Characteristics of the Regional Meteorological Drought Events in Southwest China

时间：2024-09-03

LI YunjieREN FuminLI YipingWANG Pengling and YAN Hongming

1College of Atmospheric Science,Nanjing University of Information Science&Technology,Nanjing210044

2State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing100081

3Jiangsu Collaborative Innovation Center for Climate Change,Nanjing210008

4Institute of Arid Meteorology,China Meteorological Administration,Lanzhou730020

5Beijing Climate Center,Beijing100081

6Yunnan Climate Center,Kunming650034

Characteristics of the Regional Meteorological Drought Events in Southwest China During 1960–2010

LI Yunjie1,2,REN Fumin2,3∗,LI Yiping4,WANG Pengling5, and YAN Hongming6

1College of Atmospheric Science,Nanjing University of Information Science&Technology,Nanjing210044

2State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing100081

3Jiangsu Collaborative Innovation Center for Climate Change,Nanjing210008

4Institute of Arid Meteorology,China Meteorological Administration,Lanzhou730020

5Beijing Climate Center,Beijing100081

6Yunnan Climate Center,Kunming650034

An objective identi fi cation technique for regional extreme events(OITREE)and the daily compositedrought index(CI)at 101 stations in Southwest China(including Sichuan,Yunnan,Guizhou,and Chongqing) are used to detect regional meteorological drought events between 1960 and 2010.Values of the parameters of the OITREE method are determined.A total of 87 drought events are identi fi ed,including 9 extreme events.The 2009–2010 drought is the most serious in Southwest China during the past 50 years.The regional meteorological drought events during 1960–2010 generally last for 10–80 days,with the longest being 231 days.Droughts are more common from November to next April,and less common in the remaining months. Droughts occur more often and with greater intensity in Yunnan and southern Sichuan than in other parts of Southwest China.Strong(extreme and severe)regional meteorological drought events can be divided into fi ve types.The southern type has occurred most frequently,and Yunnan is the area most frequently stricken by extreme and severe drought events.The regional meteorological drought events in Southwest China have increased in both frequency and intensity over the study period,and the main reason appears to be a signi fi cant decrease in precipitation over this region,but a simultaneous increase in temperature also contributes.

Southwest China,regional meteorological drought events,temporal characteristics,spatial distribution

1.Introduction

Droughts are major meteorological events with long duration and wide-ranging e ff ects(Zhang,2005). Increases in the frequency of droughts and disasters associated with droughts under global warming have caused great concern.Huang et al.(2006)analyzed decadal variations in the characteristics of droughts and fl oods in China,and identi fi ed an increase in the frequency of droughts in North China after the late 1970s.Some studies have suggested that the area affected by droughts has increased throughout China during the past half-century,although these increases are only statistically signi fi cant in North,Northeast, and Northwest China(Zhang et al.,2003;Zhai and Zou,2005;Ma and Fu,2006;Zou and Zhang,2008; Yang et al.,2013).

Droughts in Southwest China have attracted gre-at concern.A number of studies have examined the characteristics of droughts in this region.Shi et al. (2002,2003)showed that the climate in Northwest China was predominantly warm and dry from the end of the little ice age until the 1980s,but transitioned to warm and wet in 1987.Using the standardized precipitation index,Qi et al.(2006)found that droughts had become more frequent in Southwest China,the western part of Northwest China,and the northern part of North China over the last 60 years. Ma et al.(2006)analyzed the characteristics of climate change in Southwest China.They identi fi ed a signi fi cant warming trend over the Yungui Plateau,as well as decreases in precipitation in the Sichuan basin and the hilly regions of Guizhou.Peng et al.(2009) reported that droughts in Yunnan are most intense between January and March.Two recent studies revealed that precipitation has decreased and temperature has increased over Yunnan since the 1950s(Cheng and Xie,2008;Liu et al.,2010).Liu et al.(2012)revealed that the frequency of droughts increased significantly in the hilly regions of Yunnan,Guizhou,and Guangxi between 1961 and 2009.Severe droughts have been particularly prevalent in Southwest China during the past 10 years,with a severe drought across Southwest China in the summer of 2003,a rare earlyspring drought in Yunnan in 2005,an extreme summer drought in Sichuan and Chongqing in 2006(Li et al., 2011),and a long period of extreme drought in Southwest China from autumn of 2009 through spring of 2010.These trends have been especially pronounced since 2000.

Almost all of the elements or drought indices used in the above studies have been calculated at individual points(observing sites or gridded values).Droughts are typically regional phenomena with distinct durations and areas of impact.A few recent studies have focused on regional drought events.Andreadis et al. (2005)analyzed the characteristics of regional drought events in the United States during the 20th century using monthly soil moisture data.Qian et al.(2011) studied regional drought events in China.Their results indicated that droughts in South and Southwest China generally occur during winter,while droughts in northern China occur more frequently during spring. Ren et al.(2012)proposed an objective identi fi cation technique for regional extreme event(OITREE). This method has been used to identify a number of di ff erent regional extreme events in China,such as droughts,heavy precipitation events,and temperature extreme events. Cui(2010)analyzed regional meteorological drought events in China over the past 50 years.Regional drought events occur most frequently and with greatest intensity in North China, but Southwest China is also seriously in fl uenced by regional droughts(see Fig. 1). Previous studies of meteorological droughts in Southwest China have focused mainly on individual drought events.This study employs the new method proposed by Ren et al.(2012)to study Southwest China regional meteorological drought events between 1960 and 2010.

2.Data and method

2.1 Data

According to the Grade-I China Meteorological Geographic Zoning(CMA Forecast and Mitigation Division,2006),Southwest China includes Yunnan, Sichuan,and Guizhou provinces,and Chongqing Municipality.

The daily composite-drought index(CI)computed by the National Climate Center are adopted in this study.The underlying measurements are taken at 101 stations in Southwest China between 1 January1960 and 31 December 2010.Zhang et al.(2006)have described the method for calculating CI in detail.

Fig. 1. Distribution of the cumulative intensity of regional meteorological drought events in China during 1957–2008(from Cui,2010).

2.2 Method

The OITREE method(Ren et al.,2012)is applied in this study.The technique includes fi ve steps.(1)A daily index is selected for each individual station;(2) the region is partitioned into daily abnormality belts; (3)the temporal continuity of each event is veri fi ed; (4)a standard index for regional events is constructed; and(5)the extremity of each regional event is assessed.

Steps(2)and(3)in this process are the most important.The partitioning of daily abnormality belts starts from a structural analysis of the daily distribution of abnormality,which can then be separated into a set of distinct daily abnormality belts.The temporal continuity of each event is determined by analyzing the spatial overlap of the natural daily abnormality belts on adjacent dates.The index system(Table 1),which is developed based on the features of regional events, includes fi ve independent indices(extreme intensity, accumulated intensity,accumulated area,maximum impacted area,and duration)and one integrated index.

The values of the OITREE parameters are listed in Table 2.These parameters are determined by analyzing a set of Southwest China regional meteorological drought events,and can be summarized as follows. CI is the daily drought index for individual stations. This parameter is required for step(1)as described above.Step(2)requires four parameters.The threshold for abnormality is set to the threshold CI for moderate drought(–1.2),the threshold distance for de fi ning neighboring sites is set to 250 km,the thresholdfor the minimum number of sites in a daily drought belt is set to 11,and the threshold ratio for overlapping sites in a daily drought belt is set to 0.3.For the third step,the threshold for the number of days allowed for a gap in an event is set to zero,so that all events are continuous.

Table 1.The index system for regional weather and climate events

Table 2.Values of the parameters of the OITREE method for identifying Southwest China regional meteorological drought events(adapted from Ren et al.(2012))

The fourth step requires a number of parameters. The direction indicator is set to –1 because of the nature of CI.Ren et al.(2012)cautioned that the fi ve weighting coefficients used to calculate the integrated indexZmust be determined objectively and carefully. Here,we use the following objective method to calculate these fi ve coefficients.First,the absolute values of the standardized drought indices are ranked in descending order.Second,the ratio of the sum of the top 10%percentile of values to the sum of all values in each series is calculated.Third,the fi ve coefficients are calculated by multiplying the direction indicator for each index and the ratio for each series divided by the sum of the fi ve ratios.The fi ve coefficients obtained using this approach are –0.06, –0.32,0.28,0.10,and 0.24, respectively.More than 200 events with durations of 10 days or longer are identi fi ed during 1960–2010.

Step(5)involves a comparison of di ff erent re-gional drought events.Figure 2 shows the frequency distribution of di ff erent values of the integrated indexZ.Small values ofZ(Z＜0.3,corresponding to weak events with small impacted areas)occur with the greatest frequency.The threshold for de fi ning a Southwest China regional meteorological drought event is therefore set to 0.3.A total of 87 Southwest China regional meteorological drought events are identi fi ed. Following Ren et al.(2012),these 87 events are divided into extreme droughts(9 events,about 10%of the total),severe droughts(17 events,about 20%), moderate droughts(35 events,about 40%)and slight droughts(26 events,about 30%).The threshold values of the integrated indexZfor distinguishing these four classes of drought events are 3.63,1.72,and 0.70, respectively.

3.Examples of the identi fi ed drought events

According to the integrated intensity,the three strongest drought events between 1960 and 2010 are identi fi ed as follows.The fi rst strongest event lasted for 231 days from September 2009 through April 2010 and covered most of Southwest China.The second strongest event lasted for 217 days from November 1962 through June 1963.The third strongest event lasted for 134 days from January to June 1969,and in fl uenced all of Southwest China.

Previous studies have shown that these three extreme drought events were associated with serious economic losses(Ding,2008;Zhang et al.,2009). The methodology also identi fi es other recent severe drought events,such as the serious autumn drought in Yunnan during 2003(number 15)and the extreme drought that occurred in Sichuan and Chongqing during 2006(number 14).

Figure 3 shows a variety of indices calculated for an extreme meteorological drought event that persisted from 4 September 2009 to 22 April 2010.The maximum area impacted by this event is 118.4×104km2.Most regions of Yunnan,western Guizhou,and southern Sichuan are seriously a ff ected(Fig.3a).The daily variations of integrated intensity,extreme intensity,accumulated intensity,and impacted area are largely consistent(Figs.3b–e).The drought event began in September 2009 and strengthened quickly during October before reaching its fi rst peak in intensity around the mid November.The drought then entered a relatively weak phase until January 2010,before strengthening again and reaching a second peak in February and March 2010.The drought eased and ended in April 2010.These results are well consistent with the drought monitorings.

4.Analysis of results

4.1 Temporal characteristics

Fig.2.Frequency(time)distribution of the integrated indexZfor Southwest China drought events during 1960–2010.

Fig.3.(a)The spatial distribution of accumulated intensity of the 2009–2010 Southwest China regional extreme meteorological drought event and daily variations of(b)integrated intensity,(c)extreme intensity,(d)accumulated intensity,and(e)impacted area of this event.Dotted lines indicate polynomial trends.

Figure 4 shows frequency distributions of the duration and maximum impacted area of the 87 Southwest China regional meteorological drought events. These events generally last for 10–80 days,with a peak at 30–40 days.The frequency of the events that last longer than 80 days is very small.Only three events last for more than 200 days,with the longest being 231 days.All of these long-duration events are also extreme drought events,ranking fi rst,second,and fourth out of the 87 events.The impacted area of these events varies between 30×104and 130×104km2,with a peak at 70×104–100×104km2.

Regional drought events generally start between late autumn and early spring and end before mid summer( fi gure omitted).Figure 5 shows seasonal variations of the regional meteorological drought frequency. Droughts are most frequent during November–April, and relatively infrequent within May–October.We therefore de fi ne November to April as the drought season and May to October as the non-drought season.

Figure 6 shows interannual variations of Southwest China regional meteorological drought event indices.The frequency of droughts has increased at a rate of 0.19 per decade(statistically signi fi cant at the 0.05 level according to Student’sttest).The largest number of droughts occurs in 1994(5 times),1992(4 times),and 1988(4 times).No droughts occur in 1965 or 1967.Although the frequencies in 2010 and 1963 are both listed as 0,both years include the drought events that started the year before.The frequency statistics only account for the start time of drought events. Although accumulated integrated intensity(Fig.6b) and accumulated maximum impacted area(Fig.6c) both increase during 1960–2010,these increases are not statistically signi fi cant.The top three values of accumulated integrated intensity occur in 2009,1978, and 1962.The top three values of accumulated maximum impacted area occur in 1994,1992,and 1988. The top three years in terms of frequency are the same as those for accumulated maximum impacted area,but are completely di ff erent from those for accumulated integrated intensity.Severe drought events often last for a long time,so the frequency is low during the years within which they occur(such as 2009). Weak droughts generally last for shorter periods and are therefore more likely to repeat during a single year (such as 1994).

Fig.4.Frequency distributions for(a)duration and(b)maximum impacted area of Southwest China regional meteorological drought events between1960 and 2010.

Fig.5.Seasonal variations of Southwest China regional meteorological drought event frequency between 1960 and 2010.

The interannual variations of precipitation and mean temperature between 1960 and 2010(Fig.7) are analyzed to explore the reasons why regional meteorological drought events have become stronger and more frequent in Southwest China.Annual precipitation has decreased signi fi cantly at a rate of 12.63 mm per decade,while annual mean temperature has increased at a rate of 0.15℃ per decade.

The correlations between the time series of annual precipitation(Fig.7a)and the time series of drought frequency and accumulated integrated intensity(Figs. 6a and 6b)are –0.44 and –0.53,respectively.Both values are statistically signi fi cant at the 0.01 level based on Student’sttest.The correlations between the time series of mean temperature(Fig.7b)and the time series of drought frequency and accumulated integrated intensity(Figs.6a and 6b)are 0.19 and 0.25,respectively.This result suggests that the main reason why droughts have become stronger and more frequent lies in the signi fi cant decrease in precipitation,although the increase in temperature also contributes.Analysis of the accumulated integrated intensity on decadal timescales( fi gure omitted)indicates that the intensity of droughts in the early 21st century has been signi ficantly stronger than the intensity of droughts in other periods.The average annual precipitation is smaller after 2000 than during other periods in the past 50 years(Fig.7),while the temperature is much higher. These may be important factors in droughts becoming more serious in the fi rst decade of the 21st century.

Southwest China regional meteorological drought events vary on seasonal timescales,with drought season between November and next April.Correlation coefficients have been calculated for the drought season to further examine the relationships between precipitation,temperature,and drought intensity(Table 3). Accumulated integrated intensity is negatively correlated with precipitation during all analyzed time periods,with statistically signi fi cant relationships for annual precipitation,drought season precipitation,and September–April precipitation.The strongest correlation(–0.68)is for September–April precipitation. Accumulated integrated intensity is positively correlated with temperature during all analyzed time periods,but none of the correlations is statistically signi ficant.To summarize,the intensity of Southwest China regional meteorological drought events between 1960 and 2010 is most closely related to precipitation during the September–April season.

Fig.6.Interannual variations of the(a)frequency,(b)accumulated integrated intensity,and(c)accumulated maximum impacted area of Southwest China regional meteorological drought events between 1960 and 2010.

Table 3.Correlation coefficients between the annual accumulated integrated intensity of Southwest China regional meteorological drought events and regional mean precipitation and temperature over Southwest China during di fferent time periods

4.2 Spatial distribution characteristics

Figure 8 shows spatial distributions of the frequency and accumulated intensity of Southwest China regional meteorological drought events between 1960 and 2010.Drought frequency is generally greater than 20 across Southwest China.The highest frequencies occur in Yunnan and southern Sichuan,with peak values of greater than 70 near the Yunnan–Sichuan border.The spatial distribution of accumulated intensity is similar,with peak values in eastern-northern Yunnan and southern Sichuan.The spatial distribution of drought in Southwest China over the past 50 years is characterized by two typical patterns of “strong south and weak north” and “strong west and weak east”. To summarize,Yunnan and southern Sichuan are themost heavily impacted areas by droughts in Southwest China.

Figure 9 shows the spatial distribution of trends in drought event frequency in Southwest China during 1960–2010.The frequency of droughts has increased across Southwest China,with signi fi cant increasingtrends in central and northern Yunnan and northern and western Guizhou.

Fig.7.Variations of(a)annual precipitation and(b)mean temperature over Southwest China between 1960 and 2010. The dotted line represents the linear trend,and the short thick lines represent average values for each decade.

Fig. 8. Spatial distributions of(a)frequency(time)and(b)accumulated intensity of Southwest China regional meteorological drought events between 1960 and 2010.

Fig.9.Spatial distribution of linear trends in the annual frequency(per decade)of Southwest China regional meteorological drought events between 1961 and 2010.The shaded area indicates the trends that are statistically signi fi cant at the 0.05 level based on Student’sttest.

Among the 87 identi fi ed drought events,26 can be categorized as extreme and severe drought events (9 extreme events and 17 severe events).These 26 events can then be divided into fi ve types based on the spatial distribution of their accumulated intensity: eastern type,southern type,western type,northern type,and whole-area type.Figure 10 shows spatial distributions of the accumulated intensity for representative cases of the fi ve distribution types.Eastern type events mainly cover Guizhou and Chongqing (Fig.10a).Southern type events are concentrated in Yunnan(Fig.10b).Western type events are generally located in western Sichuan and Yunnan(Fig. 10c).Northern type events mainly occur in northern Sichuan and Chongqing(Fig.10d).Whole-area type events extend across Southwest China(Fig.10e).Figure 11 shows frequency of the fi ve-type extreme andsevere droughts.Thirteen of the total 26 events and 6 of the 9 extreme events are southern type events. The second most frequent type is the whole-area type (8,including the remaining 3 extreme events).Two of the events are western type,two are northern type, and one is eastern type.Yunnan is the area most frequently impacted by the extreme and severe drought events in Southwest China.

Fig.10.Spatial distributions of accumulated intensity for representative cases of the fi ve distribution types of Southwest China regional meteorological drought events:(a)eastern type(19 November 1988–2 December 1989),(b)southern type (3 December 1978–25 June 1979),(c)western type(11 October 1981–2 February 1982),(d)northern type(11 July–17 September 2006),and(e)whole-area type(23 January–5 June 1969).

Fig.11.Frequency of the extreme and severe Southwest China regional meteorological drought events in the fi ve distribution types.

5.Conclusions and discussion

The results of the above analysis can be summarized as follows.

(1)The OITREE method shows a good capability for identifying Southwest China regional meteorological drought events.A total of 87 events are identi fi ed between 1960 and 2010,including 9 extreme events and 17 severe events.The 2009–2010 extreme drought is the most serious meteorological drought during the past 50 years.

(2)SouthwestChina regionalmeteorological drought events generally last for 10–80 days,with the longest being 231 days.The maximum impacted area varies between 70×104and 100×104km2. The drought season in Southwest China runs from November to next April and the non-drought season runs from May to October.

(3)Droughts are most frequent and most intense in Yunnan and southern Sichuan.Extreme and severe drought events can be divided into fi ve types based on their spatial distributions:southern type,western type,northern type,eastern type,and whole-area type.The southern type occurs the most frequent. Yunnan is the area most frequently stricken by extreme and severe drought events in Southwest China.

(4)SouthwestChina regionalmeteorological drought events have become more frequent and stronger over the past 50 years.The main reason for these increases appears to be a signi fi cant decrease in precipitation in Southwest China,although an increase in temperature also contributes.

This study has analyzed the characteristics of regional meteorological drought events in Southwest China.Further studies are needed to reveal the causes of and speci fi c mechanisms that lead to droughts in Southwest China.

Acknowledgments.The language editor of this manuscript is Dr.Jonathon S.Wright.

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(Received October 12,2013;in fi nal form March 27,2014)

Supported by the National Natural Science Foundation of China(41175075)and Climate Change Special Fund of the China Meteorological Administration(CCSF201333).

∗Corresponding author:fmren@163.com.