State of the climate in the Three Gorges Region of the Yangtze River basin in 2020

Tong Cui , Xinyn Chen , , Xuki Zou , Qing Zhng , Shui Li , Hongling Zeng

a National Climate Center, China Meteorological Administration, Beijing, China

b Operation and Administration Center for River Basin Hydro Complex, China Three Gorges Corporation, Yichang, China

Keywords:Three Gorges Region Yangtze River Climate state Weather and climate events Meteorological disasters

ABSTRACT In 2020, the average air temperature in the Three Gorges Region (TGR) of the Yangtze River basin was 17.2°C,which was close to normal, there were exceptionally fewer days than normal with high temperatures, and the high-temperature events mainly occurred in August. Meanwhile, the average precipitation was 1530.8 mm, which was a remarkable 29% more than usual, and the second-highest since 1961. The precipitation was obviously above-normal in summer, and the precipitation in both June and July was the second-highest of the same period in history. The average number of rainstorm days was higher than normal, and the second-highest since 1961.The average wind speed in the TGR was apparently higher than normal; the average relative humidity was slightly higher than normal; and there were no instances of acid rain, with the rain acidity showing a significant weakening trend over the previous 15 years. In the summer of 2020, the TGR experienced heavy rainstorms and flood disasters. Analysis shows that the frequent southward movement of cold air and abundant warm water vapor from the southwest were the direct causes of the abnormally high precipitation in the TGR from June to July. After the spring of 2020, the continuously high sea surface temperature in the Indian Ocean led to a continuously strong western Pacific subtropical high and its average location being situated more to the south than normal, which might have been an important cause for the abnormal climate conditions in the Yangtze River basin from June to July.

1. Introduction

The Three Gorges Region (TGR) of the Yangtze River basin is located in the hinterland of China, spanning from Jiangjin District of Chongqing City in the west to Yichang City of Hubei Province in the east. Geographically, it includes the Three Gorges Reservoir and its surrounding areas.The main function of the Three Gorges Reservoir, whose normal water level is 175 m, is flood control in the rainy season. The flood-limiting water level in the flood season is 145 m, and the reservoir’s flood control capacity is 2.215 × 1010m3.

Studies have shown that interannual climate variations in the TGR,especially changes in precipitation, have an important impact on the reservoir’s runoff, the dam’s power generation, and the ecological environment around the reservoir ( Huang et al. 2020 ; Qin et al. 2020 ;Zhu et al. 2021 ). Therefore, it is very important to know and understand the basic climatic characteristics of important weather and climate events in the TGR each year. Up to 2020, the Three Gorges Reservoir had fulfilled the 175-m experimental impoundment for the 11th consecutive year. Climate monitoring has become more precise and accurate than before the Three Gorges Reservoir started to impound water. The Meteorological Department has obtained first-hand observational data on the basic meteorological variables such as temperature and precipitation. Including the present report, we have released an annual climate report ( Zou et al. 2020 ; Chen et al. 2021 )on the TGR for three consecutive years, providing information on climate monitoring, meteorological disasters, and climate impact, in the hope of providing references for climate-related scientific research in the region.

This paper provides climate information on the TGR for the year 2020, with an analysis of the interannual fluctuations and seasonal changes of major meteorological variables such as temperature, precipitation, wind speed and relative humidity, as well as an analysis of the major weather and climate events throughout the year.

2. Data and methods

Utilized in this report are the daily observational data from 33 representative national meteorological stations (see Chen et al. (2021) for details about the distribution of the stations) in the TGR from 1961 to 2020, including temperature, precipitation, wind speed, fog, relative humidity, and other variables. In order to compare the regional climate state of the TGR with the climate state of the surrounding areas like the upper regions of the Yangtze River and Southwest China, this paper also uses data from a total of 716 stations in the aforementioned two regions.The above data are provided by the National Meteorological Information Center of the China Meteorological Administration, Chongqing Meteorological Bureau, and Hubei Province Meteorological Bureau. The atmospheric circulation data are from the ERA5 global reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts,and they include data on monthly mean zonal wind, meridional wind,specific humidity, atmospheric pressure at the Earth’s surface, and the geopotential height field at 500 hPa. These data are used to calculate the water vapor flux and associated divergence, geopotential height, and their anomalies.

In this paper, the period for which the average values for normal years are calculated refers to 1981 to 2010. Expressions like “above normal ”, “below normal ”, and “close to normal ” refer to comparisons with the average values for those 30 years (from 1981 to 2010). In addition, the other time periods used in the statistics of this paper are as follows: annual values refer to the period January to December; spring is from March to May; summer is from June to August; autumn is from September to November; and winter is from December of the previous year to February of the current year. For example, the winter of 2019/20 is from December 2019 to February 2020.

3. Result s

3.1. Temperature and precipitation

3.1.1.Temperature

In 2020, the average temperature in the TGR was 17.2 °C, which was close to normal, and 0.3 °C lower than that in 2019. The average annual temperature (from 2011 to 2020, after the Three Gorges Reservoir started to impound water) was 17.4 °C, and the abnormal temperature situation changed from year to year. Except 2012, in which the temperature was below normal, and 2011, 2014, and 2020, in which the temperature was close to normal, the average annual temperature in each of the remaining six years was above normal. The average annual temperatures in 2006, 2013, and 1998 had ranked as the top three since 1961, and were 1.1 °C, 0.9 °C, and 0.7 °C higher than normal, respectively. Since the mid to late 1990s, the rate of increase in the average annual temperature in the TGR had increased. On the whole, the decadal and interannual fluctuations in the annual mean temperature anomaly in the TGR were relatively consistent with those in the upper regions of the Yangtze River and Southwest China ( Fig. 1 (a)), indicating that the temperature in the TGR was mainly affected by the macroclimate.Since 2014, the positive temperature anomalies in the aforementioned two regions had been slightly higher than those in the TGR.

Compared with that of normal years, the average annual temperature was generally higher by 0.2—0.4 °C in most of the central area and the northwestern area of the TGR, and higher by 0.4—1.0 °C in some of the areas; the temperature was below normal in the eastern area and part of the southwestern area of the TGR, and obviously lower by more than 0.4 °C at some stations ( Fig. 2 (a)).

The winter of 2019/20 in the TGR was warm; the temperature in the spring of 2020 fluctuated significantly; the temperature in the summer of 2020 was the same as normal; the temperature was low at the beginning of autumn and high at the end of autumn; and cold air was frequent at the end of the year. The temperature was generally above normal in the first half of the year, but below normal in the second half of the year;the average temperatures in June and November were close to normal;the average temperatures in April, July, September, October, and December were below normal; the average temperatures in both July and October were the second-lowest of the same period since 1961; the average temperatures in January to March, May, and August were above normal; and the average temperatures in March and August were 1.7 °C and 1.5 °C higher than the same period of normal years, respectively.

3.1.2.Precipitation

In 2020, the average annual precipitation in the TGR was 1530.8 mm, 29% more than normal, and 48% more than that in 2019( Chen et al. 2021 ). The average annual precipitation in 2020 was the second-highest in history since 1961, and second only to the average annual precipitation in 1998. Before the Three Gorges Reservoir started to impound water (1961—2003), the average annual precipitation in the TGR was 1204.1 mm. During the experimental impoundment period(2004—2010), the TGR had several rainless years and less than normal precipitation. During the first few years after the impoundment in 2011,the average annual precipitation was below normal, but the average annual precipitation in 2014—2017 and 2020 was above normal. Compared with the average annual precipitation in Southwest China and the upper regions of the Yangtze River, the average annual precipitation in the TGR had a larger interannual variability. However, in obviously abnormal years, the positive and negative phases of the average annual precipitation anomalies in the TGR were basically the same as those in the aforesaid two regions, and the decadal fluctuations in the three regions were basically similar ( Fig. 1 (b)). This indicated that the precipitation in the TGR was mainly affected by the macroclimate, and that the local climate had no significant impact on the precipitation.

Compared with the average value for normal years, the precipitation in each area of the TGR in 2020 was above or close to normal( Fig. 2 (b)); the precipitation in the eastern and south-central areas was 40%—50% more than normal; the precipitation in the north-central area was 10%—20% more than normal; the precipitation anomaly in part of the northwestern area was less than 10%; and only the precipitation in Beibei District, Chongqing City, was close to normal.

Fig. 1. Variations of the annual (a) temperature anomaly (units:°C) and (b) precipitation anomaly percentage (units: %) in the TGR (black curve), the upper regions of the Yangtze River basin (red curve), and Southwest China (blue curve), from 1961 to 2020.

Fig. 2. Distributions of the (a) temperature anomalies (units:°C) and (b) precipitation anomaly percentages (units: %) in 2020 in the TGR relative to the period 1981—2010.

Fig. 3. Distributions of the (a) mean 500-hPa geopotential height field (isolines; units: gpm) and associated anomalies (shading) and (b) vertically integrated water vapor flux (arrows; units: kg m ? 1 s ? 1 ) and associated divergence (shading; units: 10 ? 5 kg m ? 2 s ? 1 ) from June to July in 2020.

In terms of temporal distribution, the precipitation in the winter of 2019/20 and the summer and autumn of 2020 were above normal, while the precipitation in the spring of 2020 was below normal. The precipitation in the winter and summer of 2020 was the second-highest of the same period in history. Considering the monthly distribution of precipitation, in the five months of April, May, August, November, and December, the precipitation was 10%—30% less than normal, and that in each of the remaining seven months was 20% more than the precipitation in the same period of normal years. The precipitation in January was 1.8 times more than normal, and was the highest of the same period since 1961. The precipitation in June and July was 71% and 93% more than normal, respectively, and the second-highest of the same period since 1961.

3.2. Wind, fog, and relative humidity

In 2020, the average annual wind speed in the TGR was 1.45 m s?1,which was 0.43 m s?1higher than normal, and the sixth-highest in history since 1961. In terms of spatial distribution, the wind speeds in some of the southwestern and eastern areas of the TGR were relatively higher than those in the other areas —generally, 1.5—2.0 m s?1, and more than 2.0 m s?1in some local areas. Meanwhile, the average annual wind speed was 1—1.5 m s?1in most of the other areas. Generally speaking, the monthly average wind speed in the TGR showed little change. The maximum monthly average wind speed appeared in August, at 1.83 m s?1,and the minimum monthly average wind speed appeared in February,at 1.31 m s?1.

The TGR is well known as a foggy area of China. By way of proof, in 2020, the number of foggy days in most of the TGR was more than 30.There were 50—100 foggy days in the southwestern and eastern areas;the number of foggy days at 12 stations, including Wulong, Nanchuan,and Qijiang stations, exceeded 100 days; and the number of foggy days at Qijiang station was the highest, at 206 days. However, the number of foggy days at Wanzhou and Kaizhou stations, located in the northcentral area, was less than 6. The number of foggy days at the other stations was generally between 10 and 50, and varied relatively significantly from place to place.

In 2020, the average annual relative humidity in the TGR was 80%,which was slightly higher than normal, and the average annual relative humidity at each station was between 72% and 85% —larger in the midwest area and smaller in the eastern area. The average annual relative humidity in most of the eastern area and some of the southwestern area of the TGR was 1%—3% higher than normal. Only in part of the western area and Wanzhou District of Chongqing City was the average annual relative humidity below normal.

In 2020, the average number of rainstorm days (with daily precipitation greater than or equal to 50 mm) in the TGR was 5.5, which was 2 days more than normal, the second-most since 1961, and second only to that in 1998. Compared with normal years, most areas of the TGR had more rainstorm days than normal, and part of the southeastern area had two to three more rainstorm days than normal.

3.3. Acid rain

In 2020, the annual average pH values of precipitation measured by the representative acid rain observation stations in six areas, i.e.,Chongqing City’s Shapingba District, Fuling District, Wanzhou District,and Fengjie County, and Hubei Province’s Badong County and Yichang City, were 5.82, 5.99, 6.26, 6.01, 5.92, and 5.76, respectively. In annual average terms, all of the aforementioned six observation stations had normal rainfall. The average pH value of precipitation at the six stations was 5.96, indicating normal rainfall, and the average rain acidity at the six stations was the lowest since 1999. In fact, the average rain acidity had continued to weaken since 2006. At Yichang, Badong, Fengjie, and Shapingba stations, the annual average pH values of precipitation had increased for many consecutive years. Since 2015, the annual average pH value at Wanzhou station had also increased significantly. On the whole, the rain acidity in the TGR tended to weaken over the previous 15 years.

3.4. Major climate events and meteorological disasters

3.4.1.Significantheavyprecipitationduringthefloodseason

In 2020, the Yangtze River basin experienced its worst flood since 1998. Rainstorms in the TGR and its surrounding areas mainly occurred in June and July, with more-frequent-than-normal heavy precipitation events, short intervals, high intensity, and wide impacts. The continuous heavy rainfall also caused geological disasters, such as waterlogging and landslides. A total of 12 regional rainstorms (daily precipitation greater than or equal to 50 mm at four or more stations) occurred in the TGR and its surrounding areas in 2020, and nine of them occurred in June and July. The heavy rainfall that happened from 15 to 19 July had high intensity and large spatial extent; the 18 stations located in Jiangjin District of Chongqing City etc. observed rainstorms on 22 occasions (including 6 heavy rainstorms, each with daily precipitation greater than or equal to 100 mm), and the maximum daily precipitation was 171.8 mm.From 25 to 26 July, the mid-west area of the TGR had widespread rainstorms, and heavy rainstorms and even extremely heavy rainstorms in some local areas. The maximum precipitation per hour reached 90 mm at Maotian Village of Jianshi County, and the daily precipitation at Jianshi station was 262.2 mm, thus breaking historical records. More than 270,000 people were affected in 11 counties of Yichang City and Enshi Autonomous Prefecture; more than 1000 houses were damaged due to the disaster; and the direct economic loss amounted to more than 300 million yuan. From 20 to 21 August, heavy rainstorms occurred in the southern and eastern parts of the Hubei section of the TGR, with the maximum precipitation intensity recorded in Yichang City. The No. 5 flood of the Yangtze River and the No. 2 flood of the Jialing River (the numbers indicate the order in which floods occur in a single year for each river) overlapped to form the largest flood in the upper regions of the Yangtze River in 2020.

3.4.2.Stronglocalconvectiveweather

In 2020, strong local convective weather events in the TGR were more frequent than normal, and mainly included synoptic phenomena like short-term heavy rainfall, thunderstorms, and gales. From 20 to 21 August, rainstorms occurred in Hubei Province, accompanied by strong convective weather; there were catastrophic gales with a wind speed at or exceeding 12 m s?1, and the maximum speed appeared in Hefeng County and Xingshan County (13.3 m s?1on 20 August). More than 32,000 people were affected in Yichang City; the area of crops affected was more than 2500 ha, and the direct economic loss amounted to approximately 120 million yuan.

3.4.3.Continuousovercastandrainyweather

In 2020, the continuous overcast and rainy weather in the TGR mainly occurred in spring and autumn, and the overall intensity was stronger than normal. From 4 to 11 May, there was continuous rainy weather with little sunshine in Hubei Province, and the average cumulative precipitation was 54.7 mm, which was 40% more than normal.The continuous rainfall affected 633 people in six villages and towns in Yichang City; five houses were destroyed by the disaster; the area of crops affected was 26 ha; and some rural roads were damaged.

3.4.4.Heatwaves

In 2020, the average number of high-temperature days in the TGR was slightly below normal, with heat waves mainly concentrated from the end of July to August. Among the 33 national meteorological stations, the extreme maximum temperature occurred at Beibei station(40.9 °C on 23 August), and the maximum consecutive high-temperature days occurred at Shapingba station of Chongqing City (lasting 17 days from 1 to 17 August). The heat waves from 1 to 7 August had a wide impact and high intensity; almost all of the meteorological stations in the TGR experienced high temperatures (exceeding 35 °C); and the highest temperature at Kaizhou station of Chongqing City reached 40.5 °C (on 5 August).

3.4.5.Lowtemperatureandextremecoolingevents

In the spring and autumn of 2020, there were low-temperature phases as well as extreme cooling events in the TGR. During the year,the amplitudes of the daily temperature drop (in 24 h) at five stations in the TGR reached the monitoring standards for extreme events, and those at Yichang and Zigui stations even broke historical records. During the cold wave from 26 to 29 March, the maximum temperature drop at each station in the TGR generally exceeded 10 °C; the maximum temperature drop at Wushan station reached 15.5 °C; the extreme minimum temperature at Yichang (3.5 °C) and Wushan (3.8 °C) stations was lower than 5 °C. In addition, from 16 September to 20 October, there were many low-temperature days in the TGR; the average temperature was 2.2 °C lower than that of the same period of normal years, and was the lowest in history since 1961.

4. Conclusions and discussion

The TGR is located in the eastern section of the upper regions of the Yangtze River basin. Its interannual climate variations are mainly affected by external factors, such as the East Asian atmospheric circulation, monsoons, and sea surface temperature (SST). The variations of these factors on different time scales together dominate the occurrence and development of drought and flood disasters in the TGR and the Yangtze River basin. As a typical river-type reservoir, the Three Gorges Reservoir is not comparable to China’s surrounding oceans and the Tibetan Plateau in terms of width and coverage; indeed, the latter two can have an impact on the climate in East Asia, while the Three Gorges Reservoir only has an impact at small scales, local meteorological conditions and ecological environments, with no significant impact on the large-scale regional climate ( Wu et al. 2012 ; Li et al. 2017 , 2021a ;Song et al. 2017 ). The interannual variations of the climate in the TGR are mainly affected by large-scale circulation and SST.

Affected by climate warming, the global hydrological cycle has increased significantly; the intensity of precipitation is becoming extreme,and the frequency and duration of global extreme events are increasing.This is an important background for the worsening of drought and flood disasters in the TGR and its surrounding areas. In 2020, the Yangtze River basin had the most annual precipitation since 1961; there was typical super-strong mei-yu rainfall in summer ( Li et al. 2021b ); and heavy precipitation mainly occurred from June to July. The reason for this was that the middle troposphere at the middle and high latitudes of the Eurasian continent during this period showed a “two ridges and one trough ” pattern ( Fig. 3 (a)), and the vicinity of the Ural Mountains was controlled by positive geopotential height anomalies, which strengthened the blocking high. The area covering the Ob Bay in northern Siberia to the east of Lake Balkhash was controlled by the trough aloft while the ridge aloft, ranging from the southeast of the Far East to northeastern China, was controlled by positive anomalies. The westerly index continued to be below normal, indicating that meridional movement dominated the middle troposphere in East Asia. In addition, the low trough along the East Asian coast was active, and there were many cold and relatively strong atmospheric activities that moved southward on five occasions ( Ding et al. 2021 ). On the other hand, the formation of meiyu depends on a sufficient supply of water vapor. From June to July, the moisture transported to the Yangtze River basin was abnormally strong,and mainly came from the southwest brought by the anomalous Northwest Pacific anticyclone ( Fig. 3 (b)). Both the TGR and the lower regions of the Yangtze River basin were areas for abnormally strong water vapor convergence, conducive to the occurrence of stable and continuous large-scale rainfall in the region. To sum up, the frequent encounters of cold air moving southwards from the north and warm air from the low latitudes was an important reason for the intense rainfall in the TGR and Yangtze River basin from June to July.

From the autumn of 2019 to the spring of 2020, a weak El Ni?o event occurred in the equatorial central and eastern Pacific, and a weak La Ni?a event was formed in the winter of 2020/21. The characteristics of the abnormal circulation during the mei-yu period in 2020 were very similar to the typical characteristics of the atmospheric circulation in the year after a central Pacific El Ni?o event. This circulation configuration was conducive to the transport of water vapor to South China. In addition, from spring to summer in 2020, the SST of the equatorial Indian Ocean continued to be warmer than normal. Affected by this, the local zonal anomalous circulation of “rising in the west and descending in the east ” was triggered in the area ranging from the tropical Indian Ocean to the western Pacific from June to July. The convection in the Indian Ocean was active, while that in the Northwest Pacific was suppressed.These are the possible reasons for the strong intensity of the subtropical high and its average location being situated more to the south than normal. As a result, under the combined effect of the warmer-than-normal Indian Ocean and the weak El Ni?o event, the subtropical high continued to remain strong, and swayed north and south at 19°—24°N from June to July, causing the precipitation in the area spanning the TGR and the lower regions of the Yangtze River to be abnormally abundant during this period.

Funding

This study was supported by the National Key R&D Program of China [grant numbers 2017YFD0300201 , 2017YFA0605004, and 2017YFC1502402 ], and 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.