, and the direct economic loss was about 780 million yuan.Droughts were alleviated in mid-October when effective precipitation was received.

6.3. Early autumn rain in West China and abundant rainfall in central and western TGR

In 2019, the onset of the autumn rains in West China was four days earlier than normal, and the precipitation in the rainy season (27 August to 30 November) was 271.7 mm, which was 34% more than normal. The precipitation of the autumn rains in the central part of West China was 200 to 400 mm, and in some areas it was more than 400 mm, which was 20% to 100% more than normal. Affected by the autumn rain in West China, the central and western regions of the TGR received more-thanaverage rainfall. The rainfall totals in the western regions to Fengjie were 300 to 500 mm, which was 20% to 50% more than normal. Also,the precipitation in Wanzhou, Yunyang, and Kaizhou was more than 500 mm, which was over 50% more than normal. The Chongqing area of the TGR was repeatedly hit by regional consecutive rains from October,within which a regional rainy spell persisted for 16 consecutive days from 16 November to 1 December, resulting in flood-related disasters.

6.4. Frequent rainstorm with comparatively weak intensity

During the year 2019, there were 15 regional rainstorm processes in the TGR. Compared with that of 2018, the number of rainstorm processes was more, but the intensity was weaker on average. During 3—5 October, heavy rains hit most areas of Chongqing in the TGR, resulting in rainstorms in Shapingba, Yubei, Changshou, Wulong, Kaizhou, Wuxi etc., which was the strongest heavy rain process within the year 2019,with a daily maximum precipitation of 130.9 mm observed at Kaizhou on 5 October. During this rainstorm event, more than 28 000 people were affected in six counties including Kaizhou, Yunyang, and Wanzhou of Chongqing; more than 100 houses were damaged; the affected area of crops was more than 300 ha; and the direct economic loss was more than 19 million yuan.

7. Conclusion and discussion

The temperature in the TGR in 2019 was 0.3 °C above normal, and the precipitation was 13% less than normal. Seasonal temperatures were higher than normal, except in the winter. The number of hot days was more than normal. Precipitation was less than normal for all four seasons this year.

Persistent and frequent heat waves hit the TGR in 2019, and summer and autumn droughts occurred in central and eastern regions. Morethan-normal rainfall was received in the central and western TGR, accompanied by an early autumn rainy season in West China, which led to rainstorms, flood disasters, and economic losses in some regions.

The intensity of acid rain in 2019 was the weakest since 1999, with an obvious weakening trend in the past 20 years in the TGR.

The TGR area is located in the middle and lower reaches of the Yangtze River; its annual climate characteristics are obviously affected by the surrounding climate of the Yangtze River, which means that it is greatly affected by the synergistic influence of large-scale atmospheric circulation, oceanic forcing, and water vapor transport. The weather extremes, including heat waves, drought, heavy rain and flooding in the surrounding areas in 2019 were not isolated events in the TGR area. The major climate events in the TGR area in 2019 described in the previous section were the responses of the weather extremes in the Yangtze River Basin, such as the high temperature in summer in the Yangtze River Basin, more precipitation in the upper reaches, and less precipitation in the middle and lower reaches of the Yangtze River in autumn( Zeng et al., 2019a ).

Although against the background of climate warming, significant changes have been found in temperature, precipitation, and other meteorological observations, as well as some extreme climate events in the TGR and its surrounding regions in the past several decades ( Guo et al.,2011 ), many studies have shown that the TGR has little influence on the local climate changes compared with environmental climate changes,based on numerical experiment simulations results and statistical comparative analysis ( Li et al., 2019 ; Zeng et al., 2019b ; Lv et al., 2016,2018 ; Wu et al., 2012 ; Huang et al., 2012 ). The atmospheric circulation that have significant impacts on the climate of both the TGR area and the Yangtze River mainly include the western Pacific subtropical high,the East Asian monsoon, the South Asian high, and the Indian Burma trough, while the external forcing factors are the sea surface temperature and the snow cover on Tibetan Plateau. The variation of these factors on different time scales affects the occurrence and development of drought and flood disasters in the TGR area and the Yangtze River Basin. Compared with the surrounding oceans and the Tibetan Plateau,the TGR has much smaller magnitude in both area and capacity, which plays a much weaker role in the weather extremes over the surrounding areas.

Funding

This study was supported by the National Key R&D Program of China [grant numbers 2017YFC1502402, 2017YFD0300201, and 2017YFA0605004], as well as by the funds of comprehensive monitoring of the Three Gorges Project, 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.