The increasing predominance of extreme precipitation in Southwest China since the late 1970s

Guowei Zheng ,Yng Li ,Qunling Chen ,Xin Zhou ,c ,Guolu Go ,Minggng Li ,Ting Dun

a Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, College of Atmospheric Science, Chengdu University of Information Technology, Chengdu,China

b Fushun Meteorological Observatory, Liaoning Meteorological Bureau, Fushun, China

c School of Earth and Environment, University of Leeds, Leeds, UK

d Ya’an Meteorological Observatory, Sichuan Meteorological Bureau, Ya’an, China

e Plateau and Basin Rainstorm, Drought and Flood Key Laboratory of Sichuan Province, Chengdu, China

Keywords:Extreme precipitation Southwest China Trend Frequency Intensity

ABSTRACT Extreme precipitation events cause severe environmental and societal damage worldwide.Southwest China (SWC)is sensitive to such effects because of its overpopulation,underdevelopment,and fragile ecosystems.Using daily observations from 108 rain-gauge stations,the authors investigated the frequency of extreme precipitation events and their contribution to total precipitation in SWC since the late 1970s.Results indicate that total precipitation is decreasing insignificantly,but rainfall-events frequency is decreasing significantly,whereas the region is experiencing more frequent and intense extreme precipitation events.Note that although fewer stations are statistically significant,about 60% of the rain-gauge stations show an increasing trend in the frequency and intensity of extreme precipitation.Furthermore,there is an increasing trend in the contribution of total extreme precipitation to total precipitation,with extreme precipitation becoming dominant in the increasingly arid SWC region.The results carry important implications for policymakers,who should place greater emphasis on extreme precipitation and associated floods and landslides when drafting water-resource management policies.

1.Introduction

Extreme precipitation is receiving increasing attention because of its devastating societal,economic,and environmental impacts (Bao et al.,2017 ;Pfahl et al.,2017).The Sixth Assessment Report of the Intergovernmental Panel on Climate Change indicates that warming of the climate system is unequivocal;global surface temperature was 1.09°C(0.95°C to 1.20°C) higher in 2011–2020 than 1850–1900,with larger increases over land (1.59°C (1.34°C to 1.83°C)) than over the ocean(0.88°C (0.68°C to 1.01°C)) (IPCC,2021).Global warming had led to an increased frequency and/or intensity of some meteorological and climatic extremes (Wang et al.,2017 ;Du et al.,2019 ;Zhang and Wang,2019 ;Hu and Sun,2020),including extreme precipitation.However,changes in extreme precipitation are highly unlikely to be uniform.While the mid–high latitudes and tropical regions become wetter,the subtropics become drier (Chou and Neelin,2004 ;Chou et al.,2009).

Previous studies have focused on changes in extreme precipitation in China (Zhai et al.,1999 ;Xu et al.,2005 ;Zhang et al.,2009,2010 ;Sun and Zhang,2017),revealing that,for example,extreme precipitation has increased significantly at a rate of 22.6% per 1°C of surface warming in China,which is higher than the global mean rate (Sun and Ao,2013).This result indicates that extreme precipitation in China might have unique characteristics.

Fig.1.Topography of Southwest China (shading: surface elevation;units: m) and the climatology of (b) total precipitation (units: mm),(c) precipitation frequency(units: days),and (d) the contribution of total extreme precipitation to total precipitation (units: %) at 108 rain-gauge stations (filled circles) during 1976–2017.

Southwest China (SWC),southeast of the Tibetan Plateau,is characterized by a complex and mountainous terrain with an average height of 1500 m above sea level,with an elevation range of 100–4000 m(Fig.1 (a)).Such complex terrain often enhances the impact of the most intense convective rainfall episodes,resulting in frequent landslides and flash floods (Xiao et al.,2018).The SWC region is sensitive to climate change because of its overpopulation,underdevelopment,and fragile ecosystems (Deng et al.,2009 ;Fan et al.,2011).Some studies have indicated a heterogeneously changing trend in extreme precipitation over SWC from 1959 to 2012,with decreasing spatial variability (Liu et al.,2014,2015a).The reported trends are often inconsistent over different periods.For example,Li et al.(2012) found that changes in extreme precipitation were relatively small during 1961–2008,while Xiao et al.(2018) detected an increasing trend in extreme precipitation in SWC from 1971 to 2013.

Global atmospheric and oceanic systems underwent a regime shift at around 1976 (Nitta and Yamada,1989 ;Trenberth,1990 ;Trenberth and Hurrell,1994 ;Mantua et al.,1997 ;Zhang et al.,1997 ;Wang,2001),and this influenced regional patterns of climate change.For example,previous studies indicate a decadal shift in precipitation over China from around 1976 (Wang et al.,2001 ;Yu et al.,2015).Li et al.(2015) reported a significant wetting trend over the Yangtze River valley since the late 1970s.Ding et al.(2010,2014) found the major monsoon rainbelt shifted southward from North China to South China since the end of the 1970s.

The above results suggest that a regime shift at around 1976 might affect the trends in extreme precipitation.To avoid the potential impacts of a decadal shift on trends and considering the trends in extreme precipitation over SWC since 1976 have yet to be investigated,the present study investigated trends in extreme precipitation over SWC since 1976 using recent rain-gauge datasets,and the contribution of extreme precipitation events to total precipitation.

2.Data and methods

2.1. Data

Southwest China covers the range (20°–35°N,97°–112°E),as shown in Fig.1 (Wang et al.,2015,2018),and includes Yunnan,Guangxi,Sichuan,Guizhou,and Chongqing.The data used in this study are from 108 rain-gauge stations (Fig.1 (a)) of the National Meteorological Information Center,China Meteorological Administration (http://data.cma.gov.cn),and cover the period 1976–2017.

Fig.2.Decadal trends (units: d/10 yr,number of wet days or very wet days per 10 years) for the period 1976–2017 of (a) annual precipitation frequency,(b)annual extreme precipitation frequency,and (c) extreme precipitation frequency as a proportion of total annual precipitation frequency (units: %/10 yr).Filled circles indicate statistically significant results at the 0.1 level.

2.2. Methods

Due to the complex and mountainous topography of SWC,we used relative rather than absolute precipitation thresholds to study extreme precipitation.Following World Meteorological Organization definitions,various precipitation indices based on daily precipitation were applied(Frich et al.,2002 ;Zou and Zhou,2013).For mean precipitation indices,annual total precipitation (hereafter,PRCPTOT) is defined as the total precipitation from wet days (daily precipitation ≥ 1 mm);precipitation frequency is defined as the number of wet days;and precipitation intensity is defined by dividing the PRCPTOT by the precipitation frequency.For extreme precipitation indices,the extreme precipitation amount is defined as the total precipitation from very wet days (daily precipitation ≥ 95th percentile of wet days);the extreme precipitation frequency is defined as the number of very wet days;and the extreme precipitation intensity is defined by dividing the extreme precipitation amount by the extreme precipitation frequency.Linear least-squares methods were used to determine trends,which were tested for significance using the Mann–Kendall (M–K)test.

3.Results

3.1. Climatological mean

Fig.1 (b–d) shows the climatological mean PRCPTOT,annual precipitation frequency,and the contribution of total extreme precipitation to PRCPTOT for the period 1976–2017.Comparing the climatological PRCPTOT (Fig.1 (b)) and altitude (Fig.1 (a)),we can see that PRCPTOT is highly negatively correlated with altitude.For the climatological precipitation frequency (Fig.1 (c)),south-central Sichuan,Guizhou,and northern Guangxi experience the most rainfall events.Fig.1 (d) shows that the climatological contribution of total extreme precipitation to PRCPTOT increases from west to east,which is negatively correlated with altitude.

3.2. Frequencies of precipitation and extreme precipitation

Decadal trends in annual precipitation and extreme precipitation frequencies in SWC during 1976–2017 are shown in Fig.2 (a,b),respectively.The annual precipitation frequencies at almost all stations have a clear decreasing trend,except for two stations in northwest Sichuan province.Most trends are statistically significant at the 0.1 level based on the M–K test,confirming that the frequency of precipitation in SWC has been decreasing since 1976.The extreme precipitation frequencies show increasing trends at 68 stations (nine statistically significant) and decreasing trends at 40 stations (three statistically significant).Fig.2 (c) shows trends in extreme precipitation frequency as a proportion of the total annual precipitation frequency (extreme precipitation frequency/precipitation frequency),with 97 stations having increasing trends (24 statistically significant) and 11 decreasing trends (none statistically significant),indicating that the extreme precipitation frequency as a proportion of the total annual precipitation frequency is increasing in SWC.These results indicate a decreasing decadal precipitation frequency and an increasing extreme precipitation frequency in SWC since 1976.

3.3. Intensities of precipitation and extreme precipitation

Spatial trends in mean annual precipitation and extreme precipitation intensities are shown in Fig.3.For the former,Fig.3 (a) indicates a widespread increasing trend,gradually enhanced from north to south,with trends at over 50% of stations being statistically significant.For the latter (Fig.3 (b)),64 stations have increasing trends (11 statistically significant) and 44 decreasing trends (four statistically significant).

Fig.3.Decadal trends for the period 1976–2017 of (a) mean annual precipitation intensity and (b) mean annual extreme precipitation intensity (units: mm d-1/10 yr).

These results indicate that the annual mean precipitation intensity in SWC increased significantly (95% stations increased) during 1976–2017,while the annual mean extreme precipitation intensity increased insignificantly (59% of stations increased).

3.4. Amounts of precipitation and extreme precipitation

With the increasing frequency and intensity of extreme precipitation since 1976,the amount of extreme precipitation and its contribution to total precipitation would also be expected to be increasing.Fig.4 shows the spatial trends in PRCPTOT,extreme precipitation amount,and the percentage contribution of extreme precipitation amount to PRCPTOT(extreme precipitation amount/PRCPTOT,%).Fig.4 (a) indicates that PRCPTOT shows decreasing trends at 57 stations (eight statistically significant) and increasing trends at 51 stations (none statistically significant).The spatial trend from northwest to southeast follows a pattern in which it first increases,then decreases,and finally increases again.Stations with decreasing trends are located mainly in Yunnan and Guizhou.The decreasing trend at eight stations is statistically significant.Fig.4 (b)indicates that 76 stations show an increasing trend in extreme precipitation amount (11 statistically significant) and 32 show a decreasing trend (three statistically significant).

Fig.4.Decadal trends for the period 1976–2017 of (a) annual PRCPTOT (units: mm/10 yr),(b) annual extreme precipitation amount (units: mm/10 yr),and (c)the contribution of annual extreme precipitation amount to annual PRCPTOT (units: %/10 yr).

Fig.5.Time series of PRCPTOT (left axis;red line;units: mm),extreme precipitation amount (first axis on the right;black line;units: mm),and the contribution of extreme precipitation amount to PRCPTOT (second axis on the right;blue line;units: %) over SWC during 1976–2017.Dashed lines are regression lines.

Fig.4 (c) indicates that 80 stations show an increasing trend (15 statistically significant) in the percentage contribution of the extreme precipitation amount to PRCPTOT,and 28 show a decreasing trend (one statistically significant).The latter stations are located mainly in eastcentral Sichuan and western Chongqing.These results indicate that the extreme precipitation amount in SWC increased significantly (70% stations increased),as did the contribution of the extreme precipitation amount to PRCPTOT (74% of stations increased),during 1976–2017.

Fig.5 shows the time series of PRCPTOT (red line),the extreme precipitation amount (black line),and the contribution of the extreme precipitation amount to PRCPTOT (blue line) over SWC since 1976.PRCPTOT shows a slowly decreasing decadal trend of -2.3 mm/10 yr (not statistically significant);and the extreme precipitation amount shows an increasing trend of+7.1 mm/10 yr (not significant),affected by some stations with decreasing trends.The contribution of the extreme precipitation amount to PRCPTOT shows a clear increasing trend (0.6%/10 yr)since 1976,which is statistically significant at the 95% level,with an average contribution during 1976–1980 of 32.56%,increasing to 35.5%during 2013–2017.These results indicate that the contribution of extreme precipitation to total precipitation in SWC has been increasing since 1976.

4.Conclusion and discussion

In this study,we investigated the trend in precipitation and extreme precipitation in SWC during 1976–2017 based on daily rain-gauge station data,including frequency,intensity,amounts,and the contribution of extreme precipitation.Results showed that,although PRCPTOT is decreasing insignificantly,the frequency of precipitation is decreasing significantly,and the annual mean precipitation intensity is increasing in most parts of SWC.While the extreme precipitation amount is increasing significantly,the frequency and the intensity of extreme precipitation are increasing insignificantly.The frequency of extreme precipitation as a proportion of rainfall-events frequency is increasing significantly.Moreover,the contribution of the extreme precipitation amount to PRCPTOT is increasing significantly,which means that extreme precipitation has been becoming more dominant in SWC since 1976.Our findings indicate that policymakers should pay more attention to extreme precipitation when formulating sustainable water-resource management policies.

The increasing frequency and intensity of extreme precipitation can partly explain the increasing frequency of floods in SWC (Zhou and Lei,2018 ;Xu et al.,2021 ;Yan et al.,2021).In contrast to previous studies of extreme precipitation (Li et al.,2012 ;Xiao et al.,2018),the present study chose the period 1976–2017 to avoid the potential impacts of a decadal shift on the trends.After testing three extreme precipitation definitions (daily precipitation ≥ 90th,95th,and 99th percentile of wet days),we found the results are not sensitive to the definition of extreme precipitation.However,due to the sparse distribution of rain-gauge stations in SWC,our conclusions need to be verified with more refined data in the future.

According to the Clausius–Clapeyron equation,atmospheric water vapor and precipitation will increase by approximately 7% K-1 increase in temperature.Global warming is certainly an important reason for the increasing extreme precipitation.In addition,there have been many studies on the causes of extreme precipitation changes,such as moisture transport (Sarkar and Himesh,2021),the water-vapor lapse rate–climate feedback effect (Held and Soden,2006 ;Liu et al.,2009,2015b),aerosols changes (Yang et al.,2011),and local/teleconnection forcing factors (Wang et al.,2015 ;Li et al.,2015).For SWC,the underlying mechanisms need to be further studied in the future.

Funding

This work was jointly supported by the National Natural Science Foundation of China [grant numbers U20A2097,42175042,41905037,and 41805054 ],the China Scholarship Council [grant numbers 201908510031 and 201908510032 ],and the Plateau and Basin Rainstorm,Drought and Flood Key Laboratory of Sichuan Province[grant number SCQXKJZD202102-6].