Observed trends of precipitation amount, frequency, and intensity in China,

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jd004864, 2005 Observed trends of precipitation amount, frequency, and intensity in China, Binhui Liu, 1,2 Ming Xu, 3 Mark Henderson, 4 and Ye Qi 5 Received 4 April 2004; revised 10 December 2004; accepted 28 January 2005; published 20 April [1] We examined the spatial and temporal variation in precipitation observed daily at 272 weather stations operated by the China Meteorological Administration from 1960 to We found that precipitation in China increased by 2% over that period, while the frequency of precipitation events decreased by 10%. Seasonally, precipitation increased in winter and summer but decreased in spring and fall. Regional differences also appeared: Precipitation decreased in the North China Plain and north central China, showed almost no change in southwest China, and increased in China s five other climatic regions. Only the increase in northwest China, however, was statistically significant ( p = 0.05). For China as a whole and its eight climatic regions these changes in precipitation can be attributed mostly to changes in the frequency and intensity of heavy precipitation events. Nationwide, the increased frequency of heavy precipitation events contributed 95% of the total increase of precipitation in that category. At the same time, there were fewer light precipitation events, accounting for 66% of the national reduction in precipitation frequency. In seven of the eight climatic regions, changes in frequency accounted for most of the changes in the amounts of precipitation from heavy precipitation events; changing intensity accounted for a larger share in the southwestern region. The frequency of precipitation has decreased in all seasons and all regions except northwest China. The increasing proportion of precipitation delivered by heavy rainfall events and the decreasing trend of light precipitation events have potentially serious ramifications for flood control and vegetation production, especially for the non-irrigated croplands in the arid and semiarid areas of China. Citation: Liu, B., M. Xu, M. Henderson, and Y. Qi (2005), Observed trends of precipitation amount, frequency, and intensity in China, , J. Geophys. Res., 110,, doi: /2004jd Introduction [2] Global climate change associated with increasing emissions of greenhouse gasses is expected to have a considerable impact on the global hydrocycle. Climate simulations predict an intensification of the hydrological cycle under increased greenhouse gas conditions at global and regional scales [McGuffie et al., 1999; Frei et al., 1998]. In particular, heavy precipitation events are expected to increase in frequency and magnitude [Fowler and Hennessy, 1995; Gong and Wang, 2000]. Analyses of precipitation observed in many regions around the world, 1 College of Forestry, Northeast Forestry University, Harbin, China. 2 Also at Center for Remote Sensing and Spatial Analysis, Department of Ecology, Evolution and Natural Resources, Rutgers University, New Brunswick, New Jersey, USA. 3 Center for Remote Sensing and Spatial Analysis, Department of Ecology, Evolution and Natural Resources, Rutgers University, New Brunswick, New Jersey, USA. 4 Department of Environmental Science, Policy, and Management, University of California, Berkeley, California, USA. 5 Global Environment Policy Center, Beijing Normal University, Beijing, China. Copyright 2005 by the American Geophysical Union /05/2004JD while using different methods to categorize precipitation, support the predictions of increasing heavy precipitation events [Karl et al., 1995; Karl and Knight, 1998; Suppiah and Hennessy, 1998; Tarhule and Woo, 1998; Zhai et al., 1999], though the opposite trend has been observed in Britain during the summer [Osborn et al., 2000]. The changing frequency of precipitation events presents an even more complex puzzle than changes in total annual or seasonal precipitation. Precipitation frequency has increased in the United States [Karl and Knight, 1998] but decreased in China [Zhai et al., 1999] and Nigeria [Tarhule and Woo, 1998]. [3] Total precipitation in a season or year is determined by both the number and intensity of precipitation events in that period. Until recently, these separate components have received insufficient attention in empirical and modeling studies [Trenberth et al., 2003]. Separating the contributions of frequency and intensity and their trends to total precipitation trends is critical to understanding the longterm effects of global warming on the global hydrological cycle. [4] Precipitation varies greatly by season and region across China, ranging from less than 100 mm/yr in the arid northwest to more than 1000 mm/yr in southeast, though a common factor is that most precipitation events are con- 1of10

2 centrated in the summer monsoon season. Given the large variation of precipitation in China, it is difficult to detect changes in the characteristics of precipitation across different regions and seasons using fixed (rather than relative) categories of precipitation intensity. Most previous studies of precipitation in China have focused on changes in annual total precipitation, generally finding no statistically significant trends. Karl et al. [1995] analyzed changes in precipitation amounts across fixed categories of precipitation intensity, but found little systematic change in China. Zhai et al. [1999] also used fixed categories of precipitation intensity (divided at 10, 50, and 100 mm/day) to analyze the change in extreme precipitation events. Their analysis showed no significant trends for extreme precipitation events in China. However, they found that precipitation decreased in frequency (there were fewer days with precipitation 0.1 mm) and increased in intensity (mm per precipitating day). In another study, Gong and Wang [2000] reported a significant decrease of precipitation in eastern China from the 1950s to the mid-1970s and an increasing trend since the late 1970s. [5] In this paper, we present an analysis of the changing character of precipitation in China and its eight climatic regions on an annual and seasonal basis. We defined relative categories of precipitation intensity following the method proposed by Karl and Knight [1998] on the basis of the frequencies at each station throughout the study period. Relative rather than fixed categories are preferable because of the large spatial and temporal variation in precipitation across China. For example, a change of precipitation of 10 mm in southeastern China is only about 1% of its annual total, but the same amount may account for up to 50% of the total annual precipitation in the deserts of northwest China. The use of relative categories facilitates the analysis of spatial trends across regions with different climate conditions. Furthermore, Karl and Knight [1998] show that this method can also be used to evaluate the relative contributions of changing frequency and intensity to the trends in total precipitation. 2. Data [6] Data including daily precipitation measurements from 305 stations for the period of was provided for this study by the China Meteorological Administration. The stations were well distributed across China including the Tibetan plateau. Measurements at all of the climate stations were made using the same standards and instrumentation, ensuring the homogeneity of data quality. [7] The consistency and completeness of the data set are of particular importance to a study of precipitation frequency and intensity. Precipitation differs from other climate variables such as temperature that change gradually from hour to hour and day to day. Missing data for such other variables can be estimated by means such as stepwise regression [Liu et al., 2004], but this would be inappropriate for precipitation because most precipitation events are more abrupt in time and space. Instead, we took strict measures to exclude stations and years with missing data from our analysis. Most of the cases where stations data were missing for three or more consecutive days occurred before 1960, so in this study we consider only the period of 1960 to 2000, excluding the earlier part of the data set from our analysis. Some 33 stations with missing data for consecutive three or more days were also excluded from our analysis, leaving 272 stations. Records from these stations in the study period are over % complete, with only 40 missing observations from 31 stations. No single station accounted for more than four missing observations or two consecutive missing observations. Therefore the missing data should have minimal impact on our results. 3. Methods [8] The first steps in our analysis involved aggregating the daily observations of the 272 stations by season, region, and categories of precipitation intensity. As others have long recognized, the conventional division of the year into four equal seasons is not always appropriate for the analysis of monsoon-dominated climates as found in China. From October to March, China is mainly influenced by the winter monsoon. By late March and early April, the winter monsoon becomes weaker and retreats from south to north, until by late May the summer monsoon reaches the south coast of China. The summer monsoon holds sway across most of China from June to August, reaching north China in July. In early September, the summer monsoon retreats quickly to the south, as the winter monsoon reaches north China moving from north to south [Ye, 1958; Samel et al., 1999; Sun et al., 1999; Zeng and Lu, 2004]. Recognizing these annual patterns, for this analysis we divided the year into the winter monsoon season (October through March), a spring transition period (April and May), the summer monsoon season (June through August), and a fall transition period (September). For convenience, hereinafter we refer to these four seasons simply as winter, spring, summer and fall. For comparison with previous studies we also present an analysis of annual trends not stratified by seasons. [9] China s eight climatic regions are defined by latitude and longitude (Figure 1) but coincide roughly with the country s socioeconomic macroregions [Qi et al., 2004]. To produce a spatially unbiased characterization of the precipitation histories of each region, we first superimposed a5 5 grid on the map of China and assigned each weather station to a grid cell. Monthly precipitation values for each cell were calculated from the arithmetic average of daily precipitation data for all of the stations in each cell. Two cells in the south central Tibetan Plateau had no weather stations; for these grids, we estimated precipitation as the arithmetic average of their four neighboring grid cells. The grid cells were then weighted by surface area within China s borders to calculate the average monthly precipitation for each region. The regional values were in turn weighted by area to derive national averages, and seasonal averages were computed from the monthly values in each season. [10] We defined categories of precipitation intensity for each station and each season following the method introduced by Karl and Knight [1998]. Daily precipitation totals 0.1 mm were treated as precipitation events [Zhai et al., 1999]. The precipitation events observed at each station over the entire study period were grouped into equal frequency deciles, similar to the twenty-category method 2of10

3 Figure 1. Geographical distribution of the 272 weather stations used in this study and the eight climatic regions of China. applied by Karl and Knight [1998]. For any given station and season, the bottom decile included the ten percent of precipitation events with the lowest intensities, while the top decile included the ten percent of events with the greatest intensities. 4. Results and Discussion 4.1. Spatial and Seasonal Character of Precipitation Change [11] Annual total precipitation increased slightly in China over the period of (Table 1). Increases amounted to 2.6mm per decade in winter and 3.5mm per decade in summer, offset partially by decreases of 1.3mm per decade in spring and 2.0mm per decade in fall. Only the decrease in the fall transition period was statistically significant (p = 0.05). The number of precipitation days decreased by 2.3 days per decade over the study period with a decreases of 1.0, 0.2, 0.5, and 0.6 days per decade in winter, spring, summer and fall respectively. The decrease was statistically significant for all seasons except spring. These findings corroborate the earlier study of China by Zhai et al. [1999]. However, the decrease in frequency of precipitation in China, both annually and seasonally, runs counter to the trend reported for the contiguous United States by Karl and Knight [1998]. [12] On a regional basis, annual precipitation decreased in the North China Plain and north central China; increased in the northeast, northwest, east, southeast, and the Tibetan Plateau; and saw almost no change in the southwest (Figure 2). The frequency of precipitation increased significantly (p = 0.05) in the northwest and decreased in the seven other regions (but not significantly in the southeast and the Tibetan Plateau). Seasonally, precipitation increased significantly in the northwest, northeast, east, and southeast in winter and east in summer. It decreased significantly in the east and north central China in fall as well as in the east and southwest in spring (Figure 3). The seasonal change of precipitation frequency for the eight climatic regions reveals a decreasing trend for all four seasons, with the exceptions of the northwest in winter and spring, the northeast in winter, the Tibetan Plateau in spring, and the east in summer. Noting that the spatial pattern of change in precipitation differs from that of frequency, we continued by analyzing the relative contributions of precipitation intensity and frequency to the trends in precipitation amount Contribution to Change of Different Precipitation Intensities [13] The rate of change in precipitation differed among the ten categories of precipitation intensity defined in this Table 1. Annual and Seasonal Trends of Precipitation Amount and Frequency in China, Trends (Per Decade) Annual Winter Monsoon (October March) Spring Transition (April May) Summer Monsoon (June August) Fall Transition (September) Precipitation amount Change rate, mm/decade a Change percentage, % a Precipitation frequency Change rate, d/decade 2.3 a 1.0 a a 0.6 a Change percentage, % 2.4 a 0.3 a a 6.1 a a Statistically significant trends ( p = 0.05). 3of10

4 Figure 2. Regional precipitation trends in China, Trends of annual total precipitation amount and frequency, respectively, are shown in each region. Stars indicate statistical significance (p = 0.05). Trends are expressed as a percent of the mean per decade. study (Figure 4). The heavy precipitation events in the top decile dominated the change in the total annual precipitation for China as a whole. This national pattern was also true for the summer and winter monsoon seasons, when the contribution of precipitation events below the top decile was very small. These heaviest precipitation events also made the largest contribution to the national trends during the spring and fall transition seasons, though other categories (all in the upper 50 percent) contributed nearly as much. Figure 3. Regional precipitation trends for the four seasons defined in this study. The charts show the trends of precipitation amount (black) and frequency (gray) for the four seasons. Stars indicate statistical significance (p = 0.05). Trends are expressed as a percent of the mean per decade. 4of10

5 Figure 4. Annual and seasonal trends of precipitation for categories (deciles) of precipitation intensity defined in this study. Trends are expressed as a percent of mean precipitation per decade. The bar chart at bottom left of each map gives the national average trend. 5of10

6 Figure 5. Percentage of annual precipitation resulting from the most intense (top decile) precipitation events, The solid line represents the linear trend. [14] We reached similar findings for the eight climatic regions, with the upper decile of precipitation events making the largest contribution to the trend annually and, for most regions, in all seasons. Exceptions were found in the North China Plain in winter and fall; north central China in spring and summer; and the northwest, southeast, and Tibetan Plateau in summer, where the contributions of the eighth or ninth deciles exceed that of the tenth (Figure 4). [15] For China as a whole, precipitation events in the upper decile of intensity constituted about 50% of annual total precipitation, with a small increase in this contribution from 1960 to 2000 (Figure 5). The annual, winter, and summer trends of increased precipitation can be attributed mostly to the changes in the upper decile, while that category accounted for only 35% of the decreasing trends in spring and fall. In other words, increases in precipitation were caused mainly by increases in extremely heavy precipitation events. The decrease in events in the upper half (five deciles) of intensity also significantly contributed to the decline in precipitation seen in the spring and fall transitions. [16] The frequency of precipitation decreased over the four-decade study period. Figure 6 indicates that the reduction in precipitation frequency was concentrated in the lowest decile of precipitation intensity. Nationwide, over 66% of the decline in precipitation frequency was due to the decrease in events in the lowest decile. That was true for all four seasons. On a regional basis, the frequency of events in the lowest decile decreased in all regions and all seasons except the northwest in winter. The percentage decline in this decile outpaced changes in all other deciles for all regions and all seasons except the northwest (in winter and spring) and the Tibetan Plateau (in spring), where the magnitudes of change in any decile were very small. Although the frequency of precipitation events showed a decreasing trend, the decrease was greatest in the lowest decile, the lightest precipitation events. By contrast, precipitation frequency saw a slight increasing trend in the upper decile for China as a whole annually. This was also true seasonally except for spring, when the precipitation frequency in the top decile saw almost no change. [17] We found that the late twentieth century trend of precipitation frequency in China was very different from that of the United States as reported by Karl and Knight [1998]. First, precipitation frequency in the contiguous United States as a whole increased both annually and seasonally; this increasing trend was also true for most regions of the United States. In China, though, precipitation frequency significantly decreased both annually and seasonally, nationwide and in seven of eight regions. Second, the increase of precipitation frequency in the contiguous United States was much more evenly distributed among different categories of precipitation intensity (see Figure 3 of Karl and Knight [1998]). However, in China the decrease of precipitation frequency was mostly related to the decrease in the lowest decile of intensity. Further investigations will be needed to explain the causal mechanisms that produce these intercontinental differences. [18] As noted above, the overall trends in precipitation amount result from changes in both frequency and intensity of precipitation events. Again following Karl and Knight [1998], we separated the contributions of these two factors to the overall trends shown in Figure 4. We found that the changing frequency of events in the heaviest precipitation category contributed the most to the national and regional precipitation trends (Figure 7). For China as a whole, the effect of the increase in frequency of events in the top decile outpaced the effect of decreasing frequency in all the other categories of intensity, resulting in the slight increase of precipitation over the past four decades. The proportional contributions of the precipitation intensity trend are shown in Figure 8. As in Figure 7, the top decile provides the largest contribution to the total precipitation trends for China as a whole, both annually and seasonally. [19] Focusing on the changing character of heavy precipitation events, we compared the relative contributions of changing frequency and intensity on annual and seasonal bases. In six regions (the northeast, north China plain, east, southeast, northwest and north central China) and nationally, the frequency and intensity of heavy precipitation events changed in tandem, both showing either increases or decreases. Nationwide, the change in frequency of these heavy precipitation events contributed 95% of the total increase of precipitation in the top decile. In these six climatic regions, the trend of heavy precipitation events accounted for 69 to 96% of the total change (increase or decrease) of precipitation in this category. The contribution of the change in intensity within the heaviest precipitation category was small, accounting for only about 5% of the change of total precipitation within this category nationally. [20] Considering events within the top decile of intensity among those six regions, the changes in intensity produced the largest contributions to the total precipitation trends in east and north central China (about 31% and 32%, respectively) and the smallest contributions in northeast China (accounting for less than 4% of the total change). In the two remaining regions the Tibetan Plateau and southwest China frequency and intensity displayed countervailing trends. The effect of precipitation frequency dominated in the Tibetan Plateau, while precipitation intensity had the greater effect in southwest China. For China as a whole and the five climatic regions with increasing precipitation, our 6of10

7 Figure 6. Annual and seasonal trends of precipitation frequency by decile. Trends are expressed as a percentage of the mean precipitation frequency per decade. 7of10

8 Figure 7. Contribution of the change in precipitation frequency by decile to the trends in Figure 4. Contributions are expressed as a percentage of the mean precipitation per decade. 8of10

9 Figure 8. Contribution of the change in precipitation intensity by decile to the trends in Figure 4. Contributions are expressed as a percentage of the mean precipitation per decade. 9of10

10 results support the view that increasing numbers of heavy precipitation events make the largest contribution to changes in total annual precipitation. In north central China and the North China Plain, regions with decreasing trend of precipitation, it likewise appears to be the rapid decline of events in heavy precipitation contributes to the decrease. 5. Conclusions [21] Previous studies based on climate model simulations projected that the Asian monsoons would weaken in winter and strengthen in summer under greenhouse gas induced global warming due to the land-sea thermal contrast [Intergovernmental Panel on Climate Change, 2001; Bueh, 2003; Bueh et al., 2003]. However, some recent studies based on instrumental measurements did not support the model projections. Kripalani et al. [2003] found that the summer monsoon rainfall from 1871 to 2001 over India did not change with global warming and that the summer monsoon in eastern Asia weakened, as evidenced by decreasing wind speeds in midlatitude Asia [Wang, 2001]. Our results that precipitation frequency and intensity in the summer in the heaviest precipitation category increased in northeast, east, southeast and southwest China in the past four decades do not support the conclusion that the summer monsoons in China have weakened as reported in a recent study [Gong and Wang, 2000]. However, our results do not support the recent modeled result that the summer monsoons have strengthened [Bueh, 2003; Bueh et al., 2003] either, because both precipitation amount and frequency have decreased in the North China Plain. Our results suggest that the response of China s monsoon to global warming may vary among different regions or the response may be modified by various local/regional factors. [22] We note that the increasing proportion of precipitation delivered by heavy rainfall events has potentially serious ramifications for flood control and water resource management. Summer monsoon rains often result in flooding in eastern and southern China. Of the regional trends reported here that reach the level of statistical significance, the marked decrease in the overall frequency of precipitation in all of China s major agricultural regions is of increasing concern. The seasonal patterns indicate shifts in the timing of precipitation on which stable crop production depends, as expressed in the traditional Chinese aphorism spring rain is as valuable as oil. Spring precipitation shows declines in the five regions (the northeast, North China Plain, east, north central and southwest) that account for most of China s harvest. These implications and the discrepancies between model projections and observations should spur further research into the responses of the Asian monsoon to anthropogenic global warming. [23] Acknowledgments. The authors thank the Climate Data Center, China Meteorological Administration, for providing the historical climate data for this study. We also thank two anonymous reviewers for their constructive comments on the manuscript. Rutgers University and New Jersey Agricultural Experiment Station provided funds and facilities for data analysis. References Bueh, C. (2003), Simulation of the future change of East Asian monsoon climate using the IPCCSRES A2 and B2 scenarios, Chin. Sci. Bull., 48, Bueh, C., U. Cubasch, and S. Hagemann (2003), Impacts of global warming on changes in the East Asian monsoon and the related river discharge in a global time-slice experiment, Clim. Res., 24, Fowler, A. M., and K. J. 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Henderson, Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA , USA. B. Liu, College of Forestry, Northeast Forestry University, Harbin , China. Y. Qi, Global Environment Policy Center, Beijing Normal University, Beijing , China. M. Xu, Center for Remote Sensing and Spatial Analysis, Department of Ecology, Evolution and Natural Resources, Rutgers University, New Brunswick, NJ 08901, USA. (mingxu@crssa.rutgers.edu) 10 of 10